支持减、乘，以及除

(defun jjcc2 (expr)
  "支持两个数的四则运算的编译器"
  (cond ((eq (first expr) '+)
         `((movl ,(second expr) %eax)
           (movl ,(third expr) %ebx)
           (addl %ebx %eax)))
        ((eq (first expr) '-)
         `((movl ,(second expr) %eax)
           (movl ,(third expr) %ebx)
           (subl %ebx %eax)))
        ((eq (first expr) '*)
         ;; 将两个数字相乘的结果放到第二个操作数所在的寄存器中
         ;; 因为约定了用EAX寄存器作为存放最终结果给continuation用的寄存器，所以第二个操作数应当为EAX
         `((movl ,(second expr) %eax)
           (movl ,(third expr) %ebx)
           (imull %ebx %eax)))
        ((eq (first expr) '/)
         `((movl ,(second expr) %eax)
           (cltd)
           (movl ,(third expr) %ebx)
           (idivl %ebx)))))

(defun stringify (asm)
  "根据jjcc2产生的S表达式生成汇编代码字符串"
  (format t "        .section __TEXT,__text,regular,pure_instructions~%")
  (format t "        .globl _main~%")
  (format t "_main:~%")
  (dolist (ins asm)
    (cond ((= (length ins) 3)
           (format t "        ~A ~A, ~A~%"
                   (first ins)
                   (if (numberp (second ins))
                       (format nil "$~A" (second ins))
                       (second ins))
                   (if (numberp (third ins))
                       (format nil "$~A" (third ins))
                       (third ins))))
          ((= (length ins) 2)
           (format t "        ~A ~A~%"
                   (first ins)
                   (if (numberp (second ins))
                       (format nil "$~A" (second ins))
                       (second ins))))
          ((= (length ins) 1)
           (format t "        ~A~%" (first ins)))))
  (format t "        movl %eax, %edi~%")
  (format t "        movl $0x2000001, %eax~%")
  (format t "        syscall~%"))