Calling Conventions

Cheatsheet

Platform	Return Value	Parameter Registers	Additional Parameters	Stack Alignment	Scratch Registers	Preserved Registers	Call List
System V i386	eax, edx	none	stack (right to left)1		eax, ecx, edx	ebx, esi, edi, ebp, esp	ebp
System V X86_642	rax, rdx	rdi, rsi, rdx, rcx, r8, r9	stack (right to left)1	16-byte at call3	rax, rdi, rsi, rdx, rcx, r8, r9, r10, r11	rbx, rsp, rbp, r12, r13, r14, r15	rbp
Microsoft x64	rax	rcx, rdx, r8, r9	stack (right to left)1	16-byte at call3	rax, rcx, rdx, r8, r9, r10, r11	rbx, rdi, rsi, rsp, rbp, r12, r13, r14, r15	rbp
ARM (32-bit)	r0, r1	r0, r1, r2, r3	stack	8 byte4	r0, r1, r2, r3, r12	r4, r5, r6, r7, r8, r9, r10, r11, r13, r14

System V i386

• Parametres are pushed to stack from right to left
  • Caller cleans the stack
  • Callee is free to modify parametres
    • Caller must not assume they stayed the same!
• Scratch registers are EAX, ECX and EDX
  • Caller must preserve if needed
• Function preserves EBX, ESI, EDI and EBP
• Return value in EAX
  • If 64bit value: higher 32bits in EDX
• Functions push ebp such that the caller-return-eip is 4 bytes above it, and set ebp to the address of the saved ebp.
  • Allows iterating through the existing stack frames.
  • Can be eliminated by specifying the -fomit-frame-pointer GCC option.

CDECL

Caller's responsibilities - Push parameters in reverse order (last parameter pushed first) - Perform the call - Pop the parameters, use them, or simply increment ESP to remove them (stack clearing) - The return value is stored in EAX

Callee's responsibilities (callee is the routine being called) - Store caller's EBP on the stack - Save current ESP in EBP - Code, storing local data on the stack - For a fast exit load the old ESP from EBP, else pop local data elements - Pop the old EBP and return – store return value in EAX

It looks like this in assembly (NASM)

SECTION .text

caller:

    ; ...

    ; Caller responsibilities:
    PUSH  3         ; push the parameters in reverse order
    PUSH  2
    CALL  callee    ; perform the call
    ADD   ESP, 8    ; stack cleaning (remove the 2 words)

    ; ... Use the return value in EAX ...


callee:

    ; Callee responsibilities:
    PUSH  EBP       ; store caller's EBP
    MOV   EBP, ESP  ; save current stack pointer in EBP

    ; ... Code, store return value in EAX ...

    ; Callee responsibilities:
    MOV   ESP, EBP  ; remove an unknown number of local data elements
    POP   EBP       ; restore caller's EBP
    RET             ; return

Sources

• https://wiki.osdev.org/Stack
• https://wiki.osdev.org/Calling_Conventions
• https://wiki.osdev.org/System_V_ABI

TODO: Check if SysV ABI and CDECL are really the same thing?