goas - a port of GNU Assembler written in go
goas is an assembler that behaves like as, GNU Assembler.
This is just a toy program to illustrate how an assembler works. Actually I learned how an assembler works by writing this program :).
It does not mean to support all syntax or instructions, but Linux x86-64 AT&T syntax only.
However, for any input it supports, it behaves exactly the same as as,
which means it produces the very same binary files (*.o) as as does.
The most interesting thing is that it can assemble my Go compiler babygo. (You can see it by running make babygo.)
Requirements
You need a linux with gcc installed.
If you are using MacOS or Windows, you can use my docker image to run goas.
docker run --rm -it -v `pwd`:/mnt/goas -w /mnt/goas dqneo/ubuntu-build-essential:go bash
How to build
$ go build
How to use
Prepare a small assembly file test.s
.text
.global _start
_start:
movq $42, %rdi # status
movq $60, %rax # sys_exit
syscallAnd you can assemble it
$ ./goas -o test.o test.s
$ ld -o test test.o
$ ./test; echo $?
42
Demo
Supported Instructions
See test files under /t and /t2 directory to know what syntax it can assemble.
Design
goas is composed of 4 files.
| File | Role |
|---|---|
| parser.go | parser |
| encoder.go | instruction encoder |
| elf_writer.go | ELF format writer |
| main.go | miscellaneous tasks |
Parser
parser.go is a simple recursive descent parser. The boundary between lexer nd parser are not clearly separated.
Each line of source code is converted into a statement object.
It produces a list of statements in the end.
Instruction encoder
encoder.go translates an instruction with operands into a piece of x86-64 binary machine code.
ELF format Writer
elf_writer.go composes an object which represents ELF file format and write it into a binary object file.
Test
$ make test
References
ELF
- https://refspecs.linuxfoundation.org/elf/elf.pdf ELF Specification
- https://man7.org/linux/man-pages/man5/elf.5.html ELF man
- https://sourceware.org/git/?p=glibc.git;a=blob;f=elf/elf.h;h=4738dfa28f6549fc11654996a15659dc8007e686;hb=HEAD libc elf.h
GNU Asssembler
- https://sourceware.org/binutils/docs-2.37/as.html Manual of GNU Assembler
X86-64 Instruction set and encoding
- https://software.intel.com/content/www/us/en/develop/articles/intel-sdm.html Intel® 64 and IA-32 Architectures Software Developer Manuals
- https://hikalium.github.io/opv86/ Opcode/Instruction finder for x86_64
License
MIT