00100	STACKS
00200	
00300	In Version 0, the P Stack will be non-relocatable and totally
00400	contiguous.  FUNARGS, REFS, and OLD will not be implemented.  The
00500	stack extends from P0 to P.
00600	
00700	The stack is in PIECES, one for each live context.  The bottom of the
00800	current piece is at PBASE.  At PBASE there is a header.  Above the
00900	header is the current piece of stack.  If the stack "underflows", the
01000	last RETURN will jump through a specially modified R.A.  to the
01100	SUCCESSBLT routine.  This routine will find the real R. A. in the
01200	header, find the next portion of stack that has to be BLT'ed from the
01300	header, BLT's it up (if its context no. is different), updates the
01400	header, modifies the lowest R. A. to SUCCESSBLT, corrects P, and
01500	continues.
01600	
01700	At a decision point, a new stack piece is created just above P, and
01800	the first portion of the stack is BLT'ed up there.  The state stack
01900	keeps a record of the original PBASE.  The header of each piece
02000	records the value of P last time it was the current piece, and the
02100	number of the context that existed just before the DPNT.
02200	
02300	Upon failure, PBASE is restored from SS and then P is restored from
02400	the piece header.
02500	
02600	PRUNE makes big holes in the P stack, but they will be passed over
02700	upon later successes.  Only small holes are made in SS; they can be
02800	garbage collected occasionally.
02900	
03000	In the special case of extendable functions, the automatics are saved
03100	at the beginning and at each ALT, but the P stack is only saved at →.
03200	(At →→, a PRUNE is done instead).
03300	
03400	The automatics include P, TP, and whatever else the user declares
03500	AUTOMATIC.  There is a table of PUSHes that is executed by the DPNT
03600	routine.  Failure performs a POP indirect loop through this table to
03700	restore the automatics.
03800	
03900	It is necessary for PRUNE to correct the context number stored in the
04000	stack piece just below the hole it makes.
     

00100	STACK PIECE
00200	
00300	
00400			|---------------|
00500		P →→→→→→→		|
00600			|---------------|
00700			|		|
00800			↓	↓	↓
00900			|		|
01000			|---------------|
01100		(old RA)|   `SUCCESSBLT'|
01200			|---------------|
01300			|		|
01400			|  (arguments)	|
01500			|		|
01600			|---------------|
01700			|place to save P|
01800			|---------------|
01900			|     BLT1	|
02000			|---------------|
02100			|     BLT0	|
02200			|---------------|
02300			|    real R.A.	|
02400			|---------------|
02500		PBASE→→→→  CONTEXT NO.	|
02600			|---------------|
02700		OLD P →→→		|
02800			↓	↓	↓
02900			|		|
03000			|---------------|
03100		        |   real R.A.	|←←←initial BLT1
03200			|---------------|
03300			|		|
03400			↓	↓	↓
03500			|		|
03600			|---------------|
03700			|   `SUCCESSBLT'|
03800			|---------------|
03900			|		|
04000			|  (arguments)	|
04100			|		|
04200			|---------------|
04300			|     OLD P	|
04400			|---------------|
04500			|   old BLT1	|
04600			|---------------|
04700			|   old BLT0	|
04800			|---------------|
04900			| real old R.A.	|
05000			|---------------|
05100	    old	PBASE→→→→  CONTEXT NO.	|←←←initial BLT0
05200			|---------------|
     

00100	STATE STACK LAYOUT
00200	----- ----- ------
00300	
00400	
00500			SS REGISTER		STATE STACK
00600			 ---------------	 * * * * * * * *
00700			|	|	|	|---------------|
00800			| COUNT	|SS TOP>>>>>>>>>|  →UNDO ROUTINE|
00900			|	|	|	|---------------|
01000			 ----------------	|   THINGS	|
01100						|     TO BE	|	
01200						|    RESTORED	|
01300						|---------------|
01400						|  →UNDO ROUTINE|
01500						|---------------|
01600						|   THINGS	|
01700						|     TO BE	|
01800						|    RESTORED	|
01900						|---------------|
02000						 ↓	↓      ↓
02100						 .	.      .
02200						 ↓	↓      ↓
02300			CTAG REGISTER		|---------------|
02400			 ---------------	|  →RSTR_CONTEXT|
02500			|CURRENT |BACKUP|	|---------------|
02600			|CONTEXT | MODE	|	|   SAVED TP	|
02700			|  TAG	 | (0-3)|	|---------------|
02800			 ---------------	|		|
02900			CBASE REGISTER		|  SAVED PBASE  |
03000			 ---------------	|		|
03100			|	|	|	|---------------|
03200			| COUNT	|SS MARK>>>>>>>>|  SAVED CBASE >>>>∨
03300			|	|	|	|---------------|  ∨
03400			 ---------------	|  SAVED CTAG	|  ∨
03500						|---------------|  ∨
03600						|FAILURE ADDRESS|  ∨
03700						|---------------|  ∨
03800						|		|  ∨
03900						 ↓   ↓  ↓  ↓   ↓   ↓
     

00100	SPECIAL LAYOUT FOR EXTENDABLE FUNCTIONS
00200	------- ------ --- ---------- ---------
00300	
00400			SS REGISTER		STATE STACK
00500			 ---------------	 * * * * * * * *
00600			|	|	|	|---------------|
00700			| COUNT	|SS TOP>>>>>>>>>|  →UNDO ROUTINE|
00800			|	|	|	|---------------|
00900			 ----------------	| THINGS TO BE	|
01000						|   RESTORED	|
01100						|---------------|
01200						 ↓	↓      ↓
01300			CTAG REGISTER		|---------------|
01400			 ---------------	|      →RSTR_DEC|<<<<<<<<
01500			|CURRENT|BACKUP |	|---------------|	∧
01600			|CONTEXT| MODE	|	|   SAVED TP	|	∧
01700			|  TAG	| (0-3)	|	|---------------|	∧
01800			 ---------------	|  SAVED PBASE	|	∧
01900						|---------------|	∧
02000						|  ITS  |	|	∧
02100						|CONTEXT|SS MARK>>>∨	∧
02200						|  TAG	|	|  ∨	∧
02300						|---------------|  ∨	∧
02400						 ↓	↓      ↓   ∨	∧
02500						|---------------|  ∨	∧
02600						|      →NEXT_ALT|  ∨	∧
02700						|---------------|  ∨	∧
02800						|   SAVED TP	|  ∨	∧
02900						|---------------|  ∨	∧
03000						|   SAVED P	|  ∨	∧
03100						|---------------|  ∨	∧
03200						| →NEXT ALT ADDR|  ∨	∧
03300						|---------------|  ∨	∧
03400						↓	↓      ↓   ∨	∧
03500						|---------------|  ∨	∧
03600			CBASE REGISTER		|         →ERASE|  ∨	∧
03700			 --------------- 	|---------------|  ∨	∧
03800			|	|	|	|     SAVED	|<<<	∧
03900			| COUNT |SS MARK>>>>>>>>|     CBASE     |	∧
04000			|  	|       |  	|    REGISTER  >>>>∨	∧
04100			 ---------------	|---------------|  ∨	∧
04200						|   SAVED CTAG	|  ∨	∧
04300					      	|---------------|  ∨	∧
04400						|    POINTER   >>>>⊗>>>>∧
04500			 			|---------------|  ∨
04600						|		|  ∨
04700						 ↓   ↓  ↓  ↓   ↓   ↓
     

00100	SELECT MACRO
00200	
00300	SELECT E0 FROM I: E1 NEXT E2 UNLESS E3 IN WHICH CASE E4
00400	
00500		BEGIN
00600		NEW I ;
00700		I ← E1 ; GO TO CHECK ;
00800		TRYNEXT:
00900		I ← E2 ;
01000		CHECK:
01100		IF E3 THEN RETURN(E4) ;
01200		DPNT('TRYNEXT) ;
01300		RETURN(E0) ;
01400		END
01500	
01600	FUNCTION CHOICE(INTEGER N) =
01700		SELECT I FROM I:1 NEXT I+1 UNLESS I>N IN WHICH CASE FAIL ;
01800	
01900	CONTEXTUAL FUNCTION DPNT(L) =	% called by PUSHJ SS, DPNT %
02000		BEGIN WORLD BLT1, BLT0, BLT2, BLTB, TEMP, X, RA, SIZE ;
02100		PUSH(SS) ← CTAG ;
02200		PUSH(SS) ← CBASE ;
02300		CBASE ← SS ;
02350		BLT1←P ; BLT0←PBASE ; BLTB←PBASE+5 ; RA←REALRA(PBASE) ;
02400		OLDP(PBASE) ← P ;  PUSH(P) ← CTAG ;
02500		CTAG ← CTAG + 8 ;
02600		PUSH(SS) ← PBASE ;
02700		PUSH(SS) ← TP ;
02800		PUSH(SS) ← 'RESTORE_CONTEXT ;
02900		SIZE ← BLT1 - BLTB + XWD(1,1) ;
03000		TEMP ← P ;
03100		IF SIZE GREATERP MAXBLT ∧ BLT2←SMALLER_PIECE() THEN
03200			BEGIN
03300			PUSH(P) ← RA ;
03400			PUSH(P) ← PBASE ;
03500			PUSH(P) ← BLT2 ;
03600			END
03700		ELSE	BEGIN
03800			PUSH(P) ← RA ;
03900			PUSH(P) ← X ← OLDBLT0(BLT0) ;
04000			PUSH(P) ← OLDP(X) ;
04050			END ;
04100		PUSH(P) ← * ;
04200		PUSH(P) ← BLOCK[BLT0 FOR SIZE] ;
04300		PBASE ← TEMP ;
04400		END ;
     

00100	LET PUSHSTACK(*,*,S,*,*,E) IDEXP =
00200		{
00300		PUSH
00400		?(
00500		<EXPR>
00600		?)
00700		?←
00800		{ALT
00900			BLOCK ?[ <EXPR> FOR <EXPR> ?]
01000		|	{REP 1 M * {?*}}
01100		|	<EXPR>
01200		}
01300		}