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author | Karl Berry <karl@freefriends.org> | 2020-03-05 00:48:46 +0000 |
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committer | Karl Berry <karl@freefriends.org> | 2020-03-05 00:48:46 +0000 |
commit | f210bce174e1f2f05305ab03e88e120a1cbfc4da (patch) | |
tree | 5c4e2ad096b5c745e859516ac3196fa0864292d5 /Master/texmf-dist/doc/context/sources/general/manuals/lowlevel/lowlevel-registers.tex | |
parent | 35fd641a3546acc0c62e0aa7f134888e36da30d4 (diff) |
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git-svn-id: svn://tug.org/texlive/trunk@54086 c570f23f-e606-0410-a88d-b1316a301751
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diff --git a/Master/texmf-dist/doc/context/sources/general/manuals/lowlevel/lowlevel-registers.tex b/Master/texmf-dist/doc/context/sources/general/manuals/lowlevel/lowlevel-registers.tex new file mode 100644 index 00000000000..8ccb0cd3ab6 --- /dev/null +++ b/Master/texmf-dist/doc/context/sources/general/manuals/lowlevel/lowlevel-registers.tex @@ -0,0 +1,251 @@ +% language=us + +\environment lowlevel-style + +\startdocument + [title=registers, + color=darkmagenta] + +\startsection[title=Preamble] + +Registers are sets of variables that are accessed by index and a such resemble +registers in a processing unit. You can store a quantity in a register, retrieve +it, and also manipulate it. + +There is hardly any need to use them in \CONTEXT\ so we keep it simple. + +\stopsection + +\startsection[title={\TEX\ primitives}] + +There are several categories: + +\startitemize +\startitem + Integers (int): in order to be portable (at the time it surfaced) there are only + integers and no floats. The only place where \TEX\ uses floats internally is + when glue gets effective which happens in the backend. +\stopitem +\startitem + Dimensions (dimen): internally these are just integers but when they are entered they + are sliced into two parts so that we have a fractional part. The internal + representation is called a scaled point. +\stopitem +\startitem + Glue (skip): these are dimensions with a few additional properties: stretch and + shrink. Being a compound entity they are stored differently and thereby a bit + less efficient than numbers and dimensions. +\stopitem +\startitem + Math glue (muskip): this is the same as glue but with a unit that adapts to + the current math style properties. It's best to think about them as being + relative measures. +\stopitem +\startitem + Token lists (toks): these contain a list of tokens coming from the input + or coming from a place where they already have been converted. +\stopitem +\stopitemize + +The original \TEX\ engine had 256 entries per set. The first ten of each set are +normally reserved for scratch purposes: the even ones for local use, and the odd +ones for global usage. On top of that macro packages can reserve some for its own +use. It was quite easy to reach the maximum but there were tricks around that. +This limitation is no longer present in the variants in use today. + +Let's set a few dimension registers: + +\startbuffer[1] +\dimen 0 = 10 pt +\dimen2=10pt +\dimen4 10pt +\scratchdimen 10pt +\stopbuffer + +\typebuffer[1][option=TEX] + +We can serialize them with: + +\startbuffer[2] +\the \dimen0 +\number \dimen2 +\meaning\dimen4 +\meaning\scratchdimen +\stopbuffer + +\typebuffer[2][option=TEX] + +The results of these operations are: + +\startlines\tt +\getbuffer[1,2] +\stoplines + +The last two is not really useful but it is what you see when tracing options are +set. Here \type {\scratchdimen} is a shortcut for a register. This is {\em not} a +macro but a defined register. The low level \type {\dimendef} is used for this +but in a macro package you should not use that one but the higher level \type +{\newdimen} macro that uses it. + +\startbuffer[1] +\newdimen\MyDimenA +\def \MyDimenB{\dimen999} +\dimendef\MyDimenC998 +\stopbuffer + +\typebuffer[1][option=TEX] + +\startbuffer[2] +\meaning\MyDimenA +\meaning\MyDimenB +\meaning\MyDimenC +\stopbuffer + +\typebuffer[2][option=TEX] + +Watch the difference: + +\startlines\tt +\getbuffer[1,2] +\stoplines + +The first definition uses a yet free register so you won't get a clash. The +second one is just a shortcut using a macro and the third one too but again +direct shortcut. Try to imagine how the second line gets interpreted: + +\starttyping[option=TEX] +\MyDimenA10pt \MyDimenA10.5pt +\MyDimenB10pt \MyDimenB10.5pt +\MyDimenC10pt \MyDimenC10.5pt +\stoptyping + +Also try to imagine what messing around with \type {\MyDimenC} will do when we +also have defined a few hundred extra dimensions with \type {\newdimen}. + +In the case of dimensions the \type {\number} primitive will make the register +serialize as scaled points without unit \type {sp}. + +Next we see some of the other registers being assigned: + +\starttyping[option=TEX] +\count 0 = 100 +\skip 0 = 10pt plus 3pt minus 2pt +\skip 0 = 10pt plus 1fill +\muskip 0 = 10mu plus 3mu minus 2mu +\muskip 0 = 10mu minus 1 fil +\toks 0 = {hundred} +\stoptyping + +When a number is expected, you can use for instance this: + +\starttyping[option=TEX] +\scratchcounter\scratchcounterone +\stoptyping + +Here we use a few predefined scratch registers. You can also do this: + +\starttyping[option=TEX] +\scratchcounter\numexpr\scratchcounterone+\scratchcountertwo\relax +\stoptyping + +There are some quantities that also qualify as number: + +\starttyping[option=TEX] +\chardef\MyChar=123 % refers to character 123 (if present) +\scratchcounter\MyChar +\stoptyping + +In the past using \type {\chardef} was a way to get around the limited number of +registers, but it still had (in traditional \TEX) a limitation: you could not go +beyond 255. The \type {\mathchardef} could fo higher as it also encodes a family +number and class. This limitation has been lifted in \LUATEX. + +A character itself can also be interpreted as number, in which case it has to be +prefixed with a reverse quote: \type {`}, so: + +\startbuffer +\scratchcounter\numexpr`0+5\relax +\char\scratchcounter +\stopbuffer + +\typebuffer[option=TEX] + +produces \quotation {\inlinebuffer} because the \type {`0} expands into the +(\ASCII\ and \UTF8) slot {\tt \number`0} which represents the character zero. In +this case the next makes more sense: + +\starttyping[option=TEX] +\char\numexpr`0+5\relax +\stoptyping + +If you want to know more about all these quantities, \quotation {\TEX\ By Topic} +provides a good summary of what \TEX\ has to offer, and there is no need to repeat +it here. + +\stopsection + +\startsection[title={\ETEX\ primitives}] + +Apart from the ability to use expressions, the contribution to registers that +\ETEX\ brought was that suddenly we could use upto 65K of them, which is more +than enough. The extra registers were not as efficient as the first 256 because +they were stored in the hash table, but that was not really a problem. In \OMEGA\ +and later \LUATEX\ regular arrays were used, at the cost of more memory which in +the meantime has become cheap. As \CONTEXT\ moved to \ETEX\ rather early its +users never had to worry about it. + +\stopsection + +\startsection[title={\LUATEX\ primitives}] + +The \LUATEX\ engine introduced attributes. These are numeric properties that are +bound to the nodes that are the result of typesetting operations. They are +basically like integer registers but when set their values get bound and when +unset they are kind of invisible. + +\startitemize +\startitem + Attribute (attribute): a numeric property that when set becomes part of the + current attribute list that gets assigned to nodes. +\stopitem +\stopitemize + +Attributes can be used to communicate properties to \LUA\ callbacks. There are +several functions available for setting them and querying them. + +\starttyping[option=TEX] +\attribute999 = 123 +\stoptyping + +Using attributes this way is dangerous (of course I can only speak for \CONTEXT) +because an attribute value might trigger some action in a callback that gives +unwanted side effects. For convenience \CONTEXT\ provides: + +\startbuffer +\newattribute\MyAttribute +\stopbuffer + +\typebuffer[option=TEX] \getbuffer + +Which currently defines \type {\MyAttribute} as {\tt \meaning\MyAttribute} and is +meant to be used as: \footnote {The low level \type {\attributedef} command is +rather useless in the perspective of \CONTEXT.} + +\starttyping[option=TEX] +\attribute\MyAttribute = 123 +\stoptyping + +Just be aware that defining attributes can have an impact on performance. As you +cannot access them at the \TEX\ end you seldom need them. If you do you can +better use the proper more high level definers (not discussed here). + +\stopsection + +\startsection[title={\LUAMETATEX\ primitives}] + +{\em todo} + +\stopsection + +\stopdocument + |