Add numbers (!)

This is the point where we get confused about 32 vs 64 bits.  We'll
handle that... later, I guess?

scm/inc/Makefile

@@ -2,7 +2,7 @@ run-scheme: scheme
	./scheme
scheme: scheme.s driver.c
	gcc -O3 -g scheme.s driver.c -o $@
	gcc -g scheme.s driver.c -o $@
scheme-static: scheme.s driver.c
	gcc -O3 -static scheme.s driver.c -o $@

scm/inc/compiler.scm

@@ -5,6 +5,8 @@
  (display (apply format instr args))
  (display "\n"))
(define wordsize 4)
(define fixnum-shift 2)
(define char-shift 8)
(define boolean-shift 7)
@@ -32,6 +34,9 @@
(define (primcall-operand1 x)
  (cadr x))
(define (primcall-operand2 x)
  (caddr x))
(define (emit-compare)
  (emit "movl $0,  %eax")                ; zero %eax to put the result of the comparison into
  (emit "sete %al")                      ; set low byte of %eax to 1 if cmp succeeded
@@ -77,11 +82,17 @@
     (emit-expr (primcall-operand1 x) si)
     (emit "andl $~a, %eax" #b1111111)
     (emit "cmpl $~a, %eax" #b0011111)
     (emit-compare))))
     (emit-compare))
    ((+)
     (emit-expr (primcall-operand2 x) si)
     (emit "movl %eax, ~a(%rbp)" si) ; move second arg on the stack
     (emit-expr (primcall-operand1 x) (- si wordsize))
     (emit "addl ~a(%rbp), %eax" si))
    ))
(define (compile-program x)
  (display ".globl scheme_entry\n\n")
  (display "scheme_entry:\n")
  (emit-expr x 0)
  (emit-expr x (- wordsize))
  (emit "ret"))

scm/inc/program.scm

@@ -1,3 +1,3 @@
(load "compiler.scm")
(compile-program '(null? 0))
(compile-program '(+ 1 2))