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spec: document the NEC PC-FX (mednafen) platform appendix c17f852c2a5a

The Doomsday One mednafen fork vendors the core as a submodule and implements
the full 0.3.0 backend for PC-FX: V810 regs, 2 MB flat RAM, RGB888-decoded
framebuffer, stereo audio drain, and pad injection over a WASD-style key map.

Git-Commit: 6d02d4eb4c08cd329b210610d2d69d0026797a7c
Git-Author: Andy McCall <[email protected]>
Author
Andy McCall @andymccall <[email protected]>
When
· 2026-07-21 20:05 UTC
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◈ sealed by Andy McCall
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- Registers (ADL): `af bc de hl ix iy sp pc mbase` + ADL-mode flag.
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- Framebuffer: from `vgabuf` (the VDP output), server swizzles to PPM RGB.
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### NEC PC-FX — `mednafen` (Doomsday One fork)

    

  
    

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

    

  
    

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

    

  
    

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

    

  
    

      
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  reuse the same core once their backends land.

    

  
    

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

    

  
    

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

    

  
    

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

    

  
    

      
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  wraps in that space and `bank` is ignored.

    

  
    

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

    

  
    

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

    

  
    

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

    

  
    

      
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  Mednafen's display-dependent pixel order.

    

  
    

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

    

  
    

      
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  teed from the per-frame `SoundBuf`.

    

  
    

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

    

  
    

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

    

  
    

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

    

  
    

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

    

  
    

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

    

  
    

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

    

  
    

      
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---

    

  
    

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## Reference implementations in this repo

    

  
    

      
      
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