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# Motorola 68000 Disassembler
> Disassemble into what `as` can assemble back
This project aims to be a disassembler that is capable to produce assembly code
that GNU AS will translate into the same original machine code. It's only use
case for now is Sega Mega Drive / Genesis ROM hacking. I failed to find any way
to disassemble SMD ROMs in such a way that it would be possible to assemble it
back with GNU AS. All disassemblers I tried produce either syntactically
incompatible asembly listing, or it is not the same as original binary after
translation into machine code. So I decided to build my own disassembler, that
will do exactly what I need with full control over the process and maybe more.
Goals of this Motorola 68000 disassembler project in this particular repo:
- Support all Motorola 68000 ISA instructions.
- Flawless compatibility with GNU AS syntax. It should always emit the code on
which GNU AS produces absolutely identical binary (with or without linkage)
without errors or warnings, unless some peculiar flags has been specified.
- Support PC trace tables. With trace tables it will disassemble traced PC
locations only, without attempt to disassemble everything, because not
everything is instruction, some code is just data.
- Mark jump locations and bind jumps and calls to them. Mark obvious ROM read
accessed locations and bind the instructions to the marked locations. To make
it possible to split and reorganize the binary.
What could become a goal (possible features):
- Other Motorola 680x0 instruction sets support, including ColdFire.
- Functions and function boundaries detection.
- Static analysis of call graph of branches and subroutine calls.
- PC trace aided static analysis of dynamic branches and subroutine calls.
- Base address other than `0x00000000`. It is only zero for now because it is
sufficient for Sega Mega Drive / Genesis ROM images.
- Support for more than 4MiB of code size.
- Sparse address space support (instead of single continuous 4MiB block that
starts at `0x00000000` offset).
- Other assembler syntaxes (e.g. ASM68K.EXE).
- Expose a library API.
What is **not** the goal (at least not in this repo):
- Decompilation into some high level language like C or C++.
- Other instruction set architectures support like MIPS, x86, amd64, ARM,
PowerPC and so on.
## Current limitations
- Motorola 68000 only supported. That means no 68010 support or any 680x0,
except 68000. No ColdFire support.
- Code must be aligned to 2 bytes boundary. All PC trace values have to be
dividable by 2 without remaining. That limitation has been put into design
from the beginning, because 68000 cannot perform unaligned `word` and `long`
memory access, including instruction fetch. This is not true for some of 680x0
variations.
- Base address is always assumed to be `0x00000000`.
- Maximum binary size is 4MiB.
## Build
```
cmake -B cmake-build -S .
cmake --build cmake-build
```
## Usage example
```
./cmake-build/m68k-disasm -t pc-trace.txt -o disasm.S original.bin
```
To get detailed help you can run:
```
./cmake-build/m68k-disasm -h`
```
## Meta
Authors:
- Vladimir Novikov – oxore@protonmail.com
This is free and unencumbered software released into the public domain. See
``UNLICENSE`` for more information.
This repository includes source code of other projects:
- Optparse ([gh:skeeto/optparse](https://github.com/skeeto/optparse)) - Unlicense
Resources used to implement the disassember (this set is more than sufficient to
support all of M68000 instructions):
- [The 68000's Instruction Set](http://wpage.unina.it/rcanonic/didattica/ce1/docs/68000.pdf) - Appendix of an unrecognized book. Basically the same information also could be found in [gh:prb28/m68k-instructions-documentation](https://github.com/prb28/m68k-instructions-documentation).
- [Motorola 68000 CPU Opcodes](http://goldencrystal.free.fr/M68kOpcodes-v2.3.pdf).
- GNU assembler (GNU Binutils) 2.40 (`m68k-none-elf-as`).
- GNU objdump (GNU Binutils) 2.40 (`m68k-none-elf-objdump`).
## Contributing
Coming soon.
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