path: root/dviware/laserjet/dviplus.web

% DVIPLUS.WEB
%
% This program is not copyrighted and may be used freely.
%
% Written by Tor Lillqvist
%            Technical Research Centre of Finland
%            Lehtisaarentie 2 A
%            SF-00340  HELSINKI
%            FINLAND
%
% E-mail: tml@@fingate.bitnet, tml@@santra.uucp, ...!mcvax!santra!tml
% Phone: +358 0 4566132
% Telex: 122972 vttha sf
  
% Here is TeX material that gets inserted after \input webmac
  
\def\hang{\hangindent 3em\indent\ignorespaces}
\font\ninerm=cmr9
\let\mc=\ninerm % medium caps for names like SAIL
\def\PASCAL{Pascal}
  
\def\(#1){} % this is used to make section names sort themselves better
\def\9#1{} % this is used for sort keys in the index
  
\def\title{DVI$\,$\lowercase{plus}}
\def\contentspagenumber{1}
\def\topofcontents{\null
  \def\titlepage{F} % include headline on the contents page
  \def\rheader{\mainfont\hfil \contentspagenumber}
  \vfill
  \centerline{\titlefont The {\ttitlefont DVIplus} processor}
  \vskip 10pt
  \centerline{\titlefont for HP LaserJet+}
  \vskip 15pt
  \centerline{(Version 1.2, April 1986)}
  \vfill}
\def\botofcontents{\vfill
  \centerline{\baselineskip9pt
    \vbox{\ninerm\noindent
    `\TeX' is a
    trademark of the American Mathematical Society.}}}
\pageno=\contentspagenumber \advance\pageno by 1
  
@* Introduction.
The \.{DVIplus} program reads binary device-independent (``\.{DVI}'')
files that are produced by document compilers such as \TeX,
and translates them for printing on a HP LaserJet+
page printer.
This program is written by Tor Lillqvist,
based on the DVItype program by Donald E.~Knuth.
  
This version is for the \PASCAL/1000 compiler on the RTE--A operating
system running on the HP1000 A--Series computers.
  
Programs for
typesetting need to be especially careful about how they do arithmetic; if
rounding errors accumulate, margins won't be straight, vertical rules
won't line up, and so on. But if rounding is done everywhere, even in the
midst of words, there will be uneven spacing between the letters, and that
looks bad. Human eyes notice differences of a thousandth of an inch in the
positioning of lines that are close together; on low resolution devices,
where rounding produces effects four times as great as this, the problem
is especially critical. Experience has shown that unusual care is needed
even on high-resolution equipment; for example, a mistake in the sixth
significant hexadecimal place of a constant once led to a difficult-to-find
bug in some software for the Alphatype CRS, which has a resolution of 5333
pixels per inch (make that 5333.33333333 pixels per inch).  The document
compilers that generate \.{DVI} files make certain assumptions about the
arithmetic that will be used by \.{DVI}-reading software, and if these
assumptions are violated the results will be of inferior quality.
Therefore the present program is intended as a guide to proper procedure
in the critical places where a bit of subtlety is involved.
  
The |banner| string defined here should be changed whenever \.{DVIplus}
gets modified.
The editor of RTE--A automagically updates this timestamp when
the file is written.
  
@d banner=='This is DVIplus, RTE-A Version 1.2 <860605.2059>'
  
@ This program is not written in standard \PASCAL, but
the \PASCAL/1000 dialect used on HP1000 A--series computers
running the RTE--A operating system;
it should be easy to convert to other reasonable \PASCAL\
dialects.
  
Places where pecliarities depndent on the RTE--A implmentation have
been used are listed in the index under ``system dependencies''.
@!@^system dependencies@>
  
One of the extensions to standard \PASCAL\ that we shall deal with is the
ability to move to a random place in a binary file; another is to
determine the length of a binary file.
In fact, in RTE--A we cannot determine the logical length of a binary file,
so another approach is used: The first doubleword in \.{PXL} and \.{DVI}
files contains the size of the file (in doublewords = integers).
The records are numbered sequentially starting from 1. The internal pointers
in \.{DVI} and \.{PXL} files start the (byte) numbering from 0, so an offset
is added in the |seek| calls.
 
\PASCAL/1000 allows writing nonprinting characters to |text| files
using a notation with a number sign followed by the decimal
ordinal number of the character. For example, |write(f, #27'E')| writes
an escape followed by a capital `E'.
  
In \PASCAL/1000, output to |text| files is line buffered, and we cannot
keep |write|ing without using |writeln| every now and then.
This terminates the record, which
normally causes a newline, but the terminal drivers in RTE--A
leave the newline out if the last byte of a write request is underscore.
This is handled by the |write_lj| macro.
  
Another extension is to use a default |case| as in \.{TANGLE}, \.{WEAVE},
etc.
  
@d othercases == otherwise {default for cases not listed explicitly}
@d endcases == @+end {follows the default case in an extended |case| statement}
@f othercases == else
@f endcases == end
  
@ Before the program heading we have some compiler options.
These specify that code should be generated for the `CDS' mode,
i.e. Code-and-Data-Separation.
  
The binary input comes from |dvi_file| and the |pxl_file|s.
@^system dependencies@>
  
@p
@=$Standard_Level 'HP1000', CDS On, Debug, Range Off$@>@/
program DVIplus;
type @<Types in the outer block@>@/
var @<Globals in the outer block@>@/
@<External routine declarations@>@/
procedure initialize; {this procedure gets things started properly}
var i:0..255;
begin
  @<Set initial values@>@/
end;
  
@ Here are some macros for common programming idioms.
  
@d do_nothing == {empty statement}
@d incr(#) == #:=#+1 {increase a variable by unity}
@d decr(#) == #:=#-1 {decrease a variable by unity}
  
@ We do our own error handling.
@^system dependencies@>
  
@d catch_errors==error_status:=true
@d dont_catch_errors==error_status:=false
  
@<Types...@>=
@!error_type=(err_run,err_ema,err_io,err_fmp,err_seg,err_wrn);
@!error_file_name=packed array [1..150] of char;
  
@ The |error_status| flag is set |true| when errors are to be catched.
If a file system related error occurs, |error_status| is set false.
  
@<Glob...@>=
@!error_status:boolean;
  
@ If an error that we are not prepared to catch occurs, we call
the standard error printing routine, and |halt|.
  
@p procedure errorprinter @=$Alias 'Pas.ErrorPrinter'$@>
(err_type: error_type; err_number, err_line: short;
var err_file: error_file_name; err_flen: short);
  external;@t\2@>@/
@#
procedure errorcatcher @=$Alias 'Pas.ErrorCatcher'$@>
(err_type: error_type; err_number, err_line: short;
var err_file: error_file_name; err_flen: short);
begin
  if err_type=err_wrn then
     do_nothing
  else if error_status and (err_type=err_fmp) then
    error_status := false
  else begin
    errorprinter(err_type,err_number,err_line,err_file,err_flen);
    halt(1); {an error we weren't prepared to catch}
  end;
end;
  
@ External routines are declared here.
|parameters| retrieves a runstring parameter.
|LURQ| is used to lock the output device.
|dcb_address| returns a pointer to the DCB (a sort of file control
block for an open file.
|fmp_interactive| tells if a DCB corresponds to an interactive device,
and |fmp_lu| returns the `logical unit' number of a device.
@^system dependencies@>
  
@<External routine...@>=
function parameters @=$Alias 'Pas.Parameters'$@>
  (pos:short; var par:file_name; len:short):short; external; @t\2@>@#
  
procedure LURQ(option:short; var lu:short; num: short; var key:short);
  external; @t\2@>@#
  
procedure dcb_address @=$Alias 'Pas.DcbAddress1'$@>
  (var p:dcb_ptr; var f:text_file); external; @t\2@>@#
  
function fmp_interactive @=$Alias 'FmpInteractive'$@>
  (var d:dcb):short; external; @t\2@>@#
  
function fmp_lu @=$Alias 'FmpLU'$@> (var d:dcb):short; external; @t\2@>@#
  
@ Types related to system routines.
  
@d name_length=64 {a file name shouldn't be longer than this}
  
@<Types...@>=
@!short=-32768..32767;
@!file_name=packed array [1..name_length] of char;
@!dcb=array [1..144] of short;
@!dcb_ptr=^dcb;
  
@ @<Globals...@>=
@!laser_dcb:dcb_ptr;
@!lu,@!key:short;
  
@ We lock the printer after opening it if it is a device.
This prevents intermixed output.
The default output file name is given here.
@^system dependencies@>
  
@d default_out=='61'
  
@<Open and lock |laser_file|@>=
if parameters(arg_index,cur_name,name_length) <= 0 then
  cur_name:=default_out;
rewrite(laser_file,cur_name,'NOCCTL,SHARED');
dcb_address(laser_dcb,laser_file);
if fmp_interactive(laser_dcb^) <> 0 then begin
  lu:=fmp_lu(laser_dcb^);
  LURQ(1,lu,1,key);
end
  
@ Labels (global and local).
  
@d done=30 {go here when finished with a subtask}
@d exit=999 {go here to leave a procedure}
@d return==goto exit
  
@ The following parameters can be changed to extend or
reduce \.{DVIplus}'s capacity.
  
@d max_fonts=75 {maximum number of distinct fonts per \.{DVI} file}
@d max_printer_fonts=20 {maximum number of fonts kept in printer}
@d max_fonts_on_page=16 {maximum number of fonts per page}
@d max_widths=9601 {|max_fonts * 128 + 1|}
  {maximum number of different characters among all fonts}
@d line_length=79 {bracketed lines of output will be at most this long}
@d terminal_line_length=128 {maximum number of characters input in a single
  line of input from the terminal}
@d stack_size=100 {\.{DVI} files shouldn't |push| beyond this depth}
@d name_size=1000 {total length of all font file names}
@d max_bops=1000 {maximum number of pages printed}
  
@ If the \.{DVI} file is badly malformed, the whole process must be aborted;
\.{DVIplus} will give up, after issuing an error message about the symptoms
that were noticed.
  
Such errors might be discovered inside of subroutines inside of subroutines,
so a procedure called |jump_out| has been introduced. This procedure simply
halts the program using the \PASCAL/1000 procedure |halt|.
@^system dependencies@>
  
@d print(#)==write(term_out,#)
@d print_ln(#)==write_ln(term_out,#)
@d print_nl==write_ln(term_out)
@d abort(#)==begin print('? ',#); jump_out;
    end
@d bad_dvi(#)==abort('Bad DVI file: ',#,'!')
@.Bad DVI file@>
@d bad_pxl(#)==begin print_nl; abort('Bad PXL file: ',#,'!'); end
@.Bad PXL file@>
  
@p procedure jump_out;
begin halt(1)
end;
  
@ We fill output lines to |laser_file|, breaking the line when
|lj_threshold| characters have been written. An underscore character
is appended to the line before the newline (record boundary).
This prevents the RTE--A terminal driver from writing a {\tt CR~LF}
pair at the end of the line.
@^system dependencies@>
  
All output to |laser_file| goes through the macros |write_lj| or
|write_lj_f|. The latter doesn't test for line break before writing.
 
@d lj_threshold=70
  
@d write_lj(#)==begin if linepos(laser_file)>lj_threshold then
    write_ln(laser_file,'_');
  write(laser_file,#)
end
@d write_lj_f(#)==write(laser_file,#)
  
@* The character set.
Like all programs written with the  \.{WEB} system, \.{DVIplus} can be
used with any character set. But it uses ASCII code internally, because
the programming for portable input-output is easier when a fixed internal
code is used, and because \.{DVI} files use ASCII code for file names
and certain other strings.
  
The next few sections of \.{DVIplus} have therefore been copied from the
analogous ones in the \.{WEB} system routines. They have been considerably
simplified, since \.{DVIplus} need not deal with the controversial
ASCII codes less than @'40. If such codes appear in the \.{DVI} file,
they will be printed as question marks.
  
@<Types...@>=
@!ASCII_code=" ".."~"; {a subrange of the integers}
  
@ The original \PASCAL\ compiler was designed in the late 60s, when six-bit
character sets were common, so it did not make provision for lower case
letters. Nowadays, of course, we need to deal with both upper and lower case
alphabets in a convenient way, especially in a program like \.{DVIplus}.
So we shall assume that the \PASCAL\ system being used for \.{DVIplus}
has a character set containing at least the standard visible characters
of ASCII code (|"!"| through |"~"|).
  
Some \PASCAL\ compilers use the original name |char| for the data type
associated with the characters in text files, while other \PASCAL s
consider |char| to be a 64-element subrange of a larger data type that has
some other name.  In order to accommodate this difference, we shall use
the name |text_char| to stand for the data type of the characters in the
output file.  We shall also assume that |text_char| consists of
the elements |chr(first_text_char)| through |chr(last_text_char)|,
inclusive. The following definitions should be adjusted if necessary.
@^system dependencies@>
  
@d text_char == char {the data type of characters in text files}
@d first_text_char=0 {ordinal number of the smallest element of |text_char|}
@d last_text_char=255 {ordinal number of the largest element of |text_char|}

@<Types...@>=
@!text_file=text_;

@ The \.{DVIplus} processor converts between ASCII code and
the user's external character set by means of arrays |xord| and |xchr|
that are analogous to \PASCAL's |ord| and |chr| functions.

@<Globals...@>=
@!xord: array [text_char] of ASCII_code;
  {specifies conversion of input characters}
@!xchr: array [0..255] of text_char;
  {specifies conversion of output characters}

@ Under our assumption that the visible characters of standard ASCII are
all present, the following assignment statements initialize the
|xchr| array properly, without needing any system-dependent changes.

@<Set init...@>=
for i:=0 to @'37 do xchr[i]:='?';
xchr[@'40]:=' ';
xchr[@'41]:='!';
xchr[@'42]:='"';
xchr[@'43]:='#';
xchr[@'44]:='$';
xchr[@'45]:='%';
xchr[@'46]:='&';
xchr[@'47]:='''';@/
xchr[@'50]:='(';
xchr[@'51]:=')';
xchr[@'52]:='*';
xchr[@'53]:='+';
xchr[@'54]:=',';
xchr[@'55]:='-';
xchr[@'56]:='.';
xchr[@'57]:='/';@/
xchr[@'60]:='0';
xchr[@'61]:='1';
xchr[@'62]:='2';
xchr[@'63]:='3';
xchr[@'64]:='4';
xchr[@'65]:='5';
xchr[@'66]:='6';
xchr[@'67]:='7';@/
xchr[@'70]:='8';
xchr[@'71]:='9';
xchr[@'72]:=':';
xchr[@'73]:=';';
xchr[@'74]:='<';
xchr[@'75]:='=';
xchr[@'76]:='>';
xchr[@'77]:='?';@/
xchr[@'100]:='@@';
xchr[@'101]:='A';
xchr[@'102]:='B';
xchr[@'103]:='C';
xchr[@'104]:='D';
xchr[@'105]:='E';
xchr[@'106]:='F';
xchr[@'107]:='G';@/
xchr[@'110]:='H';
xchr[@'111]:='I';
xchr[@'112]:='J';
xchr[@'113]:='K';
xchr[@'114]:='L';
xchr[@'115]:='M';
xchr[@'116]:='N';
xchr[@'117]:='O';@/
xchr[@'120]:='P';
xchr[@'121]:='Q';
xchr[@'122]:='R';
xchr[@'123]:='S';
xchr[@'124]:='T';
xchr[@'125]:='U';
xchr[@'126]:='V';
xchr[@'127]:='W';@/
xchr[@'130]:='X';
xchr[@'131]:='Y';
xchr[@'132]:='Z';
xchr[@'133]:='[';
xchr[@'134]:='\';
xchr[@'135]:=']';
xchr[@'136]:='^';
xchr[@'137]:='_';@/
xchr[@'140]:='`';
xchr[@'141]:='a';
xchr[@'142]:='b';
xchr[@'143]:='c';
xchr[@'144]:='d';
xchr[@'145]:='e';
xchr[@'146]:='f';
xchr[@'147]:='g';@/
xchr[@'150]:='h';
xchr[@'151]:='i';
xchr[@'152]:='j';
xchr[@'153]:='k';
xchr[@'154]:='l';
xchr[@'155]:='m';
xchr[@'156]:='n';
xchr[@'157]:='o';@/
xchr[@'160]:='p';
xchr[@'161]:='q';
xchr[@'162]:='r';
xchr[@'163]:='s';
xchr[@'164]:='t';
xchr[@'165]:='u';
xchr[@'166]:='v';
xchr[@'167]:='w';@/
xchr[@'170]:='x';
xchr[@'171]:='y';
xchr[@'172]:='z';
xchr[@'173]:='{';
xchr[@'174]:='|';
xchr[@'175]:='}';
xchr[@'176]:='~';
for i:=@'177 to 255 do xchr[i]:='?';
  
@ The following system-independent code makes the |xord| array contain a
suitable inverse to the information in |xchr|.
  
@<Set init...@>=
for i:=first_text_char to last_text_char do xord[chr(i)]:=@'40;
for i:=" " to "~" do xord[xchr[i]]:=i;
  
@ The LaserJet refuses to print other that `visible characters', that is
characters in the range @'41--@'177 or @'240--@'377.
  
@p function vis_chr(p : integer):char;
begin
  if p < @'41 then
    p:=p+@'240;
  vis_chr:=chr(p);
end;
  
@* Device-independent file format.
Before we get into the details of \.{DVIplus}, we need to know exactly
what \.{DVI} files are. The form of such files was designed by David R.
@^Fuchs, David Raymond@>
Fuchs in 1979. Almost any reasonable typesetting device can be driven by
a program that takes \.{DVI} files as input, and dozens of such
\.{DVI}-to-whatever programs have been written. Thus, it is possible to
print the output of document compilers like \TeX\ on many different kinds
of equipment.
  
A \.{DVI} file is a stream of 8-bit bytes, which may be regarded as a
series of commands in a machine-like language. The first byte of each command
is the operation code, and this code is followed by zero or more bytes
that provide parameters to the command. The parameters themselves may consist
of several consecutive bytes; for example, the `|set_rule|' command has two
parameters, each of which is four bytes long. Parameters are usually
regarded as nonnegative integers; but four-byte-long parameters,
and shorter parameters that denote distances, can be
either positive or negative. Such parameters are given in two's complement
notation. For example, a two-byte-long distance parameter has a value between
$-2^{15}$ and $2^{15}-1$.
@.DVI {\rm files}@>

A \.{DVI} file consists of a ``preamble,'' followed by a sequence of one
or more ``pages,'' followed by a ``postamble.'' The preamble is simply a
|pre| command, with its parameters that define the dimensions used in the
file; this must come first.  Each ``page'' consists of a |bop| command,
followed by any number of other commands that tell where characters are to
be placed on a physical page, followed by an |eop| command. The pages
appear in the order that they were generated, not in any particular
numerical order. If we ignore |nop| commands and \\{fnt\_def} commands
(which are allowed between any two commands in the file), each |eop|
command is immediately followed by a |bop| command, or by a |post|
command; in the latter case, there are no more pages in the file, and the
remaining bytes form the postamble.  Further details about the postamble
will be explained later.

Some parameters in \.{DVI} commands are ``pointers.'' These are four-byte
quantities that give the location number of some other byte in the file;
the first byte is number~0, then comes number~1, and so on. For example,
one of the parameters of a |bop| command points to the previous |bop|;
this makes it feasible to read the pages in backwards order, in case the
results are being directed to a device that stacks its output face up.
Suppose the preamble of a \.{DVI} file occupies bytes 0 to 99. Now if the
first page occupies bytes 100 to 999, say, and if the second
page occupies bytes 1000 to 1999, then the |bop| that starts in byte 1000
points to 100 and the |bop| that starts in byte 2000 points to 1000. (The
very first |bop|, i.e., the one that starts in byte 100, has a pointer of $-1$.)

@ The \.{DVI} format is intended to be both compact and easily interpreted
by a machine. Compactness is achieved by making most of the information
implicit instead of explicit. When a \.{DVI}-reading program reads the
commands for a page, it keeps track of several quantities: (a)~The current
font |f| is an integer; this value is changed only
by \\{fnt} and \\{fnt\_num} commands. (b)~The current position on the page
is given by two numbers called the horizontal and vertical coordinates,
|h| and |v|. Both coordinates are zero at the upper left corner of the page;
moving to the right corresponds to increasing the horizontal coordinate, and
moving down corresponds to increasing the vertical coordinate. Thus, the
coordinates are essentially Cartesian, except that vertical directions are
flipped; the Cartesian version of |(h,v)| would be |(h,-v)|.  (c)~The
current spacing amounts are given by four numbers |w|, |x|, |y|, and |z|,
where |w| and~|x| are used for horizontal spacing and where |y| and~|z|
are used for vertical spacing. (d)~There is a stack containing
|(h,v,w,x,y,z)| values; the \.{DVI} commands |push| and |pop| are used to
change the current level of operation. Note that the current font~|f| is
not pushed and popped; the stack contains only information about
positioning.

The values of |h|, |v|, |w|, |x|, |y|, and |z| are signed integers having up
to 32 bits, including the sign. Since they represent physical distances,
there is a small unit of measurement such that increasing |h| by~1 means
moving a certain tiny distance to the right. The actual unit of
measurement is variable, as explained below.

@ Here is a list of all the commands that may appear in a \.{DVI} file. Each
command is specified by its symbolic name (e.g., |bop|), its opcode byte
(e.g., 139), and its parameters (if any). The parameters are followed
by a bracketed number telling how many bytes they occupy; for example,
`|p[4]|' means that parameter |p| is four bytes long.

\yskip\hang|set_char_0| 0. Typeset character number~0 from font~|f|
such that the reference point of the character is at |(h,v)|. Then
increase |h| by the width of that character. Note that a character may
have zero or negative width, so one cannot be sure that |h| will advance
after this command; but |h| usually does increase.

\yskip\hang|set_char_1| through |set_char_127| (opcodes 1 to 127).
Do the operations of |set_char_0|; but use the character whose number
matches the opcode, instead of character~0.

\yskip\hang|set1| 128 |c[1]|. Same as |set_char_0|, except that character
number~|c| is typeset. \TeX82 uses this command for characters in the
range |128<=c<256|.

\yskip\hang|set2| 129 |c[2]|. Same as |set1|, except that |c|~is two
bytes long, so it is in the range |0<=c<65536|. \TeX82 never uses this
command, which is intended for processors that deal with oriental languages;
but \.{DVIplus} will allow character codes greater than 255, assuming that
they all have the same width as the character whose code is $c \bmod 256$.
@^oriental characters@>@^Chinese characters@>@^Japanese characters@>

\yskip\hang|set3| 130 |c[3]|. Same as |set1|, except that |c|~is three
bytes long, so it can be as large as $2^{24}-1$.

\yskip\hang|set4| 131 |c[4]|. Same as |set1|, except that |c|~is four
bytes long, possibly even negative. Imagine that.

\yskip\hang|set_rule| 132 |a[4]| |b[4]|. Typeset a solid black rectangle
of height |a| and width |b|, with its bottom left corner at |(h,v)|. Then
set |h:=h+b|. If either |a<=0| or |b<=0|, nothing should be typeset. Note
that if |b<0|, the value of |h| will decrease even though nothing else happens.
Programs that typeset from \.{DVI} files should be careful to make the rules
line up carefully with digitized characters, as explained in connection with
the |rule_pixels| subroutine below.

\yskip\hang|put1| 133 |c[1]|. Typeset character number~|c| from font~|f|
such that the reference point of the character is at |(h,v)|. (The `put'
commands are exactly like the `set' commands, except that they simply put out a
character or a rule without moving the reference point afterwards.)

\yskip\hang|put2| 134 |c[2]|. Same as |set2|, except that |h| is not changed.

\yskip\hang|put3| 135 |c[3]|. Same as |set3|, except that |h| is not changed.

\yskip\hang|put4| 136 |c[4]|. Same as |set4|, except that |h| is not changed.

\yskip\hang|put_rule| 137 |a[4]| |b[4]|. Same as |set_rule|, except that
|h| is not changed.

\yskip\hang|nop| 138. No operation, do nothing. Any number of |nop|'s
may occur between \.{DVI} commands, but a |nop| cannot be inserted between
a command and its parameters or between two parameters.

\yskip\hang|bop| 139 $c_0[4]$ $c_1[4]$ $\ldots$ $c_9[4]$ $p[4]$. Beginning
of a page: Set |(h,v,w,x,y,z):=(0,0,0,0,0,0)| and set the stack empty. Set
the current font |f| to an undefined value.  The ten $c_i$ parameters can
be used to identify pages, if a user wants to print only part of a \.{DVI}
file; \TeX82 gives them the values of \.{\\count0} $\ldots$ \.{\\count9}
at the time \.{\\shipout} was invoked for this page.  The parameter |p|
points to the previous |bop| command in the file, where the first |bop|
has $p=-1$.

\yskip\hang|eop| 140.  End of page: Print what you have read since the
previous |bop|. At this point the stack should be empty. (The \.{DVI}-reading
programs that drive most output devices will have kept a buffer of the
material that appears on the page that has just ended. This material is
largely, but not entirely, in order by |v| coordinate and (for fixed |v|) by
|h|~coordinate; so it usually needs to be sorted into some order that is
appropriate for the device in question. \.{DVIplus} does not do such sorting.)

\yskip\hang|push| 141. Push the current values of |(h,v,w,x,y,z)| onto the
top of the stack; do not change any of these values. Note that |f| is
not pushed.

\yskip\hang|pop| 142. Pop the top six values off of the stack and assign
them to |(h,v,w,x,y,z)|. The number of pops should never exceed the number
of pushes, since it would be highly embarrassing if the stack were empty
at the time of a |pop| command.

\yskip\hang|right1| 143 |b[1]|. Set |h:=h+b|, i.e., move right |b| units.
The parameter is a signed number in two's complement notation, |-128<=b<128|;
if |b<0|, the reference point actually moves left.

\yskip\hang|right2| 144 |b[2]|. Same as |right1|, except that |b| is a
two-byte quantity in the range |-32768<=b<32768|.

\yskip\hang|right3| 145 |b[3]|. Same as |right1|, except that |b| is a
three-byte quantity in the range |@t$-2^{23}$@><=b<@t$2^{23}$@>|.

\yskip\hang|right4| 146 |b[4]|. Same as |right1|, except that |b| is a
four-byte quantity in the range |@t$-2^{31}$@><=b<@t$2^{31}$@>|.

\yskip\hang|w0| 147. Set |h:=h+w|; i.e., move right |w| units. With luck,
this parameterless command will usually suffice, because the same kind of motion
will occur several times in succession; the following commands explain how
|w| gets particular values.

\yskip\hang|w1| 148 |b[1]|. Set |w:=b| and |h:=h+b|. The value of |b| is a
signed quantity in two's complement notation, |-128<=b<128|. This command
changes the current |w|~spacing and moves right by |b|.

\yskip\hang|w2| 149 |b[2]|. Same as |w1|, but |b| is a two-byte-long
parameter, |-32768<=b<32768|.

\yskip\hang|w3| 150 |b[3]|. Same as |w1|, but |b| is a three-byte-long
parameter, |@t$-2^{23}$@><=b<@t$2^{23}$@>|.

\yskip\hang|w4| 151 |b[4]|. Same as |w1|, but |b| is a four-byte-long
parameter, |@t$-2^{31}$@><=b<@t$2^{31}$@>|.

\yskip\hang|x0| 152. Set |h:=h+x|; i.e., move right |x| units. The `|x|'
commands are like the `|w|' commands except that they involve |x| instead
of |w|.

\yskip\hang|x1| 153 |b[1]|. Set |x:=b| and |h:=h+b|. The value of |b| is a
signed quantity in two's complement notation, |-128<=b<128|. This command
changes the current |x|~spacing and moves right by |b|.

\yskip\hang|x2| 154 |b[2]|. Same as |x1|, but |b| is a two-byte-long
parameter, |-32768<=b<32768|.

\yskip\hang|x3| 155 |b[3]|. Same as |x1|, but |b| is a three-byte-long
parameter, |@t$-2^{23}$@><=b<@t$2^{23}$@>|.

\yskip\hang|x4| 156 |b[4]|. Same as |x1|, but |b| is a four-byte-long
parameter, |@t$-2^{31}$@><=b<@t$2^{31}$@>|.

\yskip\hang|down1| 157 |a[1]|. Set |v:=v+a|, i.e., move down |a| units.
The parameter is a signed number in two's complement notation, |-128<=a<128|;
if |a<0|, the reference point actually moves up.

\yskip\hang|down2| 158 |a[2]|. Same as |down1|, except that |a| is a
two-byte quantity in the range |-32768<=a<32768|.

\yskip\hang|down3| 159 |a[3]|. Same as |down1|, except that |a| is a
three-byte quantity in the range |@t$-2^{23}$@><=a<@t$2^{23}$@>|.

\yskip\hang|down4| 160 |a[4]|. Same as |down1|, except that |a| is a
four-byte quantity in the range |@t$-2^{31}$@><=a<@t$2^{31}$@>|.

\yskip\hang|y0| 161. Set |v:=v+y|; i.e., move down |y| units. With luck,
this parameterless command will usually suffice, because the same kind of motion
will occur several times in succession; the following commands explain how
|y| gets particular values.

\yskip\hang|y1| 162 |a[1]|. Set |y:=a| and |v:=v+a|. The value of |a| is a
signed quantity in two's complement notation, |-128<=a<128|. This command
changes the current |y|~spacing and moves down by |a|.

\yskip\hang|y2| 163 |a[2]|. Same as |y1|, but |a| is a two-byte-long
parameter, |-32768<=a<32768|.

\yskip\hang|y3| 164 |a[3]|. Same as |y1|, but |a| is a three-byte-long
parameter, |@t$-2^{23}$@><=a<@t$2^{23}$@>|.

\yskip\hang|y4| 165 |a[4]|. Same as |y1|, but |a| is a four-byte-long
parameter, |@t$-2^{31}$@><=a<@t$2^{31}$@>|.

\yskip\hang|z0| 166. Set |v:=v+z|; i.e., move down |z| units. The `|z|' commands
are like the `|y|' commands except that they involve |z| instead of |y|.

\yskip\hang|z1| 167 |a[1]|. Set |z:=a| and |v:=v+a|. The value of |a| is a
signed quantity in two's complement notation, |-128<=a<128|. This command
changes the current |z|~spacing and moves down by |a|.

\yskip\hang|z2| 168 |a[2]|. Same as |z1|, but |a| is a two-byte-long
parameter, |-32768<=a<32768|.

\yskip\hang|z3| 169 |a[3]|. Same as |z1|, but |a| is a three-byte-long
parameter, |@t$-2^{23}$@><=a<@t$2^{23}$@>|.

\yskip\hang|z4| 170 |a[4]|. Same as |z1|, but |a| is a four-byte-long
parameter, |@t$-2^{31}$@><=a<@t$2^{31}$@>|.

\yskip\hang|fnt_num_0| 171. Set |f:=0|. Font 0 must previously have been
defined by a \\{fnt\_def} instruction, as explained below.

\yskip\hang|fnt_num_1| through |fnt_num_63| (opcodes 172 to 234). Set
|f:=1|, \dots, |f:=63|, respectively.

\yskip\hang|fnt1| 235 |k[1]|. Set |f:=k|. \TeX82 uses this command for font
numbers in the range |64<=k<256|.

\yskip\hang|fnt2| 236 |k[2]|. Same as |fnt1|, except that |k|~is two
bytes long, so it is in the range |0<=k<65536|. \TeX82 never generates this
command, but large font numbers may prove useful for specifications of
color or texture, or they may be used for special fonts that have fixed
numbers in some external coding scheme.

\yskip\hang|fnt3| 237 |k[3]|. Same as |fnt1|, except that |k|~is three
bytes long, so it can be as large as $2^{24}-1$.

\yskip\hang|fnt4| 238 |k[4]|. Same as |fnt1|, except that |k|~is four
bytes long; this is for the really big font numbers (and for the negative ones).

\yskip\hang|xxx1| 239 |k[1]| |x[k]|. This command is undefined in
general; it functions as a $(k+2)$-byte |nop| unless special \.{DVI}-reading
programs are being used. \TeX82 generates |xxx1| when a short enough
\.{\\special} appears, setting |k| to the number of bytes being sent. It
is recommended that |x| be a string having the form of a keyword followed
by possible parameters relevant to that keyword.

\yskip\hang|xxx2| 240 |k[2]| |x[k]|. Like |xxx1|, but |0<=k<65536|.

\yskip\hang|xxx3| 241 |k[3]| |x[k]|. Like |xxx1|, but |0<=k<@t$2^{24}$@>|.

\yskip\hang|xxx4| 242 |k[4]| |x[k]|. Like |xxx1|, but |k| can be ridiculously
large. \TeX82 uses |xxx4| when |xxx1| would be incorrect.

\yskip\hang|fnt_def1| 243 |k[1]| |c[4]| |s[4]| |d[4]| |a[1]| |l[1]| |n[a+l]|.
Define font |k|, where |0<=k<256|; font definitions will be explained shortly.

\yskip\hang|fnt_def2| 244 |k[2]| |c[4]| |s[4]| |d[4]| |a[1]| |l[1]| |n[a+l]|.
Define font |k|, where |0<=k<65536|.

\yskip\hang|fnt_def3| 245 |k[3]| |c[4]| |s[4]| |d[4]| |a[1]| |l[1]| |n[a+l]|.
Define font |k|, where |0<=k<@t$2^{24}$@>|.

\yskip\hang|fnt_def4| 246 |k[4]| |c[4]| |s[4]| |d[4]| |a[1]| |l[1]| |n[a+l]|.
Define font |k|, where |@t$-2^{31}$@><=k<@t$2^{31}$@>|.

\yskip\hang|pre| 247 |i[1]| |num[4]| |den[4]| |mag[4]| |k[1]| |x[k]|.
Beginning of the preamble; this must come at the very beginning of the
file. Parameters |i|, |num|, |den|, |mag|, |k|, and |x| are explained below.

\yskip\hang|post| 248. Beginning of the postamble, see below.

\yskip\hang|post_post| 249. Ending of the postamble, see below.

\yskip\noindent Commands 250--255 are undefined at the present time.

@ @d set_char_0=0 {typeset character 0 and move right}
@d set1=128 {typeset a character and move right}
@d set_rule=132 {typeset a rule and move right}
@d put1=133 {typeset a character}
@d put_rule=137 {typeset a rule}
@d nop=138 {no operation}
@d bop=139 {beginning of page}
@d eop=140 {ending of page}
@d push=141 {save the current positions}
@d pop=142 {restore previous positions}
@d right1=143 {move right}
@d w0=147 {move right by |w|}
@d w1=148 {move right and set |w|}
@d x0=152 {move right by |x|}
@d x1=153 {move right and set |x|}
@d down1=157 {move down}
@d y0=161 {move down by |y|}
@d y1=162 {move down and set |y|}
@d z0=166 {move down by |z|}
@d z1=167 {move down and set |z|}
@d fnt_num_0=171 {set current font to 0}
@d fnt1=235 {set current font}
@d xxx1=239 {extension to \.{DVI} primitives}
@d xxx4=242 {potentially long extension to \.{DVI} primitives}
@d fnt_def1=243 {define the meaning of a font number}
@d pre=247 {preamble}
@d post=248 {postamble beginning}
@d post_post=249 {postamble ending}
@d undefined_commands==250,251,252,253,254,255

@ The preamble contains basic information about the file as a whole. As
stated above, there are six parameters:
$$\hbox{|@!i[1]| |@!num[4]| |@!den[4]| |@!mag[4]| |@!k[1]| |@!x[k]|.}$$
The |i| byte identifies \.{DVI} format; currently this byte is always set
to~2. (Some day we will set |i=3|, when \.{DVI} format makes another
incompatible change---perhaps in 1992.)

The next two parameters, |num| and |den|, are positive integers that define
the units of measurement; they are the numerator and denominator of a
fraction by which all dimensions in the \.{DVI} file could be multiplied
in order to get lengths in units of $10^{-7}$ meters. (For example, there are
exactly 7227 \TeX\ points in 254 centimeters, and \TeX82 works with scaled
points where there are $2^{16}$ sp in a point, so \TeX82 sets |num=25400000|
and $|den|=7227\cdot2^{16}=473628672$.)
@^sp@>

The |mag| parameter is what \TeX82 calls \.{\\mag}, i.e., 1000 times the
desired magnification. The actual fraction by which dimensions are
multiplied is therefore $mn/1000d$. Note that if a \TeX\ source document
does not call for any `\.{true}' dimensions, and if you change it only by
specifying a different \.{\\mag} setting, the \.{DVI} file that \TeX\
creates will be completely unchanged except for the value of |mag| in the
preamble and postamble. (Fancy \.{DVI}-reading programs allow users to
override the |mag|~setting when a \.{DVI} file is being printed.)

Finally, |k| and |x| allow the \.{DVI} writer to include a comment, which is not
interpreted further. The length of comment |x| is |k|, where |0<=k<256|.

@d dvi_id=2 {identifies the kind of \.{DVI} files described here}

@ Font definitions for a given font number |k| contain further parameters
$$\hbox{|c[4]| |s[4]| |d[4]| |a[1]| |l[1]| |n[a+l]|.}$$
The four-byte value |c| is the check sum that \TeX\ (or whatever program
generated the \.{DVI} file) found in the \.{TFM} file for this font;
|c| should match the check sum of the font found by programs that read
this \.{DVI} file.
@^check sum@>

Parameter |s| contains a fixed-point scale factor that is applied to the
character widths in font |k|; font dimensions in \.{TFM} files and other
font files are relative to this quantity, which is always positive and
less than $2^{27}$. It is given in the same units as the other dimensions
of the \.{DVI} file.  Parameter |d| is similar to |s|; it is the ``design
size,'' and it is given in \.{DVI} units that have not been corrected for
the magnification~|mag| found in the preamble.  Thus, font |k| is to be
used at $|mag|\cdot s/1000d$ times its normal size.

The remaining part of a font definition gives the external name of the font,
which is an ASCII string of length |a+l|. The number |a| is the length
of the ``area'' or directory, and |l| is the length of the font name itself;
the standard local system font area is supposed to be used when |a=0|.
The |n| field contains the area in its first |a| bytes.

Font definitions must appear before the first use of a particular font number.
Once font |k| is defined, it must not be defined again; however, we
shall see below that font definitions appear in the postamble as well as
in the pages, so in this sense each font number is defined exactly twice,
if at all. Like |nop| commands and \\{xxx} commands, font definitions can
appear before the first |bop|, or between an |eop| and a |bop|.

@ The last page in a \.{DVI} file is followed by `|post|'; this command
introduces the postamble, which summarizes important facts that \TeX\ has
accumulated about the file, making it possible to print subsets of the data
with reasonable efficiency. The postamble has the form
$$\vbox{\halign{\hbox{#\hfil}\cr
  |post| |p[4]| |num[4]| |den[4]| |mag[4]| |l[4]| |u[4]| |s[2]| |t[2]|\cr
  $\langle\,$font definitions$\,\rangle$\cr
  |post_post| |q[4]| |i[1]| 223's$[{\G}4]$\cr}}$$
Here |p| is a pointer to the final |bop| in the file. The next three
parameters, |num|, |den|, and |mag|, are duplicates of the quantities that
appeared in the preamble.

Parameters |l| and |u| give respectively the height-plus-depth of the tallest
page and the width of the widest page, in the same units as other dimensions
of the file. These numbers might be used by a \.{DVI}-reading program to
position individual ``pages'' on large sheets of film or paper.

Parameter |s| is the maximum stack depth (i.e., the largest excess of
|push| commands over |pop| commands) needed to process this file. Then
comes |t|, the total number of pages (|bop| commands) present.

The postamble continues with font definitions, which are any number of
\\{fnt\_def} commands as described above, possibly interspersed with |nop|
commands. Each font number that is used in the \.{DVI} file must be defined
exactly twice: Once before it is first selected by a \\{fnt} command, and once
in the postamble.

@ The last part of the postamble, following the |post_post| byte that
signifies the end of the font definitions, contains |q|, a pointer to the
|post| command that started the postamble.  An identification byte, |i|,
comes next; this currently equals~2, as in the preamble.

The |i| byte is followed by four or more bytes that are all equal to
the decimal number 223 (i.e., @'337 in octal). \TeX\ puts out four to seven of
these trailing bytes, until the total length of the file is a multiple of
four bytes, since this works out best on machines that pack four bytes per
word; but any number of 223's is allowed, as long as there are at least four
of them. In effect, 223 is a sort of signature that is added at the very end.
@^Fuchs, David Raymond@>

This curious way to finish off a \.{DVI} file makes it feasible for
\.{DVI}-reading programs to find the postamble first, on most computers,
even though \TeX\ wants to write the postamble last. Most operating
systems permit random access to individual words or bytes of a file, so
the \.{DVI} reader can start at the end and skip backwards over the 223's
until finding the identification byte. Then it can back up four bytes, read
|q|, and move to byte |q| of the file. This byte should, of course,
contain the value 248 (|post|); now the postamble can be read, so the
\.{DVI} reader discovers all the information needed for typesetting the
pages. Note that it is also possible to skip through the \.{DVI} file at
reasonably high speed to locate a particular page, if that proves
desirable. This saves a lot of time, since \.{DVI} files used in production
jobs tend to be large.
  
@* Input from binary files.
We have seen that a \.{DVI} file is a sequence of 8-bit bytes. The bytes
appear physically in what is called a `|packed file of 0..255|'
in \PASCAL\ lingo.
  
Packing is system dependent, and many \PASCAL\ systems fail to implement
such files in a sensible way (at least, from the viewpoint of producing
good production software).  For example, some systems treat all
byte-oriented files as text, looking for end-of-line marks and such
things. Therefore some system-dependent code is often needed to deal with
binary files.
@^system dependencies@>
  
In the RTE--A implementation, \.{DVI} and \.{PXL} files are files of
|integer|s,
with the first |integer| containing the number of |integer|s to follow.
(It is impossible to know the logical size of a random-access file).
The bytes of the file are accessed through a variant record.
  
@d pxl_id=1001 {identifies the kind of \.{PXL} files handled}
  
@<Types...@>=
@!eight_bits=0..255; {unsigned one-byte quantity}
four=0..3;
@!i2c=packed record case four of 0: (i:integer);@/
1:(i0,i1:short);@/
2:(b:packed array[0..3] of eight_bits);
3:(c:packed array[0..3] of char) end;
@!int_file=file of integer;
@!byte_file=record {files that contain binary data}
x:i2c;i:0..3;m,s:integer;
f:int_file;
end;
  
@ The program deals with the binary file variables |dvi_file| which is the
main input file that we are printing, and |pxl_file| is an array of
font image files from which character images is downloaded
to the printer.
  
@<Glob...@>=
@!laser_file:text_file;
@!dvi_file:byte_file; {the stuff we are \.{DVI}typing}
@!pxl_file:array[0..max_fonts] of file of i2c; {font image files}
@!s_pxl_file:integer; {size of |pxl_file|}
  
@ To prepare these files for input, we |reset| them. An extension of
\PASCAL\ is needed in the case of |pxl_file|, since we want to associate
it with external files whose names are specified dynamically (i.e., not
known at compile time). The following code assumes that `|reset(f,s)|'
does this, when |f| is a file variable and |s| is a string variable that
specifies the file name.
The function result is false if the file couldn't be opened.
@^system dependencies@>
  
@p function open_dvi_file:boolean;
var i,j:short;
begin
i:=parameters(arg_index,cur_name,name_length);
incr(arg_index);
j:=1;
while (j<=i)and(cur_name[j]<>'.') do incr(j);
if j>i then begin
  cur_name[j]:='.'; incr(j);
  cur_name[j]:='d'; incr(j);
  cur_name[j]:='v'; incr(j);
  cur_name[j]:='i'; incr(j);
end;
catch_errors;
reset(dvi_file.f,cur_name,'SHARED');
open_dvi_file:=error_status;
if error_status then begin
  dont_catch_errors;
  close(dvi_file.f);
  open(dvi_file.f,cur_name,'SHARED');
  get(dvi_file.f);
  dvi_file.s:=dvi_file.f^;
  get(dvi_file.f);
  dvi_file.x.i:=dvi_file.f^;
  dvi_file.i:=0;
end;
cur_loc:=0;
end;
@#
function open_pxl_file(i:short):boolean;
begin
catch_errors;
reset(pxl_file[i],cur_name,'SHARED');
open_pxl_file:=error_status;
if error_status then begin
  dont_catch_errors;
  close(pxl_file[i]);
  open(pxl_file[i],cur_name,'SHARED');
  get(pxl_file[i]);
  s_pxl_file:=pxl_file[i]^.i;
  get(pxl_file[i]);
  if pxl_file[i]^.i <> pxl_id then
    bad_pxl('bad header id');
end;
end;
  
@ If you looked carefully at the preceding code, you probably asked,
``What are |cur_loc| and |cur_name|?'' Good question. They're global
variables: |cur_loc| is the number of the byte about to be read next from
|dvi_file|, and |cur_name| is a string variable that will be set to the
current pixel image file name before |open_pxl_file| is called.
  
@<Glob...@>=
@!cur_loc:integer; {where we are about to look, in |dvi_file|}
@!cur_name:file_name; {external name,
  with no lower case letters}
  
@ It turns out to be convenient to read four bytes at a time, when we are
inputting from \.{PXL} files. The input goes into global variables
|b0|, |b1|, |b2|, and |b3|, with |b0| getting the first byte and |b3|
the fourth.
  
@<Glob...@>=
@!b0,@!b1,@!b2,@!b3: integer; {four bytes input at once}
  
@ The |read_pxl_word| procedure sets |b0| through |b3| to the next
four bytes in the current \.{TFM} file.
@^system dependencies@>
  
@p procedure read_pxl_word(i:short);
begin
b0:=pxl_file[i]^.b[0];b1:=pxl_file[i]^.b[1];
b2:=pxl_file[i]^.b[2];b3:=pxl_file[i]^.b[3];
get(pxl_file[i]);
end;
  
@ We shall use another set of simple functions to read the next byte or
bytes from |dvi_file|. There are seven possibilities, each of which is
treated as a separate function in order to minimize the overhead for
subroutine calls.
@^system dependencies@>
  
This is the best way to check ``eof'' in RTE--A, as the logical length of
the file is unknown (except from the first word).
  
@d eof_dvi_file==(cur_loc>dvi_file.s*4)
  
@p function get_byte:integer; {returns the next byte, unsigned}
var b:eight_bits;
begin if eof_dvi_file then get_byte:=0
else  begin
    get_byte:=dvi_file.x.b[dvi_file.i];
    if dvi_file.i=3 then begin
        get(dvi_file.f);
        dvi_file.x.i:=dvi_file.f^;
        dvi_file.i:=0; end
    else
        incr(dvi_file.i);
  end;
  incr(cur_loc);
end;
@#
function signed_byte:integer; {returns the next byte, signed}
var b:eight_bits;
begin b:=get_byte;
if b<128 then signed_byte:=b @+ else signed_byte:=b-256;
end;
@#
function get_two_bytes:integer; {returns the next two bytes, unsigned}
var a,@!b:integer;
begin a:=get_byte; b:=get_byte;
get_two_bytes:=a*256+b;
end;
@#
function signed_pair:integer; {returns the next two bytes, signed}
var a,@!b:integer;
begin a:=get_byte; b:=get_byte;
if a<128 then signed_pair:=a*256+b
else signed_pair:=(a-256)*256+b;
end;
@#
function get_three_bytes:integer; {returns the next three bytes, unsigned}
var a,@!b,@!c:eight_bits;
begin a:=get_byte; b:=get_byte; c:=get_byte;
get_three_bytes:=(a*256+b)*256+c;
end;
@#
function signed_trio:integer; {returns the next three bytes, signed}
var a,@!b,@!c:integer;
begin a:=get_byte; b:=get_byte; c:=get_byte;
if a<128 then signed_trio:=(a*256+b)*256+c
else signed_trio:=((a-256)*256+b)*256+c;
end;
@#
function signed_quad:integer; {returns the next four bytes, signed}
var a,@!b,@!c,@!d:integer;
begin a:=get_byte; b:=get_byte; c:=get_byte; d:=get_byte;
if a<128 then signed_quad:=((a*256+b)*256+c)*256+d
else signed_quad:=(((a-256)*256+b)*256+c)*256+d;
end;
  
@ Finally we come to the routines that are used to acces the |dvi_file|
randomly. The driver program below needs two such routines: |dvi_length|
should compute the total number of bytes in |dvi_file|, possibly also
causing |eof_dvi_file| to be true; and |move_to_byte(n)|
should position |dvi_file| so that the next |get_byte| will read byte |n|,
starting with |n=0| for the first byte in the file.
@^system dependencies@>
Such routines are, of course, highly system dependent.
  
@d dvi_length==(dvi_file.s*4)
@#
@d move_to_byte(#)==begin seek(dvi_file.f,((#) div 4)+2); get(dvi_file.f);
dvi_file.x.i:=dvi_file.f^; dvi_file.i:=(#) mod 4; cur_loc:=#; end
  
@
Font files
should contain exactly the same character width data that is
found in the corresponding \.{TFM}s; check sums are used to help
ensure this. In addition, font files also contain the widths of
characters in pixels, since the device-independent character widths of
\.{TFM} files are generally not perfect multiples of pixels.
  
The |pixel_width| array contains this information; when |width[k]| is the
device-independent width of some character in \.{DVI} units, |pixel_width[k]|
is the corresponding width of that character in an actual font.
The macro |char_pixel_width| is set up to be analogous to |char_width|.
  
The |status| array tells if a charater has been downloaded to the
laser printer, or if it is too large, and must be transferred as raster
graphics. It might also contain a positive value that indicates a shift
up of the reference point (because the ``baseline'' of the characters
is set at |baseline| pixels from the bottom, characters can't be deeper than
|baseline| pixels).
 
The large arrays are located in
EMA (Extended Memory Area) (outside the 32 page range addressable
with one-word addresses).
The \.{Ema\_Var} compiler options specify which variables are in EMA.
@^system dependencies@>
 
@d char_end_width(#)==#]
@d char_pixel_width(#)==pixel_width[width_base[#]+char_end_width
@d char_status(#)==status[width_base[#]+char_end_width
@d not_loaded=0
@d too_large=-1
@d loaded_ok=-2
@d baseline=55
  
@<Glob...@>=
@=$Ema_Var On$@>
@!pixel_width:array[0..max_widths] of short; {actual character widths,
  in pixels}
@!status:array[0..max_widths] of short; {character statuses}
@=$Ema_Var Off$@>
@!conv:real; {converts \.{DVI} units to pixels}
@!true_conv:real; {converts unmagnified \.{DVI} units to pixels}
@!numerator,@!denominator:integer; {stated conversion ratio}
@!mag:integer; {magnification factor times 1000}
@!desired_mag:integer;

@* Reading the font information.
The current number of known fonts is |nf|. Each known font has
an internal number |f|, where |0<=f<nf|; the external number of this font,
i.e., its font identification number in the \.{DVI} file, is
|font_num[f]|, and the external name of this font is the string that
occupies positions |font_name[f]| through |font_name[f+1]-1| of the array
|names|. The latter array consists of |ASCII_code| characters, and
|font_name[nf]| is its first unoccupied position.  A horizontal motion
in the range |-4*font_space[f]<h<font_space[f]|
will be treated as a `kern'.
A given character |c| is valid in font |f| if and only if
|char_width(f)(c)<>invalid_width|.
Finally, |char_width(f)(c)=width[width_base[f]+c]|, and |width_ptr| is the
first unused position of the |width| array.
  
@d char_width(#)==width[width_base[#]+char_end_width
@d invalid_width==@'17777777777
  
@<Glob...@>=
@!font_num:array [0..max_fonts] of integer; {external font numbers}
@!font_name:array [0..max_fonts] of 0..name_size; {starting positions
  of external font names}
@!names:array [0..name_size] of ASCII_code; {characters of names}
@!font_check_sum:array [0..max_fonts] of integer; {check sums}
@!font_scaled_size:array [0..max_fonts] of integer; {scale factors}
@!font_design_size:array [0..max_fonts] of integer; {design sizes}
@!font_mag:array [0..max_fonts] of integer;
@!font_space:array [0..max_fonts] of integer; {boundary between ``small''
  and ``large'' spaces}
@!font_used_on:array[0..max_fonts] of integer; {on which page last used}
@!width_base:array [0..max_fonts] of integer; {index into |width| table}
@=$Ema_Var On$@>
@!width:array [0..max_widths] of integer; {character widths, in \.{DVI} units}
@=$Ema_Var Off$@>
@!nf:0..max_fonts; {the number of known fonts}
@!width_ptr:0..max_widths; {the number of known character widths}
  
@ @<Set init...@>=
nf:=0; width_ptr:=0; font_name[0]:=0; font_space[0]:=0;
font_used_on[0]:=0; 
  
@ It is, of course, a simple matter to print the name of a given font.
  
@p procedure print_font(@!f:integer); {|f| is an internal font number}
var k:0..name_size; {index into |names|}
begin if f=nf then print('UNDEFINED!')
@.UNDEFINED@>
else  begin for k:=font_name[f] to font_name[f+1]-1 do
    print(xchr[names[k]]);
  end;
end;
  
@ The global variabls |pxl_check_sum|,
|pxl_design_size| are set from the \.{PXL} file.
  
@<Glob...@>=
@!pxl_check_sum:integer; {check sum found in |pxl_file|}
@!pxl_dptr:array [0..max_fonts] of integer; {directory pointers}
@!pxl_design_size:integer;
 
@ Here is a procedure that absorbs the necessary information from a
\.{PXL} file, assuming that the file has just been successfully opened.
(A complete description of
\.{PXL} file format appears elsewhere and will
not be repeated here.) The procedure does not check the \.{PXL} file
for validity, nor does it give explicit information about what is
wrong with a \.{PXL} file that proves to be invalid; \.{DVI}-reading
programs need not do this, since \.{PXL} files are almost always valid.
The procedure simply returns |false| if it
detects anything amiss in the \.{PXL} data.
  
There is a parameter, |z|, which represents the scaling factor being
used to compute the font dimensions; it must be in the range $0<z<2^{27}$.
  
@d read_pxl(#)==begin #:=pxl_file[nf]^.i; get(pxl_file[nf]); end
  
@p procedure in_PXL(@!z:integer); {input \.{PXL} data}
var i,j,k,n:short; {indices etc.}
@!wp:0..max_widths; {new value of |width_ptr| after successful input}
@!raster_address:integer;
@!alpha,@!beta:integer; {quantities used in the scaling computation}
begin
  if width_ptr+128>max_widths then begin
    print_nl;
    abort('Need larger width table');
  end;
  width_base[nf]:=width_ptr;
  wp:=width_ptr;
  @<Check the header ID@>;
  @<Read the trailer@>;
  @<Read the font directory@>;
  width_ptr:=wp;
end;
  
@ @<Check the header ID@>=
begin
  seek_pxl(nf)(0);
  get(pxl_file[nf]);
  if pxl_file[nf]^.i <> pxl_id then
    bad_pxl('bad header id');
end
  
@ @<Read the trailer@>=
begin
  seek(pxl_file[nf],s_pxl_file+1);
  get(pxl_file[nf]);
  if pxl_file[nf]^.i <> pxl_id then
    bad_pxl('bad trailer id');
  seek(pxl_file[nf],s_pxl_file-3);
  get(pxl_file[nf]);
  read_pxl(pxl_check_sum);
  read_pxl(font_mag[nf]);
  read_pxl(pxl_design_size);
  read_pxl(pxl_dptr[nf]);
end;
  
@ One important part of |in_PXL| is the width computation, which
involves multiplying the relative widths in the \.{PXL} file by the
scaling factor in the \.{DVI} file. This fixed-point multiplication
must be done with precisely the same accuracy by all \.{DVI}-reading programs,
in order to validate the assumptions made by \.{DVI}-writing programs
like \TeX82.
  
Let us therefore summarize what needs to be done. Each width in a \.{PXL}
file appears as a four-byte quantity called a |fix_word|.  A |fix_word|
whose respective bytes are $(a,b,c,d)$ represents the number
$$x=\left\{\vcenter{\halign{$#$,\hfil\qquad&if $#$\hfil\cr
b\cdot2^{-4}+c\cdot2^{-12}+d\cdot2^{-20}&a=0;\cr
-16+b\cdot2^{-4}+c\cdot2^{-12}+d\cdot2^{-20}&a=255.\cr}}\right.$$
(No other choices of $a$ are allowed, since the magnitude of a \.{TFM}
dimension must be less than 16.)  We want to multiply this quantity by the
integer~|z|, which is known to be less than $2^{27}$. Let $\alpha=16z$.
If $|z|<2^{23}$, the individual multiplications $b\cdot z$, $c\cdot z$,
$d\cdot z$ cannot overflow; otherwise we will divide |z| by 2, 4, 8, or
16, to obtain a multiplier less than $2^{23}$, and we can compensate for
this later. If |z| has thereby been replaced by $|z|^\prime=|z|/2^e$, let
$\beta=2^{4-e}$; we shall compute
$$\lfloor(b+c\cdot2^{-8}+d\cdot2^{-16})\,z^\prime/\beta\rfloor$$ if $a=0$,
or the same quantity minus $\alpha$ if $a=255$.  This calculation must be
done exactly, for the reasons stated above; the following program does the
job in a system-independent way, assuming that arithmetic is exact on
numbers less than $2^{31}$ in magnitude.
  
The following code computes pixel widths by simply rounding the \.{PXL}
widths to the nearest integer number of pixels, based on the conversion factor
|conv| that converts \.{DVI} units to pixels. However, such a simple
formula will not be valid for all fonts, and it will often give results that
are off by $\pm1$ when a low-resolution font has been carefully
hand-fitted. For example, a font designer often wants to make the letter `m'
a pixel wider or narrower in order to make the font appear more consistent.
\.{DVI}-to-printer programs should therefore input the correct pixel width
information from font files whenever there is a chance that it may differ.
A warning message may also be desirable in the case that at least one character
is found whose pixel width differs from |conv*width| by more than a full pixel.
  
Those characters that are too large to be downloadable, are marked as such,
and will be transferred using raster graphics. We must be especially careful
in the y dimension, as the character must fit into the 255*255 box when
the reference point is placed on the baseline. The baseline was set to
$255-$|baseline| pixel rows down from the top.
@^system dependencies@>
  
@d pixel_round(#)==round(conv*(#))
  
@<Read the font dir...@>=
@<Replace |z| by $|z|^\prime$ and compute $\alpha,\beta$@>;
for k:=0 to 127 do begin
  seek(pxl_file[nf],pxl_dptr[nf]+k*4+2);
  get(pxl_file[nf]);
  {is the charater too large ?}
  if (pxl_file[nf]^.i0 >= 128) or (pxl_file[nf]^.i1 >= 128) then
    status[wp]:=too_large
  else
    status[wp]:=not_loaded;
  get(pxl_file[nf]);
  get(pxl_file[nf]);
  if pxl_file[nf]^.i=0 then begin
    width[wp]:=invalid_width;
    pixel_width[wp]:=0  end
  else begin
    get(pxl_file[nf]);
    read_pxl_word(nf);
    width[wp]:=(((((b3*z)div@'400)+(b2*z))div@'400)+(b1*z))div beta;
    if b0>0 then if b0<255 then bad_pxl('strange width for char ', k:1)
      else width[wp]:=width[wp]-alpha;
    pixel_width[wp]:=pixel_round(width[wp]);
  end;
  incr(wp);
end
  
@ @<Replace |z|...@>=
begin alpha:=16*z; beta:=16;
while z>=@'40000000 do
  begin z:=z div 2; beta:=beta div 2;
  end;
end
 
@* Downloading information to the printer.
The procedure |update_pos| is used to update the printer cursor position.
This is the only place where the cursor is explicitely positioned.
  
@p procedure update_pos;
label done;
begin if (lj_h<>hh)or(lj_v<>vv) then begin
  write_lj(@=#27'*p'@>);
  if (lj_h<>hh) then begin
    if abs(lj_h-hh) > hh div 10 then
      write_lj_f(hh:1)
    else if (lj_h<hh) then
      write_lj_f('+', hh-lj_h:1)
    else
      write_lj_f('-', lj_h-hh:1);
    if (lj_v<>vv) then
      write_lj_f('x')
    else begin
      write_lj_f('X');
      goto done;
    end
  end;
  if abs(lj_v-vv) > vv div 10 then
    write_lj_f(vv:1)
  else if (lj_v<vv) then
    write_lj_f('+', vv-lj_v:1)
  else
    write_lj_f('-', lj_v-vv:1);
  write_lj_f('Y');
end;
done:
lj_h:=hh; lj_v:=vv;
end;
 
@ This procedure downloads a single character from a \.{PXL} file.
@^system dependencies@>
  
@p procedure download_char(p:integer);
var i,j:short;
@!pixel_wd,@!pixel_ht:integer;
@!offsets,char_wd,char_ht,delta_x:i2c;
@!raster_address:integer;
  
begin
  seek(pxl_file[cur_font],pxl_dptr[cur_font]+p*4+2);
  get(pxl_file[cur_font]);
  pixel_wd:=pxl_file[cur_font]^.i0;
  pixel_ht:=pxl_file[cur_font]^.i1;
  get(pxl_file[cur_font]);
  offsets:=pxl_file[cur_font]^;
  get(pxl_file[cur_font]);
  raster_address:=pxl_file[cur_font]^.i;
  @<Download character header@>;
  seek(pxl_file[cur_font],raster_address+2);
  for j:=1 to pixel_ht do begin
    for i:=1 to (pixel_wd-1) div 8 + 1 do begin
      if (i-1) mod 4 = 0 then
        get(pxl_file[cur_font]);
      write_lj(pxl_file[cur_font]^.c[(i-1) mod 4]);
    end;
  end
end;
  
@ Download a character header.
  
@d data_bytes==(pixel_ht*(((pixel_wd-1) div 8)+1))
  
@<Download character header@>=
begin
  if pixel_ht - offsets.i1 > baseline then begin
    char_status(cur_font)(p):=pixel_ht-offsets.i1;
    offsets.i1:=pixel_ht;  end
  else
    char_status(cur_font)(p):=loaded_ok;
  offsets.i0:=-offsets.i0;
  char_wd.i0:=pixel_wd;
  char_ht.i0:=pixel_ht;
  delta_x.i0:=char_pixel_width(cur_font)(p)*4;
  write_lj(@=#27'*c'@>,font_num[cur_font]:1,'d',
    ord(vis_chr(p)):1,@='E'#27'(s'@>,
    data_bytes+16:1,'W'+
    @=#4#0#14#1#0#0@>,
    offsets.c[0], offsets.c[1],
    offsets.c[2], offsets.c[3],
    char_wd.c[0], char_wd.c[1],
    char_ht.c[0], char_ht.c[1],
    delta_x.c[0], delta_x.c[1]);
end
  
@ This procedure transfers a character in raster form.
This is used for characters which are too large to be stored in
the font memory of the printer.
@^system dependencies@>
  
@p procedure raster_char(p:integer);
var
i,j,n:short;
@!pixel_ht,@!pixel_wd,@!x_offset,@!y_offset:short;
@!raster_address:integer;
  
begin
  seek(pxl_file[cur_font],pxl_dptr[cur_font]+p*4+2);
  get(pxl_file[cur_font]);
  pixel_wd:=pxl_file[cur_font]^.i0;
  pixel_ht:=pxl_file[cur_font]^.i1;
  get(pxl_file[cur_font]);
  x_offset:=pxl_file[cur_font]^.i0;
  y_offset:=pxl_file[cur_font]^.i1;
  get(pxl_file[cur_font]);
  raster_address:=pxl_file[cur_font]^.i;
  hh:=hh-x_offset; vv:=vv-y_offset;
  update_pos;
  write_lj(@=#27'*r1A'@>);
  seek(pxl_file[cur_font],raster_address+2);
  for j:=1 to pixel_ht do begin
    write_lj(#27'*b',(pixel_wd-1) div 8 + 1:1, 'W');
    for i:=1 to (pixel_wd-1) div 8 + 1 do begin
      if (i-1) mod 4 = 0 then
        get(pxl_file[cur_font]);
      write_lj(pxl_file[cur_font]^.c[(i-1) mod 4]);
    end;
  end;
  lj_v:=lj_v+pixel_ht;
  hh:=hh+char_pixel_width(cur_font)(p); vv:=vv+y_offset;
  write_lj(@=#27'*rB'@>);
end;
  
@* Optional modes of output.
The starting page is specified by giving a sequence of 1 to 10 numbers or
asterisks separated by dots. For example, the specification `\.{1.*.-5}'
can be used to refer to a page output by \TeX\ when $\.{\\count0}=1$
and $\.{\\count2}=-5$. (Recall that |bop| commands in a \.{DVI} file
are followed by ten `count' values.) An asterisk matches any number,
so the `\.*' in `\.{1.*.-5}' means that \.{\\count1} is ignored when
specifying the first page. If several pages match the given specification,
\.{DVIplus} will begin with the earliest such page in the file. The
default specification `\.*' (which matches all pages) therefore denotes
the page at the beginning of the file.
  
Another option is the page offset. That is the point on the physical
printer page where the point (0,0) in the DVI file is mapped.
  
Normally \.{DVIplus} uses default values for the options.
It can be started in such a way that it engages
the user in a brief dialog so that the
options will be specified.
@^system dependencies@>
  
@<Glob...@>=
@!max_pages:integer; {at most this many |bop..eop| pages will be printed}
@!resolution:real; {pixels per inch}
@!new_mag:integer; {if positive, overrides the postamble's magnification}
@!copies:integer;
@!h_offset,@!v_offset:integer; {offset where to put (0,0) on physical page}
  
@ The starting page specification is recorded in two global arrays called
|start_count| and |start_there|. For example, `\.{1.*.-5}' is represented
by |start_there[0]=true|, |start_count[0]=1|, |start_there[1]=false|,
|start_there[2]=true|, |start_count[2]=-5|.
We also set |start_vals=2|, to indicate that count 2 was the last one
mentioned. The other values of |start_count| and |start_there| are not
important, in this example.
  
@<Glob...@>=
@!start_count:array[0..9] of integer; {count values to select starting page}
@!start_there:array[0..9] of boolean; {is the |start_count| value relevant?}
@!start_vals:0..9; {the last count considered significant}
@!count:array[0..9] of integer; {the count values on the current page}
  
@ @<Set init...@>=
max_pages:=100; start_vals:=0; start_there[0]:=false; copies:=1;
resolution:=300.0; new_mag:=0;
h_offset:=210; v_offset:=100;
  
@ Here is a simple subroutine that tests if the current page might be the
starting page.
 
@p function start_match:boolean; {does |count| match the starting spec?}
var k:0..9;  {loop index}
@!match:boolean; {does everything match so far?}
begin match:=true;
for k:=0 to start_vals do
  if start_there[k]and(start_count[k]<>count[k]) then match:=false;
start_match:=match;
end;
  
@ The |input_ln| routine waits for the user to type a line at his or her
terminal; then it puts ASCII-code equivalents for the characters on that line
into the |buffer| array. The |term_in| file is used for terminal input,
and |term_out| for terminal output.
@^system dependencies@>
  
@<Glob...@>=
@!buffer:array[0..terminal_line_length] of ASCII_code;
@!term_in:text_file; {the terminal, considered as an input file}
@!term_out:text_file; {the terminal, considered as an output file}
@!arg_index:short; {which command line argument is being processed}
@!interactive:boolean;
  
@ Since the terminal is being used for both input and output, some systems
need a special routine to make sure that the user can see a prompt message
before waiting for input based on that message. (Otherwise the message
may just be sitting in a hidden buffer somewhere, and the user will have
no idea what the program is waiting for.) We shall call a system-dependent
subroutine |update_terminal| in order to avoid this problem.
@^system dependencies@>
  
@d update_terminal == prompt(term_out) {empty the terminal output buffer}
  
@ During the dialog, \.{DVIplus} will treat the first blank space in a
line as the end of that line. Therefore |input_ln| makes sure that there
is always at least one blank space in |buffer|.
@^system dependencies@>
  
@p procedure input_ln; {inputs a line from the terminal}
var k:0..terminal_line_length;
begin update_terminal;
k:=0;
if eoln(term_in) then read_ln(term_in)
else begin while (k<terminal_line_length)and not eoln(term_in) do
  begin buffer[k]:=xord[term_in^]; incr(k); get(term_in);
  end;
  read_ln(term_in);
end;
buffer[k]:=" ";
end;
  
@ The global variable |buf_ptr| is used while scanning each line of input;
it points to the first unread character in |buffer|.
  
@<Glob...@>=
@!buf_ptr:0..terminal_line_length; {the number of characters read}
  
@ Here is a routine that scans a (possibly signed) integer and computes
the decimal value. If no decimal integer starts at |buf_ptr|, the
value 0 is returned. The integer should be less than $2^{31}$ in
absolute value.

@p function get_integer:integer;
var x:integer; {accumulates the value}
@!negative:boolean; {should the value be negated?}
begin if buffer[buf_ptr]="-" then
  begin negative:=true; incr(buf_ptr);
  end
else negative:=false;
x:=0;
while (buffer[buf_ptr]>="0")and(buffer[buf_ptr]<="9") do
  begin x:=10*x+buffer[buf_ptr]-"0"; incr(buf_ptr);
  end;
if negative then get_integer:=-x @+ else get_integer:=x;
end;
  
@ The selected options are put into global variables by the |dialog|
procedure, which is called just as \.{DVIplus} begins.
@^system dependencies@>
  
@p procedure dialog;
label 2,3,6,7,8,9,99;
var i,j,k:short;
begin rewrite(term_out, '1', 'NOCCTL'); {prepare the terminal for output}
reset(term_in, '1'); {and for input}
print_ln(banner);
@<Get flags, check if interactive@>;
@<Determine the desired |start_count| values@>;
@<Determine the desired |max_pages|@>;
@<Determine the number of copies@>;
@<Determine the page offset@>;
@<Print all the selected options@>;
end;
 
@ @<Get flags, check if interactive@>=
arg_index:=1; interactive:=false;
repeat
  i:=parameters(arg_index,cur_name,name_length);
  if cur_name[1]='-' then begin
    incr(arg_index);
    if i=1 then
      interactive:=true
    else case cur_name[2] of
      'i','I':interactive:=true;
    othercases begin end;
    endcases;
  end;
until cur_name[1]<>'-';
if not interactive then goto 99;
  
@ @<Determine the desired |start...@>=
2: print('Starting page (default=*): ');
input_ln; buf_ptr:=0; k:=0;
if buffer[0]<>" " then
  repeat if buffer[buf_ptr]="*" then
    begin start_there[k]:=false; incr(buf_ptr);
    end
  else  begin start_there[k]:=true; start_count[k]:=get_integer;
    end;
  if (k<9)and(buffer[buf_ptr]=".") then
    begin incr(k); incr(buf_ptr);
    end
  else if buffer[buf_ptr]=" " then start_vals:=k
  else  begin print('Type, e.g., 1.*.-5 to specify the ');
    print_ln('first page with \count0=1, \count2=-5.');
    goto 2;
    end;
  until start_vals=k
  
@ @<Determine the desired |max_pages|@>=
3: print('Maximum number of pages (default=100): ');
input_ln; buf_ptr:=0;
if buffer[0]<>" " then
  begin max_pages:=get_integer;
  if max_pages<=0 then
    begin print_ln('Please type a positive number.');
    goto 3;
    end;
  end
  
@ @<Determine the number of copies@>=
6: print('Number of copies (default=1): ');
input_ln; buf_ptr:=0;
if buffer[0]<>" " then
  if (buffer[0]>="0")and(buffer[0]<="9") then copies:=get_integer
  else  begin print('Type a positive integer to specify ');
    print_ln('the number of copies.');
    goto 6;
    end
 
@ @<Determine the page offset@>=
7: print('Page offset in dots (default=210,100): ');
input_ln; buf_ptr:=0;
if buffer[0]=" " then goto 9;
if ((buffer[0]>="0")and(buffer[0]<="9")) or (buffer[0]="-") then
 h_offset:=get_integer
else goto 8;
if (buffer[buf_ptr]=",") then incr(buf_ptr);
if (buffer[buf_ptr]=" ") then incr(buf_ptr);
if ((buffer[buf_ptr]>="0")and(buffer[buf_ptr]<="9")) or
   (buffer[buf_ptr]="-") then begin
  v_offset:=get_integer;
  goto 9; end
else goto 8;
8:print('Specify a dot coordinate pair where the (0,0) ');
  print_ln('point will be placed.');
  goto 7;
9:
  
@ After the dialog is over, we print the options so that the user
can see what \.{DVIplus} thought was specified.
  
@<Print all the selected options@>=
99:print_ln('Options selected:');
@.Options selected@>
print('  Starting page = ');
for k:=0 to start_vals do
  begin if start_there[k] then print(start_count[k]:1)
  else print('*');
  if k<start_vals then print('.')
  else print_nl;
  end;
print_ln('  Maximum number of pages = ',max_pages:1);
print_ln('  Page offset = (', h_offset:1, ',', v_offset:1, ')');
  
@* Defining fonts.
\.{DVIplus} reads the postamble first and loads
all of the fonts defined there; then it processes the pages.
  
@ Approximate the desired magnification to an available one.
The ``magig numbers'' that the desired magnification
is compared to are calculated as
$1500*1.2^m$, where $m = 1/4,3/4,1.5,2.5,3.5,4.5$
  
@p function approx_mag(f:integer;d_mag:integer):integer;
  
begin
  if d_mag < 1569 then
    approx_mag := 1500
  else if d_mag < 1720 then
    approx_mag := 1643
  else if d_mag < 1971 then
    approx_mag := 1800
  else if d_mag < 2366 then
    approx_mag := 2160
  else if d_mag < 2839 then
    approx_mag := 2592
  else if d_mag < 3407 then
    approx_mag := 3110
  else if d_mag < 4089 then
    approx_mag := 3732
  else
    approx_mag:= 4479;
end;
  
@ The following subroutine does the necessary things when a \\{fnt\_def}
command is being processed.
  
@p procedure define_font(@!e:integer); {|e| is an external font number}
var f:0..max_fonts;
@!p:integer; {length of the area/directory spec}
@!n:integer; {length of the font name proper}
@!c,@!q,@!d:integer; {check sum, scaled size, and design size}
@!m:integer; {|mag| corrected for 300 pixels/inch}
@!r:0..name_length; {index into |cur_name|}
@!j,@!k:0..name_size; {indices into |names|}
@!mismatch:boolean; {do names disagree?}
begin if nf=max_fonts then abort('DVIplus capacity exceeded (max fonts=',
    max_fonts:1,')!');
@.DVIplus capacity exceeded...@>
font_num[nf]:=e; f:=0;
while font_num[f]<>e do incr(f);
@<Read the font parameters into position for font |nf|, and
  print the font name@>;
  if f<nf then print_ln('---this font was already defined!');
@.this font was already defined@>
  @<Load the new font, unless there are problems@>
end;
  
@ @<Read the font parameters into position for font |nf|...@>=
c:=signed_quad; font_check_sum[nf]:=c;@/
q:=signed_quad; font_scaled_size[nf]:=q;@/
d:=signed_quad; font_design_size[nf]:=d;@/
p:=get_byte; n:=get_byte;
if font_name[nf]+n+p>name_size then
  abort('DVIplus capacity exceeded (name size=',name_size:1,')');
@.DVIplus capacity exceeded...@>
font_name[nf+1]:=font_name[nf]+n+p;
if n+p=0 then abort('Null font name')
@.Null font name@>
else for k:=font_name[nf] to font_name[nf+1]-1 do names[k]:=get_byte;
font_used_on[nf+1]:=0;
incr(nf);
if f=nf-1 then begin
  print('Font ', e:1, ': '); print_font(nf-1);
  update_terminal;
end;
decr(nf)
 
@ @<Load the new font, unless there are problems@>=
begin
@<Compute |desired_mag|@>;
@<Move font name into the |cur_name| string@>;
if not open_pxl_file(nf) then begin
  print_nl;
  abort('Cannot open PXL file ', cur_name); end
@.Cannot open PXL file@>
else  begin if (q<=0)or(q>=@'1000000000) then begin
    print_nl;
    abort('PXL file not loaded, bad scale (',q:1,')!'); end
@.bad scale@>
  else if (d<=0)or(d>=@'1000000000) then begin
    print_nl;
    abort('PXL file not loaded, bad design size (',d:1,')!'); end
@.bad design size@>
  else begin
    in_PXL(q);
    @<Finish loading the new font info@>;
  end
end
end
  
@ @<Compute |desired_mag|@>=
desired_mag:=round((mag * q * (300.0/200.0))/d+0.5);
  
@ @<Finish loading...@>=
begin font_space[nf]:=q div 6; {this is a 3-unit ``thin space''}
if (c<>0)and(pxl_check_sum<>0)and(c<>pxl_check_sum) then
  begin print(' ---beware: check sums do not agree!');
@.beware: check sums do not agree@>
@.check sums do not agree@>
  print(' (',c:1,' vs. ',pxl_check_sum:1,')');
  end;
d:=round((100.0*conv*q)/(true_conv*d));
if d<>100 then
  print(' (magnified ',d:1,'%)');
@.this font is magnified@>
incr(nf); {now the new font is officially present}
font_space[nf]:=0; {for |out_space| and |out_vmove|}
end
  
@ If |p=0|, i.e., if no font directory has been specified, \.{DVIplus}
uses the default font directory, which is a
system-dependent place where the standard fonts are kept.
  
In RTE--A, the \.{PXL} files are kept in directories called
\.{/TeX/Fonts/MagXXXX}, where \.{XXXX} is the magnification.
The string variable |default_prefix| contains the prefix of these names.
@^system dependencies@>
  
@d default_prefix_name=='/TeX/Fonts/Mag'
@d default_prefix_length=14 {change this to the correct length}
  
@<Glob...@>=
@!default_prefix:packed array[1..default_prefix_length] of char;
  
@ @<Set init...@>=
default_prefix:=default_prefix_name;
  
@ The string |cur_name| is set to the external name of the
\.{PXL} file for the current font and magnification.
@^system dependencies@>
  
@<Move font name into the |cur_name| string@>=
for k:=1 to name_length do cur_name[k]:=' ';
if p=0 then
  begin for k:=1 to default_prefix_length do
    cur_name[k]:=default_prefix[k];
  r:=default_prefix_length;
  incr(r);
  best_mag:=approx_mag(font_name[nf],desired_mag);
  m:=best_mag;
  cur_name[r]:=xchr[ m div 1000 + xord['0']];
  incr(r);
  cur_name[r]:=xchr[ (m div 100) mod 10 + xord['0']];
  incr(r);
  cur_name[r]:=xchr[ (m div 10) mod 10 + xord['0']];
  incr(r);
  cur_name[r]:=xchr[ m mod 10 + xord['0']];
  incr(r);
  cur_name[r]:='/'; end
else
  r:=0;
  
for k:=font_name[nf] to font_name[nf+1]-1 do
  begin incr(r);
  if r+4>name_length then
    abort('DVIplus capacity exceeded (max font name length=',
      name_length:1,')!');
@.DVIplus capacity exceeded...@>
  cur_name[r]:=xchr[names[k]];
  end;
cur_name[r+1]:='.'; cur_name[r+2]:='P'; cur_name[r+3]:='X'; cur_name[r+4]:='L'
 
@* Interpreting the {\tentex DVI} file.
The main work of \.{DVIplus} is accomplished by the |do_page| procedure,
which produces the output for an entire page, assuming that the |bop|
command for that page has already been processed. This procedure is
essentially an interpretive routine that reads and acts on the \.{DVI}
commands.
  
@ The definition of \.{DVI} files refers to six registers,
$(h,v,w,x,y,z)$, which hold integer values in \.{DVI} units.  In practice,
we also need registers |hh| and |vv|, the pixel analogs of $h$ and $v$,
since it is not always true that |hh=pixel_round(h)| or
|vv=pixel_round(v)|.
The |lj_h| and |lj_v| registers hold the current actual cursor position
of the printer.
  
The stack of $(h,v,w,x,y,z)$ values is represented by eight arrays
called |hstack|, \dots, |zstack|, |hhstack|, and |vvstack|.
  
@<Glob...@>=
@!h,@!v,@!w,@!x,@!y,@!z,@!hh,@!vv:integer; {current state values}
@!lj_h,@!lj_v:integer; {current cursor position}
@!hstack,@!vstack,@!wstack,@!xstack,@!ystack,@!zstack:
  array [0..stack_size] of integer; {pushed down values in \.{DVI} units}
@!hhstack,@!vvstack:
  array [0..stack_size] of integer; {pushed down values in pixels}
  
@ Three characteristics of the pages (their |max_v|, |max_h|, and
|max_s|) are specified in the postamble, and a warning message
is printed if these limits are exceeded. Actually |max_v| is set to
the maximum height plus depth of a page, and |max_h| to the maximum width,
for purposes of page layout. Since characters can legally be set outside
of the page boundaries, it is not an error when |max_v| or |max_h| is
exceeded. But |max_s| should not be exceeded.

The postamble also specifies the total number of pages; \.{DVIplus}
checks to see if this total is accurate.

@<Glob...@>=
@!max_v:integer; {the value of |abs(v)| should probably not exceed this}
@!max_h:integer; {the value of |abs(h)| should probably not exceed this}
@!max_s:integer; {the stack depth should not exceed this}
@!max_v_so_far,@!max_h_so_far,@!max_s_so_far:integer; {the record high levels}
@!total_pages:integer; {the stated total number of pages}
@!page_count:integer; {the total number of pages seen so far}

@ @<Set init...@>=
max_v:=@'17777777777-99; max_h:=@'17777777777-99; max_s:=stack_size+1;@/
max_v_so_far:=0; max_h_so_far:=0; max_s_so_far:=0; page_count:=0;

@ Before we get into the details of |do_page|, it is convenient to
consider a simpler routine that computes the first parameter of each
opcode.

@d four_cases(#)==#,#+1,#+2,#+3
@d eight_cases(#)==four_cases(#),four_cases(#+4)
@d sixteen_cases(#)==eight_cases(#),eight_cases(#+8)
@d thirty_two_cases(#)==sixteen_cases(#),sixteen_cases(#+16)
@d sixty_four_cases(#)==thirty_two_cases(#),thirty_two_cases(#+32)

@p function first_par(o:eight_bits):integer;
begin case o of
sixty_four_cases(set_char_0),sixty_four_cases(set_char_0+64):
  first_par:=o-set_char_0;
set1,put1,fnt1,xxx1,fnt_def1: first_par:=get_byte;
set1+1,put1+1,fnt1+1,xxx1+1,fnt_def1+1: first_par:=get_two_bytes;
set1+2,put1+2,fnt1+2,xxx1+2,fnt_def1+2: first_par:=get_three_bytes;
right1,w1,x1,down1,y1,z1: first_par:=signed_byte;
right1+1,w1+1,x1+1,down1+1,y1+1,z1+1: first_par:=signed_pair;
right1+2,w1+2,x1+2,down1+2,y1+2,z1+2: first_par:=signed_trio;
set1+3,set_rule,put1+3,put_rule,right1+3,w1+3,x1+3,down1+3,y1+3,z1+3,
  fnt1+3,xxx1+3,fnt_def1+3: first_par:=signed_quad;
nop,bop,eop,push,pop,pre,post,post_post,undefined_commands: first_par:=0;
w0: first_par:=w;
x0: first_par:=x;
y0: first_par:=y;
z0: first_par:=z;
sixty_four_cases(fnt_num_0): first_par:=o-fnt_num_0;
end;
end;

@ Here is another subroutine that we need: It computes the number of
pixels in the height or width of a rule. Characters and rules will line up
properly if the sizes are computed precisely as specified here.  (Since
|conv| is computed with some floating-point roundoff error, in a
machine-dependent way, format designers who are tailoring something for a
particular resolution should not plan their measurements to come out to an
exact integer number of pixels; they should compute things so that the
rule dimensions are a little less than an integer number of pixels, e.g.,
4.99 instead of 5.00.)

@p function rule_pixels(x:integer):integer;
  {computes $\lceil|conv|\cdot x\rceil$}
var n:integer;
begin n:=trunc(conv*x);
if n<conv*x then rule_pixels:=n+1 @+ else rule_pixels:=n;
end;

@ Strictly speaking, the |do_page| procedure is really a function with
side effects, not a `\&{procedure}'; it returns the value |false| if
\.{DVIplus} should be aborted because of some unusual happening. The
subroutine is organized as a typical interpreter, with a multiway branch
on the command code followed by |goto| statements leading to routines that
finish up the activities common to different commands. We will use the
following labels:
 
@d fin_set=41 {label for commands that set or put a character}
@d fin_rule=42 {label for commands that set or put a rule}
@d move_right=43 {label for commands that change |h|}
@d move_down=44 {label for commands that change |v|}
@d change_font=46 {label for commands that change |cur_font|}
  
@ Some \PASCAL\ compilers severely restrict the length of procedure bodies,
so we shall split |do_page| into two parts, one of which is
called |special_cases|. The different parts communicate with each other
via the global variables mentioned above, together with the following ones:
  
@<Glob...@>=
@!s:integer; {current stack size}
@!ss:integer; {stack size to print}
@!cur_font:integer; {current internal font number}
@!prev_font:integer;
@!fonts_in_use:integer; {how many are currently loaded}
@!fonts_on_page:integer; {how many fonts used on current page}
  
@ @<Set init...@>=
fonts_in_use:=0;
  
@ Here is the overall setup.
  
@p @t\4@>@<Declare the function called |special_cases|@>@;
procedure do_page;
label fin_set,fin_rule,move_right,done,exit;
var o:eight_bits; {operation code of the current command}
@!p,@!q:integer; {parameters of the current command}
pp,qq:integer;
i,j,k:integer;
@!a:integer; {byte number of the current command}
@!hhh:integer; {|h|, rounded to the nearest pixel}
@!height,pitch:i2c;
begin cur_font:=nf; {set current font undefined}
prev_font:=-1;
s:=0; h:=round(h_offset/conv); v:=round(v_offset/conv);
w:=0; x:=0; y:=0; z:=0; hh:=pixel_round(h); vv:=pixel_round(v);
lj_h:=-10000; lj_v:=-10000;
  {initialize the state variables}
while true do @<Translate the next command in the \.{DVI} file@>;
exit:
end;
  
@
  
@d error(#)==print_ln('% ',#)
  
@<Translate the next command...@>=
begin a:=cur_loc;
o:=get_byte; p:=first_par(o);
if eof_dvi_file then bad_dvi('file ended prematurely');
@.the file ended prematurely@>
@<Start translation of command |o| and |goto| the appropriate label to
  finish the job@>;
fin_set: @<Finish a command that either sets or puts a character, then
    |goto move_right| or |done|@>;
fin_rule: @<Finish a command that either sets or puts a rule, then
    |goto move_right| or |done|@>;
move_right: @<Finish a command that sets |h:=h+q|, then |goto done|@>;
done:
end
  
@ The multiway switch in |first_par|, above, was organized by the length
of each command; the one in |do_page| is organized by the semantics.
  
@<Start translation...@>=
if o<set_char_0+128 then @<Translate a |set_char| command@>
else case o of
  four_cases(set1),
  four_cases(put1): bad_dvi('illegal character (', p:1, ')');
  set_rule,
  put_rule: goto fin_rule;
  @t\4@>@<Cases for commands |nop|, |bop|, \dots, |pop|@>@;
  @t\4@>@<Cases for horizontal motion@>@;
  othercases if special_cases(o,p,a) then goto done
             else bad_dvi(' ')
  endcases
  
@ @<Declare the function called |special_cases|@>=
function special_cases(@!o:eight_bits;@!p,@!a:integer):boolean;
label change_font,move_down,done,9998;
var q:integer; {parameter of the current command}
q1,q2:integer; {dummies for |fnt_def|}
@!k:integer; {loop index}
@!bad_char:boolean; {has a non-ASCII character code appeared in this \\{xxx}?}
@!pure:boolean; {is the command error-free?}
@!vvv:integer; {|v|, rounded to the nearest pixel}
begin pure:=true;
case o of
@t\4@>@<Cases for vertical motion@>@;
@t\4@>@<Cases for fonts@>@;
four_cases(xxx1): @<Translate an |xxx| command and |goto done|@>;
pre: begin error('preamble command within a page!'); goto 9998;
  end;
@.preamble command within a page@>
post,post_post: begin error('postamble command within a page!'); goto 9998;
@.postamble command within a page@>
  end;
othercases begin error('undefined command ',o:1,'!');
  goto done;
@.undefined command@>
  end
endcases;
move_down: @<Finish a command that sets |v:=v+p|, then |goto done|@>;
change_font: @<Finish a command that changes the current font,
  then |goto done|@>;
9998: pure:=false;
done: special_cases:=pure;
end;
  
@ @<Cases for commands |nop|, |bop|, \dots, |pop|@>=
nop: goto done;
bop: begin error('bop occurred before eop'); bad_dvi(' ');
@.bop occurred before eop@>
  end;
eop: begin
  write_lj(#12);
  if s<>0 then error('stack not empty at end of page (level ',
    s:1,')!');
@.stack not empty...@>
  return;
  end;
push: begin
  if s=max_s_so_far then
    begin max_s_so_far:=s+1;
    if s=max_s then error('deeper than claimed in postamble!');
@.deeper than claimed...@>
@.push deeper than claimed...@>
    if s=stack_size then
      abort('DVIplus capacity exceeded (stack size=',
        stack_size:1,')');
    end;
  hstack[s]:=h; vstack[s]:=v; wstack[s]:=w;
  xstack[s]:=x; ystack[s]:=y; zstack[s]:=z;
  hhstack[s]:=hh; vvstack[s]:=vv; incr(s); ss:=s-1; goto done;
  end;
pop: begin
  if s=0 then error('(illegal at level zero)!')
  else  begin decr(s); hh:=hhstack[s]; vv:=vvstack[s];
    h:=hstack[s]; v:=vstack[s]; w:=wstack[s];
    x:=xstack[s]; y:=ystack[s]; z:=zstack[s];
    end;
  ss:=s; goto done;
  end;
  
@ Rounding to the nearest pixel is best done in the manner shown here, so as
to be inoffensive to the eye: When the horizontal motion is small, like a
kern, |hh| changes by rounding the kern; but when the motion is large, |hh|
changes by rounding the true position |h| so that accumulated rounding errors
disappear. We allow a larger space in the negative direction than in
the positive one, because \TeX\ makes comparatively
large backspaces when it positions accents.
  
@d out_space==begin
  if (p>=font_space[cur_font])or(p<=-4*font_space[cur_font]) then
    hh:=pixel_round(h+p)
  else hh:=hh+pixel_round(p);
  q:=p; goto move_right;
end
  
@<Cases for horizontal motion@>=
four_cases(right1):out_space;
w0,four_cases(w1):begin w:=p; out_space;
  end;
x0,four_cases(x1):begin x:=p; out_space;
  end;
  
@ Vertical motion is done similarly, but with the threshold between
``small'' and ``large'' increased by a factor of five. The idea is to make
fractions like ``$1\over2$'' round consistently, but to absorb accumulated
rounding errors in the baseline-skip moves.
  
@d out_vmove==begin
  if abs(p)>=5*font_space[cur_font] then vv:=pixel_round(v+p)
  else vv:=vv+pixel_round(p);
  goto move_down;
end
  
@<Cases for vertical motion@>=
four_cases(down1):out_vmove;
y0,four_cases(y1):begin y:=p; out_vmove;
  end;
z0,four_cases(z1):begin z:=p; out_vmove;
  end;
  
@ @<Cases for fonts@>=
sixty_four_cases(fnt_num_0),
four_cases(fnt1):
  goto change_font;
four_cases(fnt_def1): begin
  @<Skip a |fnt_def| command@>;
  goto done;
  end;
  
@ @<Skip a |fnt_def| command@>=
q:=signed_quad; q:=signed_quad; q:=signed_quad;
q1:=get_byte; q2:=get_byte;
for k:=1 to q1+q2 do q:=get_byte;
  
@ @<Translate an |xxx| command and |goto done|@>=
begin bad_char:=false;
for k:=1 to p do
  begin q:=get_byte;
  if not ((q>=" ")and(q<="~")) then
    bad_char:=true;
  end;
if bad_char then error('non-ASCII character in xxx command!');
@.non-ASCII character...@>
goto done;
end
  
@ @<Translate a |set_char|...@>=
goto fin_set
  
@ This is the code that checks whether the next character to be
printed in |cur_font| has occurred previously.
If not, the character data is downloaded to the printer. If the character
is too large, the data is sent as a raster image.
If |cur_font| is also new, the font header is first downloaded.
The number of fonts stored in the printer is kept at |max_printer_fonts|
maximum. When more fonts are needed, the least recently used one is
deleted.
  
The value of |max_printer_fonts| was choosed by waving a magig rod;
it would be more appropriate to calculate exactly how many bytes of
the user-available memory in the LaserJet+ is in use, and base the
decisions whether we must delete some fonts on that.
(Why can't you ask the printer how much memory it has left?).
  
There should also be a test whether the number of fonts on a page
exceeds the maximum 16.
  
@<Finish a command that either sets or puts a character...@>=
if p<0 then p:=255-((-1-p) mod 256)
else if p>=256 then p:=p mod 256; {width computation for oriental fonts}
@^oriental characters@>@^Chinese characters@>@^Japanese characters@>
if (p>127) then q:=invalid_width
else q:=char_width(cur_font)(p);
if q=invalid_width then
  begin error('character ',p:1,' invalid in font ');
@.character $c$ invalid...@>
  print_font(cur_font);
  if cur_font<>nf then print('!'); {font |nf| has `\.!' in its name}
  end;
if font_used_on[cur_font]=0 then begin
  if fonts_in_use=max_printer_fonts then
    @<Delete least recently used font@>;
  incr(fonts_on_page);
  @<Download font header@> end
else if font_used_on[cur_font]<page_count then
  incr(fonts_on_page);
if fonts_on_page>max_fonts_on_page then
  error('too many fonts on this page');
font_used_on[cur_font]:=page_count;
if cur_font <> prev_font then
  write_lj(@=#27'('@>,font_num[cur_font]:1,'X');
prev_font:=cur_font;
if char_status(cur_font)(p) = too_large then
  raster_char(p)
else begin
  if char_status(cur_font)(p) = not_loaded then
    download_char(p);
  if char_status(cur_font)(p) > 0 then begin
    vv:=vv+char_status(cur_font)(p);
    update_pos;
    write_lj(vis_chr(p));
    vv:=vv-char_status(cur_font)(p); end
  else begin
    update_pos;
    write_lj(vis_chr(p));
  end;
  lj_h:=lj_h+char_pixel_width(cur_font)(p);
end;
if o>=put1 then begin
  hh:=hh-char_pixel_width(cur_font)(p);
  goto done;
end;
if q=invalid_width then q:=0
else hh:=hh+char_pixel_width(cur_font)(p);
goto move_right
 
@ @<Download font header@>=
begin
  height.i0:=4*round(font_design_size[cur_font]*
                     font_mag[cur_font]/1000.0*conv);
  if (height.i0 < 0) or (height.i0>10922) then
    height.i0:= 10922;
  pitch.i0:=height.i0-20; {is this value used for anything in the printer??}
  
  write_lj(@=#27'*c'@>,font_num[cur_font]:1,
  {char cell 255*255 pixels max}
  {baseline distance set to $255-$|baseline| }
    @='D'#27')s26W'#0#26#0#1#0#0#0@>, chr(255-baseline),
    @=#0#255#0#255#0#1#1#21@>,
    pitch.c[0], pitch.c[1], height.c[0], height.c[1],
    @=#0#0#0#0#0#0#27'*c4F'@>);
  incr(fonts_in_use);
end
  
@ @<Delete least recently...@>=
begin
  j:=9999;
  k:=nf;
  for i:=0 to nf-1 do
    if font_used_on[i] < j then begin
      j:=font_used_on[i];
      k:=i;
    end;
  write_lj(@=#27'*c'@>,font_num[k]:1,'d2F');
  font_used_on[k]:=0;
  for i:=0 to 127 do
    if (char_status(k)(i) > 0) or (char_status(k)(i) = loaded_ok) then
      char_status(k)(i):=not_loaded;
  decr(fonts_in_use);
end
  
@ @<Finish a command that either sets or puts a rule...@>=
q:=signed_quad;
qq:=rule_pixels(q);
pp:=rule_pixels(p);
if (p>0) and (q>0) then begin
  vv:=vv-pp;
  update_pos;
  write_lj(@=#27'*c'@>,qq:1,'a',pp:1,'b0P');
  vv:=vv+pp;
end;
if o=put_rule then goto done;
hh:=hh+qq;
goto move_right
  
@ A sequence of consecutive rules, or consecutive characters in a fixed-width
font whose width is not an integer number of pixels, can cause |hh| to drift
far away from a correctly rounded value. \.{DVIplus} ensures that the
amount of drift will never exceed |max_drift| pixels.
  
Since \.{DVIplus} is intended to diagnose strange errors, it checks
carefully to make sure that |h| and |v| do not get out of range.
Normal \.{DVI}-reading programs need not do this.
  
@d infinity==@'17777777777 {$\infty$ (approximately)}
@d max_drift=2 {we insist that abs|(hh-pixel_round(h))<=max_drift|}
  
@<Finish a command that sets |h:=h+q|, then |goto done|@>=
if (h>0)and(q>0) then if h>infinity-q then
  begin error('arithmetic overflow! parameter changed from ',
@.arithmetic overflow...@>
    q:1,' to ',infinity-h:1);
  q:=infinity-h;
  end;
if (h<0)and(q<0) then if -h>q+infinity then
  begin error('arithmetic overflow! parameter changed from ',
    q:1, ' to ',(-h)-infinity:1);
  q:=(-h)-infinity;
  end;
hhh:=pixel_round(h+q);
if abs(hhh-hh)>max_drift then begin
  if hhh>hh then hh:=hhh-max_drift
  else hh:=hhh+max_drift;
end;
h:=h+q;
if abs(h)>max_h_so_far then
  begin if abs(h)-round(h_offset/conv)>max_h+99 then
    begin error('warning: h > ',max_h:1,'!');
@.warning: |h|...@>
    max_h:=abs(h);
    end;
  max_h_so_far:=abs(h);
  end;
goto done
  
@ @<Finish a command that sets |v:=v+p|, then |goto done|@>=
if (v>0)and(p>0) then if v>infinity-p then
  begin error('arithmetic overflow! parameter changed from ',
@.arithmetic overflow...@>
    p:1,' to ',infinity-v:1);
  p:=infinity-v;
  end;
if (v<0)and(p<0) then if -v>p+infinity then
  begin error('arithmetic overflow! parameter changed from ',
    p:1, ' to ',(-v)-infinity:1);
  p:=(-v)-infinity;
  end;
vvv:=pixel_round(v+p);
if abs(vvv-vv)>max_drift then begin
  if vvv>vv then vv:=vvv-max_drift
  else vv:=vvv+max_drift;
end;
v:=v+p;
if abs(v)>max_v_so_far then
  begin if abs(v)-round(v_offset/conv)>max_v+99 then
    begin error('warning: v > ',max_v:1,'!');
@.warning: |v|...@>
    max_v:=abs(v);
    end;
  max_v_so_far:=abs(v);
  end;
goto done
  
@ @<Finish a command that changes the current font...@>=
begin
  font_num[nf]:=p; cur_font:=0;
  while font_num[cur_font]<>p do incr(cur_font);
  goto done
end
  
@* Using the backpointers.
First comes a routine that illustrates how to find the postamble quickly.
  
@<Find the postamble, working back from the end@>=
n:=dvi_length;
if n<53 then bad_dvi('only ',n:1,' bytes long');
@.only n bytes long@>
m:=n-4;
repeat if m=0 then bad_dvi('all 223s');
@.all 223s@>
move_to_byte(m); k:=get_byte; decr(m);
until k<>223;
if k<>dvi_id then bad_dvi('ID byte is ',k:1);
@.ID byte is wrong@>
move_to_byte(m-3); q:=signed_quad;
if (q<0)or(q>m-33) then bad_dvi('post pointer ',q:1,' at byte ',m-3:1);
@.post pointer is wrong@>
move_to_byte(q); k:=get_byte;
if k<>post then bad_dvi('byte ',q:1,' is not post');
@.byte n is not post@>
post_loc:=q; first_backpointer:=signed_quad
  
@ Note that the last steps of the above code save the locations of the
the |post| byte and the final |bop|.  We had better declare these global
variables, together with another one that we will need shortly.
  
@<Glob...@>=
@!post_loc:integer; {byte location where the postamble begins}
@!first_backpointer:integer; {the pointer following |post|}
@!start_loc:integer; {byte location of the first page to process}
@!start_inx:integer; {index into |page_start| for first page}
@!last_loc:integer; {byte localtion of last page to process}
@!page_start:array[1..max_bops] of integer; {pointers to |bop|s}
  
@ The next routines follow the backpointers
to move through a \.{DVI} file in reverse order. \.{DVIplus} does this because
it wants to print the pages backwards as the LaserJet stacks
them with the printed side up.
 
First we search for the starting page and the last page.
  
@<Scan for page range to print@>=
q:=post_loc; p:=first_backpointer; start_loc:=-1;
repeat
  {now |q| points to a |post| or |bop| command; |p>=0| is prev pointer}
  if p>q-46 then
    bad_dvi('page link ',p:1,' after byte ',q:1);
@.page link wrong...@>
  q:=p; move_to_byte(q);
  k:=get_byte;
  if k=bop then incr(page_count)
  else bad_dvi('byte ',q:1,' is not bop');
@.byte n is not bop@>
  if page_count>max_bops then bad_dvi('there are too many pages');
@.there are too many pages@>
  page_start[page_count]:=q;
  for k:=0 to 9 do count[k]:=signed_quad;
  if start_match then begin start_loc:=q; start_inx:=page_count; end;
  p:=signed_quad;
until p<0;
if start_loc<0 then abort('starting page number could not be found!');
@.starting page number...@>
if (start_inx > max_pages) then
  last_loc:=page_start[start_inx-max_pages+1]
else
  last_loc:=page_start[1];
if page_count<>total_pages then
  print_ln('there are really ',page_count:1,
    ' pages, not ',total_pages:1,'!');
@.there are really n pages@>
 
@ This is the code that really goes through the pages to be printed
(in reverse order). It starts from the page pointed to by |last_loc| and
proceeds up to |start_loc|.
  
The code shown here uses a convention that has proved to be useful:
If the starting page was specified as, e.g., `\.{1.*.-5}', then
all page numbers in the file are displayed by showing the values of
counts 0, 1, and~2, separated by dots. Such numbers can, for example,
be displayed on the console of a printer when it is working on that
page.
  
@<Translate the page range in reverse order@>=
page_count:=0;
q:=last_loc;
repeat
  move_to_byte(q); k:=get_byte;
  incr(page_count);
  fonts_on_page:=0;
  for k:=0 to 9 do count[k]:=signed_quad;
  q:=signed_quad;
  print('[');
  for k:=0 to start_vals do
    begin print(count[k]:1);
      if k<start_vals then print('.');
    end;
  update_terminal;
  do_page;
  print('] ');
  update_terminal;
until q<start_loc;
  
@* Reading the postamble.
Now imagine that we are reading the \.{DVI} file and positioned just
four bytes after the |post| command. That, in fact, is the situation,
when the following part of \.{DVIplus} is called upon to read, translate,
and check the rest of the postamble.
  
@p procedure read_postamble;
var k:integer; {loop index}
@!p,@!q,@!m:integer; {general purpose registers}
begin post_loc:=cur_loc-5;
@.Postamble starts at byte n@>
if signed_quad<>numerator then
  print_ln('numerator doesn''t match the preamble!');
@.numerator doesn't match@>
if signed_quad<>denominator then
  print_ln('denominator doesn''t match the preamble!');
@.denominator doesn't match@>
if signed_quad<>mag then if new_mag=0 then
  print_ln('magnification doesn''t match the preamble!');
@.magnification doesn't match@>
max_v:=signed_quad; max_h:=signed_quad;@/
max_s:=get_two_bytes; total_pages:=get_two_bytes;@/
if total_pages>max_bops then
  bad_dvi('enormous number of pages (', total_pages:1, ')');
@.enormous number of pages@>
@<Process the font definitions of the postamble@>;
@<Make sure that the end of the file is well-formed@>;
end;
  
@ No warning is given when |max_h_so_far| exceeds |max_h| by less than~100,
since 100 units is invisibly small; it's approximately the wavelength of
visible light, in the case of \TeX\ output. Rounding errors can be expected
to make |h| and |v| slightly more than |max_h| and |max_v|, every once in
a~while; hence small discrepancies are not cause for alarm.
  
@ When we get to the present code, the |post_post| command has
just been read.

@<Make sure that the end of the file is well-formed@>=
q:=signed_quad;
if q<>post_loc then
  print_ln('bad postamble pointer in byte ',cur_loc-4:1,'!');
@.bad postamble pointer@>
m:=get_byte;
if m<>dvi_id then print_ln('identification in byte ',cur_loc-1:1,
@.identification...should be n@>
    ' should be ',dvi_id:1,'!');
k:=cur_loc; m:=223;
while (m=223)and not eof_dvi_file do m:=get_byte;
if not eof_dvi_file then bad_dvi('signature in byte ',cur_loc-1:1,
@.signature...should be...@>
    ' should be 223')
else if cur_loc<k+4 then
  print_ln('not enough signature bytes at end of file (',
@.not enough signature bytes...@>
    cur_loc-k:1,')');

@ @<Process the font definitions...@>=
repeat k:=get_byte;
if (k>=fnt_def1)and(k<fnt_def1+4) then
  begin p:=first_par(k); define_font(p); print_nl; k:=nop;
  end;
until k<>nop;
if k<>post_post then
  print_ln('byte ',cur_loc-1:1,' is not postpost!')
@.byte n is not postpost@>
  
@* The main program.
Now we are ready to put it all together. This is where \.{DVIplus} starts,
and where it ends.
  
@p begin dont_catch_errors; initialize; {get all variables initialized}
dialog; {set up all the options}
@<Process the preamble@>;
@<Find the postamble, working back from the end@>;
read_postamble;
print_ln('Total of ', nf:1, ' fonts.');
@<Scan for page range to print@>;
write_lj(@=#27'E'#27'&l'@>, copies:1, @='X'#27'*t300R'@>);
@<Translate the page range...@>;
write_lj(@=#27'&l1X'@>);
write_ln(laser_file,'_');
print_nl
end.
  
@ The main program needs a few global variables in order to do its work.
  
@<Glob...@>=
@!k,@!m,@!n,@!p,@!q:integer; {general purpose registers}
  
@ A \.{DVI}-reading program that reads the postamble first need not look at the
preamble; but \.{DVIplus} looks at the preamble in order to do error
checking, and to display the introductory comment.
  
@<Process the preamble@>=
if not open_dvi_file then
  bad_dvi('cannot open file');
@<Open and lock |laser_file|@>;
p:=get_byte; {fetch the first byte}
if p<>pre then bad_dvi('first byte isn''t start of preamble!');
@.First byte isn't...@>
p:=get_byte; {fetch the identification byte}
if p<>dvi_id then
  print_ln('identification in byte 1 should be ',dvi_id:1,'!');
@.identification...should be n@>
@<Compute the conversion factor@>;
p:=get_byte; {fetch the length of the introductory comment}
print('''');
while p>0 do
  begin decr(p); print(xchr[get_byte]);
  end;
print_ln('''')

@ The conversion factor |conv| is figured as follows: There are exactly
|n/d| \.{DVI} units per decimicron, and 254000 decimicrons per inch,
and |resolution| pixels per inch. Then we have to adjust this
by the stated amount of magnification.
  
@<Compute the conversion factor@>=
numerator:=signed_quad; denominator:=signed_quad;
if numerator<=0 then bad_dvi('numerator is ',numerator:1);
@.numerator is wrong@>
if denominator<=0 then bad_dvi('denominator is ',denominator:1);
@.denominator is wrong@>
conv:=(numerator/254000.0)*(resolution/denominator);
mag:=signed_quad;
if new_mag>0 then mag:=new_mag
else if mag<=0 then bad_dvi('magnification is ',mag:1);
@.magnification is wrong@>
true_conv:=conv; conv:=true_conv*(mag/1000.0);
  
@* System-dependent changes.
This section sho