Assemblersky

x86-64 assembly decoding the Bluesky firehose

CBOR → CAR → DAG-CBOR → post text. No runtime. No abstractions. Just registers and the protocol.

Part of the Fortransky ecosystem — an AT Protocol client written in Fortran.

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What it does

The AT Protocol relay (com.atproto.sync.subscribeRepos) speaks binary WebSocket frames. Each frame contains three nested binary formats before you reach a post:

Raw WebSocket frame
  └─ CBOR envelope     {op, t, seq, repo, rev, ops, blocks}
       └─ CARv1 block store
            └─ DAG-CBOR record   {$type, text, createdAt, ...}

Assemblersky decodes all of it in x86-64 NASM assembly — four functions, four stages, no allocator, no garbage collector, no language runtime:

asb_decode_envelope()     CBOR envelope → seq, repo, rev, ops slice, blocks
asb_find_create_post_op() ops array → collection, rkey, CID
asb_car_find_block()      CARv1 → record block bytes
asb_extract_post_record() DAG-CBOR → $type, text, createdAt

A thin Rust harness wraps the assembly via a C ABI and emits normalized NDJSON — the same schema as Fortransky's Rust firehose bridge, so both decoders are interchangeable.

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Architecture

assemblersky-harness (Rust CLI)
  └─ ffi.rs           Rust FFI bindings → C ABI
       └─ assemblersky_exports.c   C glue
            └─ cbor_scan.asm       asb_decode_envelope
            └─ car_scan.asm        asb_car_find_block
            └─ post_extract.asm    asb_find_create_post_op + asb_extract_post_record
            └─ util.asm            asb_mem_eq

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SysV ABI discipline

The assembly is strict about the x86-64 SysV calling convention. Getting this wrong is the main class of bug in assembly code that calls other assembly:

• Callee-save registers (rbx, r12–r15, rbp) — pushed on entry, popped on exit for every function
• Caller-save registers (rsi, rdi, rcx, rdx, r8, r9) — reloaded from callee-save registers before every call that needs them; cannot be assumed to survive across a call
• rsi (end pointer) — must be reloaded before every helper call since nearly all helpers use it as a bounds limit
• Output pointer arguments (r8, r9)** — saved to the stack on entry since _decode_varint and similar helpers clobber both

The bring-up debugging session that fixed these is documented in the git history.

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Build dependencies

• NASM (sudo apt install nasm / sudo pacman -S nasm)
• Rust toolchain >= 1.78 (rustup or distro package)
• C compiler (gcc or clang, via the cc crate)

Build

make

Or manually:

cd rust-harness
ASB_ROOT=$(pwd)/.. cargo build --release

Test against fixture

make test

Expected output:

{
    "kind": "commit",
    "op": "create",
    "collection": "app.bsky.feed.post",
    "seq": 26653242501,
    "repo": "did:plc:fortranskyfixture000000000000",
    "rev": "3lmfixture-rev",
    "rkey": "3lmfixturepost",
    "record": {
        "$type": "app.bsky.feed.post",
        "text": "Synthetic raw relay commit fixture: hello from Fortransky.",
        "created_at": "2026-03-19T00:00:00.000Z"
    }
}

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Integrated with Fortransky already, but if you ever accidently delete it :)

mkdir -p ../fortransky/bridge/assemblersky/bin
cp rust-harness/target/release/assemblersky-harness \
   ../fortransky/bridge/assemblersky/bin/assemblersky_cli

Fortransky's relay_raw_tail.py auto-detects assemblersky_cli and prefers it over the Rust firehose bridge. Detection order:

1. FORTRANSKY_RELAY_DECODER env var
2. FORTRANSKY_ASSEMBLERSKY_DECODER env var
3. bridge/assemblersky/bin/assemblersky_cli (bundled)
4. assemblersky_cli on PATH
5. Rust firehose_bridge_cli fallback

Check detection: ./scripts/check_assemblersky.sh

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Scope

• #commit frames only
• create ops only
• app.bsky.feed.post collection only
• Definite-length CBOR subset (indefinite-length items fail gracefully)
• Linux x86-64 only (SysV ABI)

The CID in commit ops is captured as raw bytes — no semantic CID decode. The CAR block scanner uses a heuristic varint walk to determine CID length.

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Files

asm/
  cbor_scan.asm      asb_decode_envelope — CBOR envelope + body map parser
  car_scan.asm       asb_car_find_block  — CARv1 varint scanner + CID match
  post_extract.asm   asb_find_create_post_op + asb_extract_post_record
  util.asm           asb_mem_eq — bounded memory compare

include/
  assemblersky.h     C ABI header — struct definitions + function prototypes

cshim/
  assemblersky_exports.c   C-visible glue (no logic)

rust-harness/
  src/main.rs        CLI entry point — calls all 4 stages, emits NDJSON
  src/ffi.rs         Rust FFI bindings
  src/normalize.rs   NormalizedEvent + NormalizedRecord output structs
  build.rs           Compiles NASM + C shim via cc crate
  Cargo.toml

tests/
  fixtures/relay_commit_frame.bin    Synthetic raw #commit relay frame
  expected/relay_commit_frame.json   Expected normalized output

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Part of the Former Lab ecosystem

Fortransky  (Fortran TUI Bluesky client)
  └─ relay_raw_tail.py
       └─ assemblersky-harness   ← this project
            └─ x86-64 NASM assembly

Fortransky: https://github.com/FormerLab/fortransky Former Lab: https://bsky.app/profile/formerlab.bsky.social