78 lines
4.4 KiB
Markdown
78 lines
4.4 KiB
Markdown
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# Calling Conventions
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## Cheatsheet
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| Platform | Return Value | Parameter Registers | Additional Parameters | Stack Alignment | Scratch Registers | Preserved Registers | Call List |
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| -------------------------------------------------------------------- | ------------ | -------------------------- | -------------------------------------------------------------------------- | -------------------------------------------------------------------- | ----------------------------------------- | ------------------------------------------- | --------- |
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| 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 |
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| 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 |
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| 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 |
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| 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 | |
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## System V i386
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- Parametres are pushed to stack from right to left
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- Caller cleans the stack
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- Callee is free to modify parametres
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- Caller must not assume they stayed the same!
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- Scratch registers are `EAX`, `ECX` and `EDX`
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- Caller must preserve if needed
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- Function preserves `EBX`, `ESI`, `EDI` and `EBP`
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- Return value in `EAX`
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- If 64bit value: higher 32bits in `EDX`
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- Functions push `ebp` such that the caller-return-eip is 4 bytes above it, and set `ebp` to the address of the saved ebp.
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- Allows iterating through the existing stack frames.
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- Can be eliminated by specifying the -fomit-frame-pointer GCC option.
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## CDECL
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**Caller's responsibilities**
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- Push parameters in reverse order (last parameter pushed first)
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- Perform the call
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- Pop the parameters, use them, or simply increment `ESP` to remove them (stack clearing)
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- The return value is stored in `EAX`
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**Callee's responsibilities (callee is the routine being called)**
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- Store caller's `EBP` on the stack
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- Save current `ESP` in `EBP`
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- Code, storing local data on the stack
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- For a fast exit load the old `ESP` from `EBP`, else pop local data elements
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- Pop the old `EBP` and return – store return value in `EAX`
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**It looks like this in assembly (NASM)**
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``` nasm
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SECTION .text
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caller:
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; ...
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; Caller responsibilities:
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PUSH 3 ; push the parameters in reverse order
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PUSH 2
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CALL callee ; perform the call
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ADD ESP, 8 ; stack cleaning (remove the 2 words)
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; ... Use the return value in EAX ...
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callee:
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; Callee responsibilities:
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PUSH EBP ; store caller's EBP
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MOV EBP, ESP ; save current stack pointer in EBP
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; ... Code, store return value in EAX ...
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; Callee responsibilities:
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MOV ESP, EBP ; remove an unknown number of local data elements
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POP EBP ; restore caller's EBP
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RET ; return
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```
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## Sources
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- [https://wiki.osdev.org/Stack](https://wiki.osdev.org/Stack)
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- [https://wiki.osdev.org/Calling_Conventions](https://wiki.osdev.org/Calling_Conventions)
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- [https://wiki.osdev.org/System_V_ABI](https://wiki.osdev.org/System_V_ABI)
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**TODO:** Check if SysV ABI and CDECL are really the same thing?
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