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# FreeChaF - RetroBIOS

Type | libretro
Classification | pure_libretro
Source | https://github.com/libretro/FreeChaF
Version | GIT
Profiled | 2026-03-23
Cores | freechaf
Systems | fairchild-channelf

Platform details

bios_mapping: - target: 0x000-0x7FF in 64K memory space - source_ref: src/memory.h:27-29, src/memory.c:29-61 - notes: MEMORY_loadSysROM_libretro() reads the file and copies it into

Memory[] at the given address offset. PSU1 is loaded at 0x000, PSU2 at 0x400. The loader caps file size to (MEMORY_SIZE - address) to prevent overflow. MEMORY_RAMStart is advanced past the loaded ROM to protect it from writes.

    **hle_fallback:**
    - source_ref: src/channelf_hle.c
    - notes: When BIOS files are missing, FreeChaF uses HLE to emulate the

PSU routines in software. This is marked as experimental and may cause compatibility issues with some games. Each PSU can independently fall back to HLE.

    **load_priority:**
    - source_ref: src/libretro.c:186-198
    - notes: PSU1 loading order: sl90025.bin (Channel F II) first, then

sl31253.bin (original Channel F) as fallback. If both fail, HLE is used. sl90025.bin is the preferred BIOS as it includes Channel F II improvements.

Technical notes

FreeChaF is a Fairchild Channel F emulator written for libretro.

BIOS loading is in src/libretro.c:186-208. The core first tries to load sl90025.bin (Channel F II PSU1) at address 0x000. If that fails, it falls back to sl31253.bin (original Channel F PSU1) at the same address. PSU2 (sl31254.bin) is always loaded at address 0x400.

Memory layout from src/memory.h:27-30: 0x000-0x3FF PSU1 ROM (sl31253 or sl90025) 0x400-0x7FF PSU2 ROM (sl31254) 0x800-0x1FFF cartridge ROM 0x2000+ VRAM

If either PSU ROM fails to load, the core switches to experimental HLE (high-level emulation) for that PSU. A warning message is shown to the user recommending real BIOS files. HLE state is tracked per-PSU via hle_state.psu1_hle and hle_state.psu2_hle (src/channelf_hle.c).

The .info file declares all three firmware files as required (opt=false), but the core does not abort on missing BIOS -- it falls back to HLE.

3 files | 0 required, 3 optional | 3 in repo, 0 missing | 3 with HLE fallback

`sl31253.bin` -optional, HLE available

- System: fairchild-channelf
- Source: `src/libretro.c:192`
- Platforms: batocera, lakka, recalbox, retroarch, retrobat, retrodeck, retropie
- Channel F PSU1 ROM (original). Fallback if sl90025.bin is missing.

`sl31254.bin` -optional, HLE available

- System: fairchild-channelf
- Source: `src/libretro.c:202`
- Platforms: batocera, lakka, recalbox, retroarch, retrobat, retrodeck, retropie
- Channel F PSU2 ROM. Always loaded at address 0x400.

`sl90025.bin` -optional, HLE available

- System: fairchild-channelf
- Source: `src/libretro.c:186`
- Platforms: batocera, lakka, recalbox, retroarch, retrobat, retrodeck, retropie
- Channel F II PSU1 ROM. Tried first, preferred over sl31253.bin.

Generated on 2026-03-25T14:56:43Z