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I was creating a rather large LaTeX project, so I had to use many packages. This gave me an error

No room for a new \dimen \newdimen \MPscratchDim

while my editor(Kile) opened the file supp-pdf.mkii, pointing at the line

\newdimen\MPscratchDim % will be assigned global

Searching through the net, I found that this could happen due to loading too many packages, as answered in No room for a new \dimen by egreg.

Indeed I loaded, etex and everything works fine now. I searched etex on CTAN to understand what does this particular package do, but I am not able to understand what is going on. As stated by CTAN

The pack­age pro­vides sup­port for LATEX doc­u­ments to use many of the ex­ten­sions of­fered by e-TEX; in par­tic­u­lar, it mod­i­fies LATEX’s reg­is­ter al­lo­ca­tion macros to make use of the ex­tended reg­is­ter range

I believe that the bold words give the answer, but I cannot understand why there is a limited register allocation (I don't even know what that is) and how can a package extend this register range.

Any idea on that will be very educational!

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1  
If a register is defined by 8 bits, you can uniquely address 2^8=256 locations. In the "old days" this may have been the norm. As the hardware became capable, upgrading a register to 16 bits would extend your addressing range to 65536 locations. (My first computer had 4096 total bytes of memory) –  Steven B. Segletes Jun 25 at 15:02
2  
Originally, TeX provided a fixed amount of 256 different "dimension registers" (which is what is allocated by \newdimen). The code of the \newdimen macro reflects this by checking that not too many registers are allocated. The current eTeX engine extends this fixed number to 32768, but the code of \newdimen needs to be adapted to the new maximum number by the etex package. –  Stephan Lehmke Jun 25 at 15:03

3 Answers 3

up vote 37 down vote accepted

TeX, as designed by Knuth, has various registers addressable with an eight bit number (that is, from 0 to 255). Registers are of type

  • \count
  • \dimen
  • \skip
  • \muskip
  • \toks
  • \insert
  • \box

Let's consider the \dimen registers, for the others the allocation mechanism is similar. Each register is addressable by its number, for instance

\dimen34=42pt
\kern\dimen34

are legal example of setting a \dimen register or retrieving its value. However, calling registers by number is a problem, because packages couldn't cooperate with each other. So all formats provide an allocation mechanism: one says

\newdimen\foo

and TeX sets up things so that \foo is the same as calling \dimen<n> where <n> represents a number that we don't need to know. Note that \newdimen is not a documented LaTeX command, but it is documented in source2e and is very useful when writing packages.

TeX/LaTeX keeps track of the most recent allocated number for each register type; since the maximum number is 255, when a \newdimen command hits the limit, the dreaded

No room for a new \dimen

error message appears. There is a slight complication because allocation of \insert registers starts from 254 down and each \newinsert also allocates the \count, \dimen, \skip and \box registers with the same number, so the upper limit is usually something less than 255, but it's not the main problem.

In case we hit the limit, there is nothing to do: we are forced to load less packages or do nasty tricks that will almost certainly bite us later on.

In order to solve this problem that became apparent several years ago when PicTeX was released, a different implementation of TeX was prepared that defines 32768 registers of each type. This is part of e-TeX, that is incorporated in pdftex, the engine used when we run LaTeX on a document.

However, the kernel of LaTeX has not been modified, because it's basically frozen apart from bug fixes. Changing the allocation mechanism might break documents or packages that have been written under the assumption that only 256 registers of each type are available.

The etex package modifies the allocation mechanism; when \newdimen would issue the No room for a new \dimen error message, it insteads jumps beyond 255 and allocates the register number 256 and goes forward from there.

Here is a simulation, where we assume that \newdimen\foo allocates the last available eight bit slot and \newdimen\baz needs to go up to fifteen bits; in the log file you'd find

\foo=\dimen233

Normal \dimen register pool exhausted, switching to extended pool.
\baz=\dimen256

provided you have \usepackage{etex} in your preamble.

Why 233 is the last? Because LaTeX allocates \insert classes from 254 to 234; the are connected with marginal notes, footnotes, figures and tables.

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1  
It's hard to see how one could break a document which assumes there are only 256 registers, at least at the allocator level. (The registers are after all active, etex package or not, while running out or registers just leads to an error in any case.) I can see you could break docs that try to alter the allocation routine themselves. However, as noted in chat there is some discussion about this very point within the team at the moment. –  Joseph Wright Jun 25 at 17:08

classic TeX has 256 registers (eg count and dimen registers as allocated by \newcounter and \newlength in LaTeX.

For some years LaTeX formats have used the extended etex (or pdf(e)tex engines rather than classic TeX, so actually there are 32768 registers available. However for compatibility reasons (or stubbornness, or apathy, depending on your point of view) the default latex macros that allocate a name to a internal register number have not been updated so still only allow numbers 0-255. So if you load etex package (or use lualatex and xelatex formats as currently constructed, which use similarly extended allocation) The macros are changed to use a different range.

Note that etex is active by default, you can use \nummexpr or a count register such as \count1000, it is just that \newcount does not "know" that you may do that so does not allocate any more than 255 .

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The format-building processes at present do not load etex even if they do other allocation-related things (all a bit strange). –  Joseph Wright Jun 25 at 16:42

Run texdoc etex and you'll get the documentation of eTeX. Two of the nice new features are \dimexpr and \numexpr. Instead of 256 available counters, dimensions, etc. there are now 2^{15}.

ε-TEX increases the number of TEX’s count, dimen, skip, muskip, box, and token registers from 256 to 32768.

eTeX is the default engine for all programs except of the original TeX. To activate the eTeX extensions one has to include \usepackage{etex}

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Thank you very much for your answer! What I don't understand is what is a counter? For every package used, the counter increases by 1, which means that you can use only 256 packages? In addition what is a register? Or what is dimen, skip, muskip, box and token registers? –  Thanos Jun 25 at 15:19
    
@Thanos latex commands such as \newcounter give counters for things like equation or section and the question is how many different such counters can you have. It is unrelated to package use. –  David Carlisle Jun 25 at 15:21
    
All those facts are not important to the "normal" user, whatever this may be. A counter counts things like pages, equations, figures, tables, a.s.o. –  Herbert Jun 25 at 15:22
    
@DavidCarlisle : I know that! That's why it's weird! My document has only a preamble with many packages and only a hello world within the document environment, so there's no need to "count" anything... –  Thanos Jun 25 at 15:24
2  
@Thanos A package might allocate one or more counters. It doesn't necessarily do. –  egreg Jun 25 at 15:33

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