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157 lines
4 KiB
Markdown
157 lines
4 KiB
Markdown
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[← Home](../../README.md) · [Reverse Engineering](../README.md)
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# Compiler-Specific Code Generation Patterns
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## Overview
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Different Amiga compilers produce distinct code signatures. Recognising these helps quickly identify compiler origin, locate `main()`, and distinguish OS glue from application logic.
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---
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## SAS/C 6.x Patterns
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### Function Prologue / Epilogue
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```asm
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; Non-leaf function with local vars:
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LINK A5, #-N ; allocate N bytes of locals on stack
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MOVEM.L D2-D7/A2-A3, -(SP) ; save preserved registers
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...
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MOVEM.L (SP)+, D2-D7/A2-A3
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UNLK A5
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RTS
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; Leaf function (no locals, no preserved regs):
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; — no LINK, pure computation, ends in RTS
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```
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### D0 Save Pattern
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SAS/C saves D0 at the start of functions that need it later:
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```asm
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MOVE.L D0, -(SP) ; save return value from previous call
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JSR another_func
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MOVE.L (SP)+, D0 ; restore
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```
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### Register Argument Passing
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SAS/C passes OS call args via `#pragma amicall` register placement. Inside application functions, SAS/C uses a **stack-based C ABI** (unlike OS calls):
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```asm
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; C function call in SAS/C: push args right-to-left
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MOVE.L arg3, -(SP)
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MOVE.L arg2, -(SP)
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MOVE.L arg1, -(SP)
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JSR _myfunction
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ADDQ.L #12, SP ; clean args (caller cleanup)
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```
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### String Constants
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SAS/C places string literals in the **data hunk**, referenced via absolute addresses requiring `HUNK_RELOC32`:
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```asm
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MOVE.L #_str_hello, D1 ; absolute address → RELOC32 entry
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MOVEA.L _DOSBase, A6
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JSR (-48,A6) ; Write(stdout, "hello", ...)
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```
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---
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## GCC (m68k-amigaos / bebbo) Patterns
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### PC-Relative String Access
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GCC uses PC-relative addressing by default, eliminating most HUNK_RELOC32 entries:
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```asm
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LEA _str_hello(PC), A0 ; PC-relative — no reloc needed
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```
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### No Frame Pointer (Default)
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```asm
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; GCC -O2 leaf function:
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MOVEM.L D2/A2, -(SP) ; only save what's used
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...
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MOVEM.L (SP)+, D2/A2
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RTS
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; No LINK/UNLK — pure register allocation
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```
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### GCC Function Prologues
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```asm
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; Non-leaf with GCC -fno-omit-frame-pointer:
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LINK A6, #-N ; note: GCC uses A6 as frame pointer here
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; (conflicts with OS library base usage — rare)
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; More common with -O2:
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SUBQ.L #N, SP ; allocate locals without frame pointer
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```
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### Integer Division / Modulo
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GCC emits calls to `__divsi3`, `__modsi3` from `libgcc`:
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```asm
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JSR ___divsi3 ; 32-bit signed divide (libgcc helper)
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; operands in D0:D1, result in D0
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```
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SAS/C uses the 68k `DIVS.L` instruction directly (available on 020+) or `DIVS.W`.
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---
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## VBCC Patterns
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VBCC generates very tight code with minimal function overhead:
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```asm
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; VBCC typical function (no frame pointer, minimal saves):
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MOVEM.L D2-D4, -(SP)
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...
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MOVEM.L (SP)+, D2-D4
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RTS
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```
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VBCC's OS call inline expansion looks identical to GCC's inline-asm stubs.
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---
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## Distinguishing Compiler Artefacts from Logic
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| Pattern | Compiler | Meaning |
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|---|---|---|
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| `LINK A5, #-N` | SAS/C | Function with locals |
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| `LINK A6, #-N` | GCC (rare) | Frame pointer mode |
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| `JSR ___divsi3` | GCC | Software 32-bit division |
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| `DIVS.L D1, D0` | SAS/C (020+) | Hardware divide |
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| `MULS.L D1, D0` | SAS/C (020+) | Hardware multiply |
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| `LEA str(PC), A0` | GCC | PC-relative string ref |
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| `MOVE.L #_str, D1` | SAS/C | Absolute string ref (reloc'd) |
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| `JSR _main` | Startup | C main() entry point |
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| `MOVE.L 4.W, A6` | Startup | SysBase load |
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| `JSR -552(A6)` | Any | exec.library OpenLibrary |
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---
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## Locating `main()` via Startup Skip
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After identifying the startup stub (`MOVE.L 4.W, A6` → `JSR _OpenLibraries`):
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1. Find the first `JSR` or `BSR` after library opens
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2. That target is `__main` or directly `_main`
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3. If `__main`: follow its internal `JSR _main` call
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4. Label the target `main` in IDA
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---
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## References
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- SAS/C 6.x manual — code generation chapter
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- GCC for m68k: https://github.com/bebbo/amiga-gcc
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- VBCC manual: http://www.compilers.de/vbcc.html
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- *Amiga ROM Kernel Reference Manual: Libraries* — register conventions
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