# M4: the live DOSBox seam — design note (2026-07-19)

The last milestone of GPU-RETARGET.md: the decoded renderer running live behind
the game. All gates are now open: geometry = emu860c (351×, checkpoint-perfect),
raster = the GPU tile path (M1/M2 conformant), spec = igc_conformance.

## The loop

```
DOSBox-X (FLYK.EXE etc., unmodified)
  │  port I/O 0x150-0x161 (C012 link)            [VELOCIRENDER_PROTOCOL.md §7]
  ▼
virtual C012 link device                          [NEW — small FSM]
  │  byte stream ⇄ message assembler (§1-§2 framing, verified vs real EXE)
  ▼
protocol server (Python, exists as emu_main)      [boot handshake done]
  │  wire commands -> the firmware receive path
  ▼
emu860c: VREND.MNG geometry stage                 [DONE — real-time 3.5×]
  │  per-draw DMA stream + payload programs (captured live from C memory)
  ▼
igc_gpu frame loop: tile dispatch + readout       [DONE for fixtures]
  │  RGBA frame
  ▼
present:  (a) vr_readpixels reply -> host SVGA    [FLYK readback path]
          (b) or the render-bridge window          [existing live_bridge seam]
```

## What exists vs what's new

| piece | status |
|---|---|
| Wire protocol + framing | verified against the shipped EXE (protocol doc §9) |
| Boot handshake (iserver ×3, BTL/MNG swallow) | implemented in emu_main |
| Geometry at real-time | **emu860c** — done |
| DMA-stream capture at draw time | the send_capture pattern — port the w32
  tap into emu860c (a C-side write-watch range on the queue pages, cheap) |
| Tile raster on GPU | igc_gpu / igc_gpu_frame — done for fixtures |
| Texel path (TXDN) | the remaining spec item (M3) — effects/bench render
  without it; texture-mapped content needs it |
| C012 DOSBox device | NEW: six ports + byte FIFO in DOSBox-X (protocol §7
  calls it "bounded and testable in isolation") |
| Present path | reuse render-bridge/live_bridge (frozen renderer window) or
  implement vr_readpixels for FLYK's own SVGA display (§6: "expected") |

## Sequencing
1. **M4b — offline end-to-end**: wire capture in, frames out, no DOSBox:
   emu860c runs a mission, the C-side DMA tap feeds igc_gpu_frame per draw,
   frames dump to disk. Validates the whole chain at speed. (next)
2. **M4c — the C012 device** in DOSBox-X + the protocol server as a socket
   bridge (the emulator process stays Python+C+GPU; DOSBox talks to it over a
   local pipe — same shape as the existing namedpipe serial work).
3. **M4d — present**: vr_readpixels or the bridge window; frame pacing.
4. M3 texel path lands whenever the TXDN spec closes — orthogonal.