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Lauren Toivanen 2025-01-24 05:13:48 +02:00 • committed by root
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# 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](https://wiki.osdev.org/Calling_Conventions#Note1) | | eax, ecx, edx | ebx, esi, edi, ebp, esp | ebp |
| System V X86_64[2](https://wiki.osdev.org/Calling_Conventions#Note2) | rax, rdx | rdi, rsi, rdx, rcx, r8, r9 | stack (right to left)[1](https://wiki.osdev.org/Calling_Conventions#Note1) | 16-byte at call[3](https://wiki.osdev.org/Calling_Conventions#Note3) | 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](https://wiki.osdev.org/Calling_Conventions#Note1) | 16-byte at call[3](https://wiki.osdev.org/Calling_Conventions#Note3) | 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 byte[4](https://wiki.osdev.org/Calling_Conventions#Note4) | 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)**
``` 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/Stack)
- [https://wiki.osdev.org/Calling_Conventions](https://wiki.osdev.org/Calling_Conventions)
- [https://wiki.osdev.org/System_V_ABI](https://wiki.osdev.org/System_V_ABI)
**TODO:** Check if SysV ABI and CDECL are really the same thing?