# Retro Remote Debug Controller — HTTP Control Contract

**Contract version: 0.1.0** (semver; clients assert on the MAJOR).

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 (later) 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.

```json

{

  "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):

```json

{ "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` / `POST /reset`  *(reserved for 0.2)*

Input injection. Proposed shape — `POST /key` body

`{ "code": <keycode>, "down": true|false }` or `{ "text": "..." }`; `/reset`

performs a cold reset. Not required in 0.1, but reserve the paths. Injection

plus `/step` is what retires scripted-autoexec input.

## 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

- Flag: `--control-port <N>`.

- Address space: eZ80 24-bit, flat 16 MB (`addr` up to `0xFFFFFF`); `bank`

  ignored. Program load base `0x040000`.

- Registers (ADL): `af bc de hl ix iy sp pc mbase` + ADL-mode flag.

- Framebuffer: from `vgabuf` (the VDP output), server swizzles to PPM RGB.

### NEC PC-FX — `mednafen` (Doomsday One fork)

- **Adoption:** vendors the shared core as a git submodule

  (`src/extern/retro-remote-debug-controller`), built in with `./configure

  --enable-rrdc`. Dispatched per running console, so PC Engine and Saturn can

  reuse the same core once their backends land.

- Control port: the `MEDNAFEN_CONTROLPORT=<N>` environment variable (Mednafen

  exposes no free-form CLI flag for a driver add-on). Unset ⇒ server off.

- Address space: V810, flat **2 MB** main RAM (`0x000000–0x1FFFFF`); `addr`

  wraps in that space and `bank` is ignored.

- Registers: `pc` + `r0`–`r31` (33 × 32-bit), from the V810 core.

- Framebuffer: the active display rect (typically 256×240), decoded to RGB888

  by the fork with the surface's channel shifts, so it is correct regardless of

  Mednafen's display-dependent pixel order.

- Audio: stereo, at Mednafen's configured output rate (`espec.SoundRate`),

  teed from the per-frame `SoundBuf`.

- Input: the PC-FX has a gamepad, not a keyboard, so `/key?text=<c>` and

  `/key?code=<n>` are both read as a character over a WASD-style pad map —

  `w/a/s/d` = up/left/down/right, space or Enter = RUN (start), `c` = SELECT,

  `1`–`6` = buttons I–VI. A held button is re-asserted each frame (the frontend

  rewrites the pad from physical input every frame), so holds persist across

  `/step`. `/reset` power-cycles the machine.

---

## Reference implementations in this repo

- `core/retro_control.h` — the portable backend vtable each fork implements.

- `core/retro_control.c` — the shared server (socket loop, HTTP parse, PPM/JSON

  encode, command marshalling). Forks vendor/submodule this and call

  `retro_control_start(port, &backend)`. *(server .c lands next.)*

- `client/emu_control.py` — the shared pytest client.

- `conformance/` — tests an emulator runs to prove it conforms. *(next.)*