Add garbage collection (now 470 bytes)

This GC wraps Eval() to create a copy of the result, and then
memcpy's it backwards to the position where the g_mem pointer
resided at the beginning of Eval() thereby discarding all the
cons cells that got created which weren't part of the result.
Overlap (or negative GC) isn't possible because we don't copy
cells beneath the low water mark.

As it turns out 44 bytes is all you need to implement garbage
collection when your language guarantees that data structures
can't have cycles, due to the lack of mutability.

sectorlisp.S

@@ -22,8 +22,8 @@
 // LISP meta-circular evaluator in a MBR
 // Compatible with the original hardware
 
-.set g_token,		%bp
-.set g_mem,		%bp
+.set g_mem,		%cx
+.set g_token,		%cx
 .set ZERO,		%bh
 .set TWO,		%bx
 
@@ -67,13 +67,12 @@ begin:	xor	%ax,%ax
 	sti					# enable interrupts
 	cld					# direction forward
 	mov	$2,TWO
-	mov	$Eval,%cx
-	mov	$0x8000,g_mem
 main:	mov	$'\n',%dl
+	mov	$0x8000,g_mem
 	call	GetToken
 	call	GetObject
 	xor	%dx,%dx
-	call	*%cx				# call Eval
+	call	Eval
 	xchg	%ax,%di
 	call	PrintObject
 	mov	$'\r',%al
@@ -162,14 +161,12 @@ GetChar:
 						# ah is bios scancode
 						# al is ascii character
 PutChar:
-	push	%bp				# original ibm pc scroll up bug
 	mov	$0x0e,%ah			# teletype output al cp437
 	int	$0x10				# vidya service
-	pop	%bp				# preserves al
-	cmp	$'\r',%al			# don't clobber stuff
+	cmp	$'\r',%al			# don't clobber
 	jne	.ret
 	mov	$'\n',%al
-	jmp	PutChar				# bx volatile
+	jmp	PutChar
 
 ////////////////////////////////////////////////////////////////////////////////
 
@@ -192,9 +189,7 @@ Evlis:	test	%di,%di				# Evlis(m:di,a:dx):ax
 	jz	1f				# jump if nil
 	push	(TWO,%di)			# save 1 Cdr(m)
 	mov	(%di),%ax
-	push	%dx				# save a
-	call	*%cx				# call Eval
-	pop	%dx				# restore a
+	call	Eval
 	pop	%di				# restore 1
 	push	%ax				# save 2
 	call	Evlis
@@ -210,6 +205,19 @@ Cons:	xchg	%ax,%si				# Cons(m:di,a:ax):ax
 1:	xchg	%di,%ax
 	ret
 
+Gc:	cmp	%dx,%di				# Gc(x:di,mark:dx,aj:bp):ax
+	jb	1b				# we assume immutable cells
+	push	(TWO,%di)			# mark prevents negative gc
+	mov	(%di),%di
+	call	Gc
+	pop	%di
+	push	%ax
+	call	Gc
+	pop	%di
+	call	Cons
+	sub	%bp,%ax				# subtract adjustment
+	ret
+
 GetList:call	GetToken
 	cmpb	$')',%al
 	je	.retF
@@ -218,21 +226,7 @@ GetList:call	GetToken
 	call	GetList
 	jmp	xCons
 
-1:	mov	(TWO,%di),%di			# di = Cdr(c)
-Evcon:	push	%di				# save c
-	mov	(%di),%si			# di = Car(c)
-	lodsw					# ax = Caar(c)
-	push	%dx				# save a
-	call	*%cx				# call Eval
-	pop	%dx				# restore a
-	pop	%di				# restore c
-	test	%ax,%ax				# nil test
-	jz	1b
-	mov	(%di),%di			# di = Car(c)
-.EvCadr:call	Cadr				# ax = Cadar(c)
-#	jmp	Eval
-
-Eval:	test	%ax,%ax				# Eval(e:ax,a:dx):ax
+.Eval:	test	%ax,%ax				# Eval(e:ax,a:dx):ax w/o gc
 	jns	Assoc				# lookup val if atom
 	xchg	%ax,%si				# di = e
 	lodsw					# ax = Car(e)
@@ -279,9 +273,7 @@ Apply:	test	%ax,%ax				# Apply(fn:ax,x:si:a:dx):ax
 .retF:	xor	%ax,%ax				# ax = nil
 	ret
 .dflt1:	push	%si				# save x
-	push	%dx				# save a
-	call	*%cx				# call Eval
-	pop	%dx				# restore a
+	call	Eval
 	pop	%si				# restore x
 	jmp	Apply
 
@@ -301,7 +293,37 @@ Assoc:	mov	%dx,%si				# Assoc(x:ax,y:dx):ax
 	mov	(TWO,%di),%ax			# ax = Cdar(y)
 	ret
 
-.type .sig,@object;
+1:	mov	(TWO,%di),%di			# di = Cdr(c)
+Evcon:	push	%di				# save c
+	mov	(%di),%si			# di = Car(c)
+	lodsw					# ax = Caar(c)
+	call	Eval
+	pop	%di				# restore c
+	test	%ax,%ax				# nil test
+	jz	1b
+	mov	(%di),%di			# di = Car(c)
+.EvCadr:call	Cadr				# ax = Cadar(c)
+#	jmp	Eval
+
+Eval:	push	%dx				# Eval(e:ax,a:dx):ax w/ gc
+	push	g_mem				# with garbage collections
+	call	.Eval				# discards non-result cons
+	pop	%dx
+	push	g_mem
+	mov	g_mem,%bp
+	sub	%dx,%bp
+	xchg	%ax,%di
+	call	Gc
+	pop	%si
+	mov	%dx,%di
+	mov	g_mem,%cx
+	sub	%si,%cx
+	rep movsb
+	mov	%di,g_mem
+	pop	%dx
+	ret
+
+.type .sig,@object
 .sig:
 .fill 510 - (. - _start), 1, 0xce
 .word 0xAA55