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Assembly Language & x86 Low-Level Systems Programming · 课时

传递参数与返回值

了解向过程传递参数和返回值的常见约定，以及如何利用寄存器和栈实现这些操作。

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传递参数与返回值 是 CoddyKit 上的免费 Assembly Language & x86 Low-Level Systems Programming 课时。 这是第 3 节课，共 4 节。 你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。 这是 Assembly Language & x86 Low-Level Systems Programming 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Assembly Language & x86 Low-Level Systems Programming 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Intro: Data Flow in Procedures

When you write a procedure (like a function in C), it often needs data to work with, and it might produce a result.

This lesson explores how data, called arguments, is sent into a procedure, and how the procedure sends a return value back out.

Why Pass Data?

Procedures are like mini-programs. To make them useful, they need to communicate with the main program or other procedures.

  • Arguments: Input data for the procedure to process.
  • Return Value: The result produced by the procedure.

Without this communication, procedures would be very limited!

Calling Conventions

To ensure procedures can talk to each other, there are rules called calling conventions.

These rules dictate:

  • How arguments are passed (registers or stack).
  • Which registers a procedure can modify.
  • How return values are transmitted.

We'll look at common ways for x86.

Passing Arguments via Registers

For a small number of arguments, it's efficient to pass them using registers.

A common convention (like System V ABI on Linux) uses specific registers for the first few arguments:

  • RDI (1st argument)
  • RSI (2nd argument)
  • RDX (3rd argument)
  • RCX (4th argument)

For 32-bit (x86), these would be EDI, ESI, etc.

Code: Register Arguments (32-bit)

Here, a procedure _add_two takes two numbers in EBX and ECX (acting as arguments) and returns their sum in EAX.

section .data
    msg db "Sum: ", 0

section .text
    global _start

_add_two:
    ; Arguments: EBX, ECX
    ; Returns: EAX
    mov eax, ebx  ; Move first arg to EAX
    add eax, ecx  ; Add second arg
    ret           ; Return to caller

_start:
    ; Prepare arguments for _add_two
    mov ebx, 5    ; First argument
    mov ecx, 3    ; Second argument
    call _add_two ; Call the procedure

    ; EAX now holds the sum (8)
    ; --- Print result (for demonstration) ---
    ; Convert EAX to ASCII (simple example)
    add eax, '0'  ; Convert 8 to ASCII '8'
    mov [result_char], al

    ; Write "Sum: "
    mov eax, 4    ; sys_write
    mov ebx, 1    ; stdout
    mov ecx, msg
    mov edx, 5    ; length of "Sum: "
    int 0x80

    ; Write the result character
    mov eax, 4    ; sys_write
    mov ebx, 1    ; stdout
    mov ecx, result_char
    mov edx, 1    ; length of '8'
    int 0x80

    ; Write newline
    mov eax, 4
    mov ebx, 1
    mov ecx, newline
    mov edx, 1
    int 0x80

    ; Exit program
    mov eax, 1    ; sys_exit
    xor ebx, ebx  ; exit code 0
    int 0x80

section .bss
    result_char resb 1
    newline resb 1

Passing Arguments via the Stack

When there are many arguments, or if registers are already in use, the stack is used to pass arguments.

  • Arguments are PUSHed onto the stack before the CALL instruction.
  • The called procedure accesses these arguments relative to the EBP (base pointer) or ESP (stack pointer).

The order of pushing matters! Often, arguments are pushed in reverse order (last argument first).

Accessing Stack Arguments

After CALL, the return address is on the stack. If arguments were pushed, they are below the return address.

Inside the procedure, EBP is typically saved and then set to ESP to create a stable base for accessing arguments and local variables.

  • [EBP+8]: First argument (after saved EBP and return address)
  • [EBP+12]: Second argument
  • And so on...

Code: Stack Arguments (32-bit)

This example shows _multiply_two taking two arguments from the stack and returning their product in EAX.

section .data
    msg db "Product: ", 0

section .text
    global _start

_multiply_two:
    push ebp          ; Save old EBP
    mov ebp, esp      ; Set new EBP to current ESP

    ; Arguments are at [ebp+8] and [ebp+12]
    mov eax, [ebp+8]  ; Get first argument
    imul dword [ebp+12] ; Multiply by second argument

    mov esp, ebp      ; Restore ESP (deallocate local vars if any)
    pop ebp           ; Restore old EBP
    ret 8             ; Return, and pop 8 bytes (2 args * 4 bytes) from stack

_start:
    ; Push arguments in reverse order
    push dword 4      ; Second argument
    push dword 6      ; First argument
    call _multiply_two ; Call the procedure

    ; EAX now holds the product (24)
    ; --- Print result (for demonstration) ---
    ; Convert EAX to ASCII (simple example)
    add eax, '0'  ; Convert 24 to ASCII (this won't work for 24, but for single digit results it does)
    mov [result_char], al

    ; (Code to print 'Product: ' and result_char, and newline omitted for brevity, similar to previous example)
    ; In a real scenario, you'd convert multi-digit numbers properly.

    ; Exit program
    mov eax, 1    ; sys_exit
    xor ebx, ebx  ; exit code 0
    int 0x80

section .bss
    result_char resb 1
    newline resb 1

Returning Values (Registers)

Just like arguments, return values are most commonly passed back in registers for simplicity and speed.

  • For 32-bit x86, the EAX register is typically used for integer return values.
  • For 64-bit x86, RAX is used.
  • Floating-point values might use ST(0) (x87 FPU) or XMM0 (SSE).

If a procedure has no explicit return value, EAX/RAX might still contain leftover data, so don't rely on it.

Returning Values (Stack/Memory)

What if you need to return a large structure or an array?

  • Memory Pointer: The caller might pass a pointer to a memory location where the procedure should store its result.
  • Stack: Less common for simple types, but complex structures could theoretically be built on the stack by the called procedure and then accessed by the caller.

For most basic cases, stick to registers for return values.

Check Your Knowledge

Consider a 32-bit x86 assembly procedure designed to take two integer arguments and return their sum. If the arguments are pushed onto the stack, and the return value is placed in EAX, what is the correct instruction to return from the procedure and clean up the stack?

在交互式课程中回答

Recap: Arguments & Returns

In this lesson, we explored how procedures communicate by passing data:

  • Arguments are inputs, passed via registers (for few) or the stack (for many).
  • Return values are outputs, typically placed in a designated register (like EAX/RAX).
  • Calling conventions provide rules for this data exchange.

Mastering these concepts is key to writing robust assembly programs!

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此课程中的所有课时

  1. 调用栈基础
  2. 定义与调用过程
  3. 传递参数与返回值
  4. 栈帧与局部变量
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