% look at TeXbook, p 246
\par
\centerline{%
\vbox{\offinterlineskip\hrule
\halign{\vrule#&\strut\quad\hfil#\hfil\quad&
        \vrule#&\strut\quad\hfil#\hfil\quad&
        \vrule#&\strut\quad#\hfil\quad&
        \vrule#\cr
height2pt&\omit&&\omit&&\omit&\cr
&Registers&&Forward&&\hfil Description&\cr
height2pt&\omit&&\omit&&\omit&\cr
\noalign{\hrule}
height2pt&\omit&&\omit&&\omit&\cr
&0&&&&zero&\cr
&1&&&&reserved for assembler (temporary)&\cr
&2&&yes&&standard argument ({\tt saved\_ptrs[0]})&\cr
&3&&yes&&standard continuation ({\tt saved\_ptrs[1]})&\cr
&4&&yes&&standard closure ({\tt saved\_ptrs[2]})&\cr
&5--19&&yes&&miscellaneous registers&\cr
&20&&&&{\tt my\_arithtemp} (internal to {\tt CMACHINE})&\cr
&21&&yes&&{\tt my\_ptrtmp} (internal to {\tt CMACHINE})&\cr
&22&&special&&store pointer (list of stored locations)&\cr
&23&&special&&data pointer (next available word on heap)&\cr
&24--25&&&&arithmetic temporaries 1~and~2 (exported by {\tt CMACHINE})&\cr
&26--27&&&&reserved for operating system kernel&\cr
&28&&&&global pointer (used by C)&\cr
&29&&&&stack pointer (used by C)&\cr
&30&&yes&&points to current exception handler&\cr
&31&&special&&reserved for the assembler (to hold program counter)&\cr
height2pt&\omit&&\omit&&\omit&\cr
}\hrule}}
\endinput


\def\*{\rlap{${}^*$}}
I've put an asterisk (\*) next to registers that are forwarded
during garbage collection.
The data pointer and store pointer are special to garbage collection
\begin{itemize}
\item[\$0] zero
\item[\$1] reserved for the assembler (as a temporary)
\item[\$2\*] standard argument; also [[saved_ptrs[0] ]]
\item[\$3\*] standard continuation; also [[saved_ptrs[1] ]]
\item[\$4\*] standard closure; also [[saved_ptrs[2] ]]
\item[\$5--\$19\*] miscellaneous registers; 
		may contain only pointers or tagged integers.
\item[\$20] arithmetic temporary number 3; 
		may contain anything except pointers
\item[\$21\*] pointer temporary 1; 
		may contain only pointers or tagged integers.
\item[\$22\*] store pointer;
		used to track mutable (assigned) cells for garbage collection
\item[\$23] data pointer;
		this points to the first available word in the 
		garbage-collectible area, the word that will be the
		descriptor of a newly-created object.
\item[\$24--\$25] arithmetic temporaries number 1 and 2;
		may contain anything except pointers
\item[\$26--\$27] reserved for the operating system kernel
\item[\$28]	the ``global pointer'' (whatever that is)
\item[\$29]	the stack pointer; mostly unused
\item[\$30\*] points to the current exception handler
\item[\$31] reserved for the assembler (to hold the program counter)
\end{itemize} 
