Previous: Byte-Code Objects, Up: Byte Compilation

16.8 Disassembled Byte-Code

People do not write byte-code; that job is left to the byte compiler. But we provide a disassembler to satisfy a cat-like curiosity. The disassembler converts the byte-compiled code into human-readable form.

The byte-code interpreter is implemented as a simple stack machine. It pushes values onto a stack of its own, then pops them off to use them in calculations whose results are themselves pushed back on the stack. When a byte-code function returns, it pops a value off the stack and returns it as the value of the function.

In addition to the stack, byte-code functions can use, bind, and set ordinary Lisp variables, by transferring values between variables and the stack.

— Command: disassemble object &optional buffer-or-name

This command displays the disassembled code for object. In interactive use, or if buffer-or-name is nil or omitted, the output goes in a buffer named *Disassemble*. If buffer-or-name is non-nil, it must be a buffer or the name of an existing buffer. Then the output goes there, at point, and point is left before the output.

The argument object can be a function name, a lambda expression or a byte-code object. If it is a lambda expression, disassemble compiles it and disassembles the resulting compiled code.

Here are two examples of using the disassemble function. We have added explanatory comments to help you relate the byte-code to the Lisp source; these do not appear in the output of disassemble.

     (defun factorial (integer)
       "Compute factorial of an integer."
       (if (= 1 integer) 1
         (* integer (factorial (1- integer)))))
          ⇒ factorial
     
     (factorial 4)
          ⇒ 24
     
     (disassemble 'factorial)
          -| byte-code for factorial:
      doc: Compute factorial of an integer.
      args: (integer)
     
     0   varref   integer      ; Get the value of integer and
                               ;   push it onto the stack.
     1   constant 1            ; Push 1 onto stack.
     2   eqlsign               ; Pop top two values off stack, compare
                               ;   them, and push result onto stack.
     3   goto-if-nil 1         ; Pop and test top of stack;
                               ;   if nil, go to 1, else continue.
     6   constant 1            ; Push 1 onto top of stack.
     7   return                ; Return the top element of the stack.
     8:1 varref   integer      ; Push value of integer onto stack.
     9   constant factorial    ; Push factorial onto stack.
     10  varref   integer      ; Push value of integer onto stack.
     11  sub1                  ; Pop integer, decrement value,
                               ;   push new value onto stack.
     12  call     1            ; Call function factorial using first
                               ;   (i.e., top) stack element as argument;
                               ;   push returned value onto stack.
     13 mult                   ; Pop top two values off stack, multiply
                               ;   them, and push result onto stack.
     14 return                 ; Return the top element of the stack.

The silly-loop function is somewhat more complex:

     (defun silly-loop (n)
       "Return time before and after N iterations of a loop."
       (let ((t1 (current-time-string)))
         (while (> (setq n (1- n))
                   0))
         (list t1 (current-time-string))))
          ⇒ silly-loop
     
     (disassemble 'silly-loop)
          -| byte-code for silly-loop:
      doc: Return time before and after N iterations of a loop.
      args: (n)
     
     0   constant current-time-string  ; Push current-time-string
                                       ;   onto top of stack.
     1   call     0            ; Call current-time-string with no
                               ;   argument, push result onto stack.
     2   varbind  t1           ; Pop stack and bind t1 to popped value.
     3:1 varref   n            ; Get value of n from the environment
                               ;   and push the value on the stack.
     4   sub1                  ; Subtract 1 from top of stack.
     5   dup                   ; Duplicate top of stack; i.e., copy the top
                               ;   of the stack and push copy onto stack.
     6   varset   n            ; Pop the top of the stack,
                               ;   and bind n to the value.
     
     ;; (In effect, the sequence dup varset copies the top of the stack
     ;; into the value of n without popping it.)
     
     7   constant 0            ; Push 0 onto stack.
     8   gtr                   ; Pop top two values off stack,
                               ;   test if n is greater than 0
                               ;   and push result onto stack.
     9   goto-if-not-nil 1     ; Goto 1 if n > 0
                               ;   (this continues the while loop)
                               ;   else continue.
     12  varref   t1           ; Push value of t1 onto stack.
     13  constant current-time-string  ; Push current-time-string
                                       ;   onto the top of the stack.
     14  call     0            ; Call current-time-string again.
     15  unbind   1            ; Unbind t1 in local environment.
     16  list2                 ; Pop top two elements off stack, create a
                               ;   list of them, and push it onto stack.
     17  return                ; Return value of the top of stack.