Retro Remote Debug Controller — HTTP Control Contract

Contract version: 0.2.0 (semver; clients assert on the MAJOR). 0.2 adds input injection (POST /key, POST /reset); it is purely additive, so 0.1 clients keep working and servers advertise their level via /status.contract.

A minimal, portable HTTP API that a retro-platform emulator exposes so an external harness can screenshot the screen, read memory and registers, step the machine deterministically, and inject input. The same contract is implemented by the forked FAB Agon Emulator, the Neo6502 emulator, and the Commander X16 emulator, so one pytest harness drives all three ports.

Principles

• Opt-in. The server is OFF by default and starts only when the emulator is launched with a control-port flag. Zero cost to normal use.
• Localhost only. The server MUST bind 127.0.0.1 (never 0.0.0.0). /mem and input injection are a memory-read / remote-control surface; they do not leave the machine.
• The contract is the HTTP surface, not the flag. Each emulator names its launch flag idiomatically (x16emu: -controlport N; FAB: --control-port N). Only the endpoints below are normative.
• Determinism is a first-class requirement. These are frame-exact ports. Every state read (/mem, /regs, /screenshot) MUST observe a consistent machine state — captured at an instruction/frame boundary on the emulator thread, never a torn mid-instruction read from the HTTP thread. See Consistency below.

Conventions

• JSON responses are application/json; charset=utf-8.
• Binary responses use the stated content-type; no chunked encoding required.
• Errors: HTTP 4xx/5xx with body {"error":"<message>"}.
• Numbers in JSON are decimal unless a field is documented as a hex string.
• Methods are exactly as listed (GET/POST). Unknown path → 404. Unknown method on a known path → 405.

Endpoints

GET /status

Liveness + contract negotiation. Always available.

{
  "contract": "0.1.0",
  "emulator": "x16emu",
  "platform": "x16",
  "frame": 12345,
  "paused": true,
  "running": true
}

• contract — this document's version. Clients MUST reject a mismatched MAJOR.
• platform — one of x16, neo6502, agon (extensible). Tells the client the memory/register model (see Per-platform appendix).
• frame — monotonic completed-frame counter since boot. The determinism anchor: a screenshot taken at frame == N shows the state after N complete frames.
• paused — true if the machine is halted (only advances via /step).

GET /screenshot

The live screen as a Portable Pixmap.

• Response: image/x-portable-pixmap, binary PPM P6, 8-bit RGB, no alpha.
• Pixels are the most recent COMPLETE frame (never a half-scanned frame). The emulator keeps a snapshot the render thread fills at frame end; the HTTP thread serves that snapshot.
• Dimensions are the platform's native framebuffer (appendix). Clients read width/height from the PPM header — do not hardcode.
• Reserved: ?frame=N (block until frame N) — not required in 0.1.

GET /mem?addr=<a>&len=<n>[&bank=<b>]

Raw memory bytes.

• addr — start address, decimal or 0x-hex. Interpreted in the platform's address space (appendix).
• len — byte count, decimal. Servers MAY cap len (recommend ≥ 65536); a capped request returns what it can with Content-Length telling the truth.
• bank — OPTIONAL bank selector for banked platforms (X16). Omitted → the machine's currently-selected bank. Ignored by flat-memory platforms.
• Response: application/octet-stream, exactly the requested bytes.
• Reads go through the emulator's debug-read path (no side effects, no I/O triggers, no access-flag mutation).

GET /regs

CPU registers as JSON. Keys are platform-specific (appendix); values are decimal unless suffixed _hex. Example (x16 / 65C02):

{ "a":0, "x":16, "y":2, "sp":505, "pc":49152, "status":32,
  "ram_bank":1, "rom_bank":0 }

POST /step?frames=<n> (recommended; required for frame-exact tests)

Advance the machine by exactly n complete frames (default 1), then halt.

• Only meaningful when the machine is paused (see /pause). On a free-running machine, servers SHOULD pause, run n frames, and stay paused.
• Response: { "frame": <new frame counter> }.
• POST /step?cycles=<n> MAY be offered for sub-frame stepping.

POST /pause / POST /resume (recommended)

Halt / free-run. { "paused": true|false }. A machine launched with a --control-pause-style option SHOULD boot paused so tests are deterministic from frame 0.

POST /key?text=<char> | ?code=<n> [&down=0|1] (0.2)

Inject a key. Query params (like /mem and /step), no body:

• text — a single character, mapped to the platform's keyboard.
• code — a raw platform key code (e.g. fabgl virtual key on Agon, an SDL/PS2 code on others). text and code are alternatives; provide one.
• down — 1 presses (holds), 0 releases; omitted taps (press then release). Holding across /step frames is how a program that samples input each frame (e.g. a game) actually sees the key.
• Response: { "injected": true }. Missing/unmapped key → 400.

Injection plus /step retires scripted-autoexec input: press → step → assert → release, deterministically.

POST /reset (0.2)

Soft/cold reset the machine. Response { "reset": true }. The frame counter MAY reset here (the one case /status.frame is allowed to go backwards).

Consistency (normative)

• /mem, /regs, and /step MUST be serviced on the emulator thread at an instruction boundary — e.g. the HTTP handler enqueues a command that the main loop drains between instructions/frames and signals back. The HTTP thread MUST NOT read CPU/RAM state directly while the CPU is executing.
• /screenshot MUST serve a complete-frame snapshot (double-buffer: render thread writes the back buffer, swaps under a lock at frame end; HTTP thread reads the front buffer).
• frame in /status MUST increment once per completed video frame and never go backwards (except across /reset).

Versioning

• Semantic. MAJOR bump = breaking change to an existing endpoint's request/response. MINOR = additive (new endpoint / optional field). PATCH = clarification.
• Clients read /status.contract and fail fast on a MAJOR mismatch.

Per-platform appendix

Commander X16 — x16emu

• Flag: -controlport <N> (single-dash, matching x16emu's -dump style).
• Framebuffer: 640×480, native order BGRA8888 (framebuffer[i*4+2]=R, +1=G, +0=B); the server swizzles to PPM RGB.
• Address space: 16-bit CPU (0x0000–0xFFFF). The banked window 0xA000–0xBFFF is selected by bank → the emulator's read6502(addr, bank) / banked RAM (BRAM). addr < 0xA000 or ≥ 0xC000 ignores bank.
• Registers: a x y (8), sp pc (16), status (8), plus ram_bank, rom_bank. Source: the regs struct + memory_get_ram_bank/rom_bank().
• Frame source: video.c frame_count.

Neo6502 — neo6502 emulator

• Flag: --control-port <N>.
• Address space: flat 64 KB; bank ignored.
• Registers: 6502 a x y sp pc status.
• Framebuffer: native resolution per the emulator's video mode.

Agon (Console8 / Light 2) — FAB Agon Emulator

• Adoption: conform-only (Rust emulator — keeps its own HTTP server, matches this contract rather than vendoring the C core).
• Flag: --control-port <N>.
• Address space: eZ80 24-bit, flat 16 MB (addr up to 0xFFFFFF); bank tolerated and ignored. Program load base 0x040000.
• Registers (ADL) — /regs example (16-bit regs; mbase 8-bit; adl 0/1):

  { "af":0, "bc":0, "de":0, "hl":0, "ix":0, "iy":0,
    "sp":0, "pc":262144, "mbase":0, "adl":1 }
  

• Framebuffer: from vgabuf (the VDP output); the offscreen path already keeps video alive (the FAB equivalent of x16's SDL_VIDEODRIVER=dummy).
• Input: /key?text=c maps the char to a fabgl virtual key (ascii2vk) and delivers it via sendVKeyEventToFabgl; ?code=<vk> passes a raw fabgl vkey. The control thread queues events; the render thread delivers them (same thread as normal keyboard input). /reset triggers the soft-reset atomic.
• Pointer: /pointer feeds PS/2 mouse packets through sendHostMouseEventToFabgl — the same device path host mouse motion uses, so it drives the real fabgl/VDP mouse rather than a shortcut. PS/2 motion is relative, so the control server tracks the absolute position itself: x/y become a delta from that tracked point (split across packets past the ±127 int9 range), dx/dy apply directly, and GET /pointer reports it exactly. Coordinates are screen space (x right, y down; y is negated into the PS/2 up-positive convention). buttons bit0/bit1 map to the PS/2 left/right (primary/secondary) button bits. Like /key, packets are queued by the control thread and delivered by the render thread at the frame boundary, so pause → pointer → step(1) → screenshot is reproducible.

Adoption paths

An emulator conforms by exposing exactly the HTTP surface above — nothing else is required. Two ways to get there:

• Vendor the shared core (core/retro_control.c + .h) — for C/C++ emulators. Implement the four backend callbacks, wire three loop hooks, call retro_control_start(port, &backend); the socket loop, HTTP parsing, PPM/JSON encoding and pause/step control come for free. The Commander X16 fork does this.
• Conform directly — for emulators in other languages (the FAB Agon emulator is Rust). Keep your own HTTP server; just match the contract and pass the conformance suite. Sharing the C core is an optimization, not a requirement — the contract is the asset.

Either way, conformance/ is the arbiter: pass it and you conform.

In this repo

• core/retro_control.{c,h} — portable server + backend vtable (vendor path).
• client/emu_control.py — the shared pytest client.
• conformance/ — a platform-agnostic pytest that proves an emulator conforms.