Shave another fifteen bytes (now 438 bytes)

%bx wasn't needed in PutChar since we're assuming the machine boots
in a non-graphical mode, i.e. BIOS teletype. In MDA mode BX will be
2 which is dark green on black. The same probably applies with CGA.

Now that BX is always two, we can now use multi-register addressing
when accessing CDR cells. That's a huge savings, since we no longer
need all those displacement bytes; we only need the modrm.

%cx wasn't needed it could be replaced with %si in GetToken thereby
making it possible to remove our use of the %fs register.

Changing the second argument Cons() from %si not %ax helped to save
even more space, since all the things that called it needed to move
the register from %ax to %si.

sectorlisp.S

@@ -21,7 +21,6 @@
 
 // LISP meta-circular evaluator in a MBR
 
-.set ONE,		%bp
 .set ATOM_NIL,		(kNil-kSymbols)<<1|1
 .set ATOM_QUOTE,	(kQuote-kSymbols)<<1|1
 .set ATOM_COND,		(kCond-kSymbols)<<1|1
@@ -33,9 +32,11 @@
 .set ATOM_T,		(kT-kSymbols)<<1|1
 
 .set g_str,		0x0
-.set g_token,		0x7800
+.set g_token,		%bp
-.set boot,		0x7c00
+.set g_mem,		%bp
-.set g_mem,		0x8000
+.set ZERO,		%ch
+.set ONE,		%cx
+.set TWO,		%bx
 
 ////////////////////////////////////////////////////////////////////////////////
 // Currently requires i386+ in real mode
@@ -61,13 +62,13 @@ kCar:	.asciz	"CAR"
 kCdr:	.asciz	"CDR"
 kCons:	.asciz	"CONS"
 kEq:	.asciz	"EQ"				# needs to be last
-_begin:	mov	$g_mem,%cx
+_begin:	push	%cs				# memory model ds=es=ss=cs
-	mov	%cx,%fs				# fs = &g_mem
-	mov	%cx,%di
-	push	%cs				# memory model ds=es=ss=cs
 	pop	%ds
 	push	%cs
 	pop	%es
+	mov	$0x8000,%cx
+	mov	%cx,g_mem
+	mov	%cx,%di
 	xor	%ax,%ax
 	cld					# clear direction flag
 	rep stosb				# memset(0x8000,0,0x8000)
@@ -76,12 +77,13 @@ _begin:	mov	$g_mem,%cx
 	pop	%ss				# disable nonmaskable interrupts
 	mov	%ax,%sp				# use null pointer as our stack
 #	sti					# enable interrupts
-	inc	%ax
+	inc	ONE				# ++cx
-	xchg	%ax,ONE				# bp = 1
+	mov	ONE,TWO
+	inc	TWO
 main:	mov	$'\n',%dl
 	call	GetToken
 	call	GetObject
-	mov	ONE,%dx
+	mov	ONE,%dx				# dx = NIL
 	call	Eval
 	call	PrintObject
 	mov	$'\r',%al
@@ -89,14 +91,14 @@ main:	mov	$'\n',%dl
 	jmp	main
 
 GetToken:					# GetToken():al, dl is g_look
-	mov	%fs,%di				# di = g_token
+	mov	g_token,%di
 	mov	%di,%si
 1:	mov	%dl,%al
 	cmp	$' ',%al
 	jbe	2f
 	stosb
-	xchg	%ax,%cx
+	xchg	%ax,%si
-2:	call	GetChar				# bh = 0 after PutChar
+2:	call	GetChar
 	xchg	%ax,%dx				# dl = g_look
 	cmp	$' ',%al
 	jbe	1b
@@ -104,8 +106,8 @@ GetToken:					# GetToken():al, dl is g_look
 	jbe	3f
 	cmp	$')',%dl
 	ja	1b
-3:	movb	%bh,(%di)
+3:	movb	ZERO,(%di)
-	xchg	%cx,%ax
+	xchg	%si,%ax
 	ret
 
 .PutObject:					# .PutObject(c:al,x:di)
@@ -128,7 +130,7 @@ PrintObject:					# PrintObject(x:ax)
 	jmp	.PrintString
 .PrintList:
 	mov	$'(',%al
-2:	push	2(%di)				# save 1 Cdr(x)
+2:	push	(TWO,%di)			# save 1 Cdr(x)
 	mov	(%di),%di			# di = Car(x)
 	call	.PutObject
 	pop	%ax				# restore 1
@@ -157,7 +159,7 @@ GetObject:					# called just after GetToken
 	jne	1b
 	jmp	5f
 2:	pop	%si				# drop 1
-	mov	%fs,%si				# si = g_token
+	mov	g_token,%si
 3:	scasb
 	jne	3b
 	cmp	(%di),%al
@@ -180,7 +182,6 @@ GetChar:
 						# al is ascii character
 PutChar:
 #	push	%bp				# original ibm pc scroll up bug
-	xor	%bx,%bx				# normal mda/cga style page zero
 	mov	$0x0e,%ah			# teletype output al cp437
 	int	$0x10				# vidya service
 #	pop	%bp				# preserves al
@@ -193,17 +194,15 @@ PutChar:
 
 Pairlis:cmp	ONE,%di				# Pairlis(x:di,y:si,a:dx):ax
 	je	1f				# it's zip() basically
-	push	2(%di)				# save 1 Cdr(x)
+	push	(TWO,%di)			# save 1 Cdr(x)
 	lodsw
 	push	(%si)				# save 2 Cdr(y)
 	mov	(%di),%di
-	xchg	%ax,%si
 	call	Cons				# preserves dx
 	pop	%si				# restore 2
 	pop	%di				# restore 1
 	push	%ax				# save 3
 	call	Pairlis
-	xchg	%ax,%si
 	pop	%di				# restore 3
 	jmp	Cons				# can be inlined here
 1:	xchg	%dx,%ax
@@ -211,7 +210,7 @@ Pairlis:cmp	ONE,%di				# Pairlis(x:di,y:si,a:dx):ax
 
 Evlis:	cmp	ONE,%di				# Evlis(m:di,a:dx):ax
 	je	1f
-	push	2(%di)				# save 1 Cdr(m)
+	push	(TWO,%di)			# save 1 Cdr(m)
 	mov	(%di),%ax
 	push	%dx				# save a
 	call	Eval
@@ -220,17 +219,14 @@ Evlis:	cmp	ONE,%di				# Evlis(m:di,a:dx):ax
 	push	%ax				# save 2
 	call	Evlis
 
-xCons:	xchg	%ax,%si
+xCons:	pop	%di				# restore 2
-	pop	%di				# restore 2
+Cons:	xchg	%ax,%si				# Cons(m:di,a:ax):ax
-#	jmp	Cons
+	xchg	%di,%ax
-Cons:	xchg	%di,%ax
+	mov	g_mem,%di
-	mov	%fs,%di
 	stosw
 	xchg	%si,%ax
 	stosw
-	xchg	%di,%ax
+	xchg	%di,g_mem
-	mov	%fs,%di
-	mov	%ax,%fs
 1:	xchg	%di,%ax
 	ret
 
@@ -242,10 +238,10 @@ GetList:call	GetToken
 	call	GetList
 	jmp	xCons
 
-1:	mov	2(%di),%di			# di = Cdr(c)
+1:	mov	(TWO,%di),%di			# di = Cdr(c)
 Evcon:	push	%di				# save c
-	mov	(%di),%di			# di = Car(c)
+	mov	(%di),%si			# di = Car(c)
-	mov	(%di),%ax			# ax = Caar(c)
+	lodsw					# ax = Caar(c)
 	push	%dx				# save a
 	call	Eval
 	pop	%dx				# restore a
@@ -274,18 +270,18 @@ Eval:	test	$1,%al				# Eval(e:ax,a:dx):ax
 Apply:	test	$1,%al				# Apply(fn:ax,x:si:a:dx):ax
 	jnz	.switch
 	xchg	%ax,%di				# di = fn
-.lambda:mov	2(%di),%di			# di = Cdr(fn)
+.lambda:mov	(TWO,%di),%di			# di = Cdr(fn)
 	push	%di				# save 1
 	mov	(%di),%di			# di = Cadr(fn)
 	call	Pairlis
 	xchg	%ax,%dx
 	pop	%di				# restore 1
 	jmp	.EvCadr
-.ifCons:mov	2(%si),%si			# si = Cdr(x)
+.ifCons:cmp	$ATOM_CONS,%al
-	mov	(%si),%si			# si = Cadr(x)
+	mov	(TWO,%si),%si			# si = Cdr(x)
-	cmp	$ATOM_CONS,%al
+	lodsw					# si = Cadr(x)
 	je	Cons
-.isEq:	cmp	%di,%si
+.isEq:	cmp	%di,%ax
 	jne	.retF
 .retT:	mov	$ATOM_T,%al			# ax = ATOM_T
 	ret
@@ -309,20 +305,20 @@ Apply:	test	$1,%al				# Apply(fn:ax,x:si:a:dx):ax
 	pop	%si				# restore x
 	jmp	Apply
 
-Cadr:	mov	2(%di),%di			# contents of decrement register
+Cadr:	mov	(TWO,%di),%di			# contents of decrement register
 	.byte	0x3C				# cmp §scasw,%al (nop next byte)
 Cdr:	scasw					# increments our data index by 2
 Car:	mov	(%di),%ax			# contents of address register!!
 	ret
 
-.Assoc:	mov	2(%si),%dx			# dx = Cdr(y)
+.Assoc:	mov	(TWO,%si),%dx			# dx = Cdr(y)
 Assoc:	cmp	ONE,%dx				# Assoc(x:ax,y:dx):ax
 	mov	%dx,%si
 	je	.retF
-	mov	(%si),%bx			# bx = Car(y)
+	mov	(%si),%di			# bx = Car(y)
-	cmp	%ax,(%bx)			# (%bx) = Caar(y)
+	cmp	%ax,(%di)			# (%di) = Caar(y)
 	jne	.Assoc
-	mov	2(%bx),%ax			# ax = Cdar(y)
+	mov	(TWO,%di),%ax			# ax = Cdar(y)
 	ret
 
 .type .sig,@object;