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afs-as

(noun): a barrel of shifted registers

Standalone ARM64 assembler for macOS. Reads .s assembly text, encodes ARM64 instructions, and emits Mach-O object files linkable with Apple's ld.

Part of ARMFORTAS, a bespoke ARM64 Fortran compiler.

Usage

# Assemble
afs-as hello.s -o hello.o

# Link and run
ld hello.o -o hello -lSystem -syslibroot $(xcrun --show-sdk-path) -e _main
./hello

Library API

use afs_as::assemble;
use afs_as::encode::Inst;
use afs_as::reg::*;

// From source text
let obj = assemble::assemble_source(".global _main\n_main:\nret\n").unwrap();

// From pre-built instructions (no parsing)
let obj = assemble::assemble_instructions(
    &[Inst::Ret { rn: X30 }],
    &["_main"],
);

Instruction Coverage

  • Data processing: ADD, SUB, MUL, SDIV, UDIV (register + immediate, 32/64-bit)
  • Logic: AND, ORR, EOR, ANDS
  • Shifts: LSL, LSR, ASR (immediate)
  • Moves: MOV, MOVZ, MOVK, MOVN
  • Compare: CMP, CMN, TST (aliases)
  • Branches: B, BL, B.cond (all 16 codes), CBZ, CBNZ, RET, BR, BLR
  • Address: ADRP (with PAGE/PAGEOFF relocations)
  • Load/Store: LDR, STR (64/32-bit, unsigned/pre/post-index), LDRB, LDRH, LDRSW, LDP, STP (all addressing modes)
  • FP: FADD, FSUB, FMUL, FDIV, FNEG, FABS, FSQRT, FCMP, FMADD (single + double)
  • FP conversion: FCVTZS, SCVTF, FMOV
  • System: SVC, NOP, BRK

Tests

365 tests across four levels:

Suite Count What it validates
Unit (encoding) 217 Each instruction encodes to the correct 4-byte value (hardcoded ground truth)
Round-trip 88 Parse text, encode, compare byte-for-byte against Apple as
System validation 55 Shell out to Apple as, compare our encoding against theirs
End-to-end 5 Assemble, link with ld, run binary, verify output 
cargo test -p afs-as

Building

cargo build -p afs-as          # build
cargo test -p afs-as           # test
cargo clippy -p afs-as         # lint

Requires macOS on ARM64 (Apple Silicon) for integration tests that invoke the system assembler and linker.

License

GPL-3.0