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188 lines
5.6 KiB
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188 lines
5.6 KiB
Markdown
[← Home](../README.md) · [Linking & Libraries](README.md)
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# Compiler Library Call Stubs
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## Overview
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This document explains how each major AmigaOS compiler generates library calls and what the resulting machine code looks like — essential for recognising compiler signatures during reverse engineering.
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---
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## The AmigaOS Calling Convention
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All library calls use the same register-based convention:
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- **A6** = library base pointer
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- Arguments in **A0–A3**, **D0–D7** as specified by the `.fd` file
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- Return value in **D0** (or D0+D1 for 64-bit returns)
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- **Callee** preserves: D2–D7, A2–A6
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- **Caller** may destroy: D0, D1, A0, A1
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The code generated by all compilers follows this — the difference is how they **set up A6** before the `JSR`.
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---
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## SAS/C 6.x
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SAS/C uses **global library base variables** (`_DOSBase`, `_SysBase`, etc.) and generates direct `JSR LVO(A6)` calls. The standard pattern:
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```asm
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; Calling Write(file, buffer, length):
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; D1=file, D2=buffer, D3=length, A6=_DOSBase
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MOVE.L _DOSBase, A6 ; load library base from global
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MOVE.L file, D1 ; arg 1
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MOVE.L buffer, D2 ; arg 2
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MOVE.L length, D3 ; arg 3
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JSR -48(A6) ; Write LVO = -48
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```
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### SAS/C Inline Pragmas
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SAS/C uses `#pragma libcall` to define inline stubs:
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```c
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#pragma libcall DOSBase Write 30 32103
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/* 30 = bias/6 = 8th function (LVO -48)
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32103 = register encoding: D3,D2,D1 → args 3,2,1 */
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```
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The pragma-generated stub wraps the JSR into a C-callable inline function.
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### SAS/C Library Base Globals
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SAS/C's `clib/dos_protos.h` + startup code declares:
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```c
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extern struct DosLibrary *DOSBase;
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extern struct ExecBase *SysBase;
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```
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These are initialized in `c.o` (the startup stub):
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```asm
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_c_start:
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MOVE.L 4.W, A6 ; SysBase from exception vector
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MOVE.L A6, _SysBase ; store in global
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LEA _dosname, A1 ; "dos.library"
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MOVEQ #0, D0 ; version 0
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JSR -552(A6) ; OpenLibrary
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MOVE.L D0, _DOSBase
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```
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---
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## GCC (m68k-amigaos / bebbo GCC)
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GCC uses a different stub mechanism — **inline functions** from `inline/` headers or the `libnix` library:
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```c
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/* inline/exec.h (generated from exec_lib.fd) */
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static __inline APTR
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AllocMem(ULONG byteSize, ULONG requirements)
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{
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register APTR _res __asm("d0");
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register struct ExecBase *const _SysBase __asm("a6") = SysBase;
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register ULONG _byteSize __asm("d0") = byteSize;
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register ULONG _requirements __asm("d1") = requirements;
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__asm volatile ("jsr a6@(-198:W)"
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: "=r"(_res)
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: "r"(_SysBase), "r"(_byteSize), "r"(_requirements)
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: "a0", "a1", "fp0", "fp1", "cc", "memory");
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return _res;
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}
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```
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The `jsr a6@(-198:W)` emits a 4-byte instruction: `JSR -198(A6)` using 16-bit word displacement.
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### GCC Code Pattern
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```asm
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; GCC calling AllocMem(1024, MEMF_CHIP):
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MOVEA.L _SysBase, A6 ; GCC uses same global SysBase
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MOVEQ #$01, D1 ; MEMF_CHIP = 2... actually:
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MOVE.L #$00000400, D0 ; byteSize = 1024
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MOVE.L #$00000002, D1 ; MEMF_CHIP
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JSR -$C6(A6) ; -198 decimal = AllocMem
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MOVE.L D0, _mybuf ; save return value
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```
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### PC-Relative Addressing (GCC Default)
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GCC with `-fpic` or `-fbaserel` generates **PC-relative** accesses:
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```asm
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LEA _SysBase(PC), A0 ; load SysBase address PC-relative
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MOVEA.L (A0), A6 ; dereference to get SysBase value
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JSR -$228(A6) ; OpenLibrary LVO = -552
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```
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This reduces relocation entries and is the default for GCC on AmigaOS.
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---
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## VBCC (vbcc m68k-amigaos)
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VBCC uses `inline.h` style stubs similar to GCC. The calling pattern is identical at the assembly level:
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```asm
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; VBCC compiled library call — indistinguishable from GCC at asm level
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MOVEA.L (_SysBase), A6
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MOVE.L D2, -(SP) ; VBCC may push/pop D2 as scratch
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MOVE.L arg, D0
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JSR -$C6(A6)
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MOVE.L (SP)+, D2
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```
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VBCC is slightly more aggressive about not clobbering D2-D7, matching the calling convention exactly.
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---
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## Recognising Compiler Signatures During RE
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### SAS/C Signature
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```asm
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; Entry prologue:
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LINK A5, #-N ; frame setup (A5 = frame pointer)
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MOVEM.L D2-D7/A2-A4, -(SP) ; save callee-saved regs
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; ...
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MOVEM.L (SP)+, D2-D7/A2-A4 ; restore
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UNLK A5 ; teardown
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RTS
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```
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SAS/C uses `A5` as frame pointer and `LINK/UNLK` for stack frames.
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### GCC Signature
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```asm
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; Entry prologue (GCC without frame pointer):
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MOVEM.L D2/A2, -(SP) ; only saves what it uses
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; ...
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MOVEM.L (SP)+, D2/A2
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RTS
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```
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GCC typically does **not** use LINK/UNLK unless required. It uses `A5` as an additional scratch register and often generates `JSR` through `A0` for indirect calls.
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### VBCC Signature
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VBCC generates very clean, minimal code — similar to GCC but with more consistent callee-save patterns and no GCC-specific idioms (e.g., no `__builtin_` calls).
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---
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## Library Call Pattern Summary
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| Pattern | Compiler | Key Signature |
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|---|---|---|
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| `LINK A5, #-N` + `MOVEM D2-D7/A2-A4` | SAS/C | Frame pointer A5, full reglist |
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| `JSR LVO(A6)` with global `_LibBase` | SAS/C / GCC | Both use global |
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| `jsr a6@(-LVO:W)` (GAS syntax) | GCC inline | 16-bit short displacement |
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| PC-relative `LEA _SysBase(PC)` | GCC -fpic | No absolute refs |
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| No LINK, minimal MOVEM | GCC / VBCC | No frame pointer |
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---
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## References
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- NDK39: `fd/`, `proto/`, `inline/`, `clib/` directories
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- SAS/C 6.x Programmer's Guide — pragma libcall chapter
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- GCC m68k-amigaos port documentation (bebbo/m68k-amigaos-toolchain)
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- VBCC documentation — register calling convention
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- *Amiga ROM Kernel Reference Manual: Libraries* — calling conventions appendix
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