組合語言的函數呼叫

• 你所不知道的C語言：函式呼叫篇

組合語言常見的幾種函數呼叫方式

C 語言

z = mult(x, y)

...

int mult(int a, int b) { ... }

組合語言

使用暫存器傳遞參數            使用堆疊傳遞參數 1       使用堆疊傳遞參數 2

LD R1, x                     PUSH R1, x             PUSH R1, x
LD R2, y                     PUSH R2, y             PUSH R2, y
CALL mult                    CALL mult              CALL mult

mult:                    
// calculate R1 = R1*R2      POP R1, b              MOV FP, SP
                             POP R2, a              // access a by FP-2
// ....                                             // access b by FP-1
  RET

Hack CPU 實作函數呼叫

使用暫存器傳遞參數
// LD R1, x
@x
D=M
@R1
M=D
// LD R2, y
@y
D=M
@R2
M=D
// CALL mult
@multNext
D=A
@SP
M=D
M=M+1

@mult
0; JMP
(multNext)

...

(mult)
// calculate R1 = R1*R2 

// RET
@multNext
0; JMP

Hack 虛擬機的函數呼叫

• https://github.com/cccbook/sp/blob/master/code/nand2tetris/08/FunctionCalls/FibonacciElement/Main.vm

// This file is part of www.nand2tetris.org
// and the book "The Elements of Computing Systems"
// by Nisan and Schocken, MIT Press.
// File name: projects/08/FunctionCalls/FibonacciElement/Main.vm

// Computes the n'th element of the Fibonacci series, recursively.
// n is given in argument[0].  Called by the Sys.init function 
// (part of the Sys.vm file), which also pushes the argument[0] 
// parameter before this code starts running.

function Main.fibonacci 0
push argument 0
push constant 2
lt                     // check if n < 2
if-goto IF_TRUE
goto IF_FALSE
label IF_TRUE          // if n<2, return n
push argument 0        
return
label IF_FALSE         // if n>=2, return fib(n-2)+fib(n-1)
push argument 0
push constant 2
sub
call Main.fibonacci 1  // compute fib(n-2)
push argument 0
push constant 1
sub
call Main.fibonacci 1  // compute fib(n-1)
add                    // return fib(n-1) + fib(n-2)
return