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.
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
(see Lambda Expressions), or a byte-code object (see Byte-Code Function Objects). 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: (arg1)
0 dup ; 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 dup ; Push value ofintegeronto stack. 9 constant factorial ; Pushfactorialonto stack. 10 stack-ref 2 ; Push value ofintegeronto stack. 11 sub1 ; Popinteger, decrement value, ; push new value onto stack. 12 call 1 ; Call functionfactorialusing 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: (arg1)
0 constant current-time-string ; Push current-time-string
; onto top of stack.
1 call 0 ; Callcurrent-time-stringwith no ; argument, push result onto stack ast1.
2:1 stack-ref 1 ; Get value of the argument n
; and push the value on the stack.
3 sub1 ; Subtract 1 from top of stack.
4 dup ; Duplicate top of stack; i.e., copy the top ; of the stack and push copy onto stack. 5 stack-set 3 ; Pop the top of the stack, ; and setnto the value. ;; (In effect, the sequencedup stack-setcopies the top of ;; the stack into the value ofnwithout 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 dup ; Push value oft1onto stack. 13 constant current-time-string ; Pushcurrent-time-string; onto the top of the stack. 14 call 0 ; Callcurrent-time-stringagain.
15 list2 ; Pop top two elements off stack, create a ; list of them, and push it onto stack. 16 return ; Return value of the top of stack.