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RetroBIOS

Frodo

Frodo emulates Commodore C64. It reads 4 files, none of them required: it starts games without them.

About this profile
Detail Value
Type libretro
Classification Community fork
Source https://github.com/libretro/frodo-libretro
Upstream https://github.com/cebix/frodo4
Version V4.2, stated in Src/Version.h
Revision libretro/frodo-libretro@891d20102a88
Profiled 2026-03-23
Profile YAML, JSON
Cores frodo
Systems Commodore C64
Install this emulator's files

The installer finds your setup and its BIOS folder, then downloads only the files this emulator reads there and checks each one.

Open the Start menu. Type PowerShell and open it. Paste the line. Press Enter.

PowerShell
&([scriptblock]::Create((irm https://abdess.github.io/retrobios/i.ps1))) --emulator frodo
Technical notes

Frodo is a Commodore 64 emulator by Christian Bauer (Frodo V4.2), ported to libretro. It emulates the C64 (MOS 6510, VIC-II, SID, CIA x2) and an optional 1541 floppy drive.

All four ROMs are embedded directly in the core binary as compiled-in byte arrays (Basic_ROM.h, Kernal_ROM.h, Char_ROM.h, 1541_ROM.h). No external BIOS files are required for normal operation.

The core attempts to load external ROM files from the current working directory (DATADIR is empty in the libretro build) at startup via load_rom_files() in main.cpp. If any file fails to open or has the wrong size, the embedded ROM is used as fallback via memcpy(). The filenames are hardcoded with spaces, no subdirectory prefix.

ROM loading order in load_rom_files():

  1. "Basic ROM" (8192 bytes, BASIC_ROM_SIZE 0x2000) -> TheC64->Basic
  2. "Kernal ROM" (8192 bytes, KERNAL_ROM_SIZE 0x2000) -> TheC64->Kernal
  3. "Char ROM" (4096 bytes, CHAR_ROM_SIZE 0x1000) -> TheC64->Char
  4. "1541 ROM" (16384 bytes, DRIVE_ROM_SIZE 0x4000) -> TheC64->ROM1541

The core does NOT construct paths relative to retro_system_directory for ROM loading. The system directory variable is fetched in retro_init() but only used for the save directory fallback, not for ROM path construction. DATADIR defaults to "" (empty string) when not defined by the build system, so the filenames resolve to bare names in the cwd. In practice, the external ROM loading path is effectively unused in the libretro build since the cwd is unpredictable and the filenames contain spaces.

The Kernal ROM is patched at runtime by PatchKernal() in C64.cpp for fast reset (addresses 0x1D84-0x1D85) and IEC bus replacement when 1541 processor emulation is enabled (multiple addresses in the 0x0D00-0x0E00 range are patched with 0xF2 trap opcodes).

The 1541 drive ROM is also patched in PatchKernal(): ROM checksum verification is disabled (0x2AE4-0x2AE9 NOPed), DOS idle loop is trapped (0x2C9B), and write sector / format track operations are intercepted (0x3594-0x3598, 0x3B0C-0x3B0D).

Source: Src/main.cpp:67-115 (ROM defines, load_rom, load_rom_files), Src/C64.h:33-37 (ROM size constants), Src/C64.cpp:221-285 (PatchKernal), Src/C64.cpp:734-746 (Run init, saves original kernal bytes, calls PatchKernal), Src/Basic_ROM.h, Src/Kernal_ROM.h, Src/Char_ROM.h, Src/1541_ROM.h (embedded ROM arrays)

Files

Section titled “Files”

4 files: 0 required, 4 optional. 4 have an HLE fallback.

In this collection: 4 in repo and 0 missing.

Optional

Section titled “Optional”

Basic ROM optional in repo HLE fallback

C64 BASIC V2 ROM

  • System: Commodore C64
  • Embedded as builtin_basic_rom[] in Basic_ROM.h. External file never loaded in practice (DATADIR is empty, path is bare filename in cwd).
Source

Kernal ROM optional in repo HLE fallback

C64 Kernal ROM

  • System: Commodore C64
  • Embedded as builtin_kernal_rom[] in Kernal_ROM.h. Patched at runtime for fast reset and 1541 IEC traps.
Source

Char ROM optional in repo HLE fallback

C64 Character Generator ROM

  • System: Commodore C64
  • Embedded as builtin_char_rom[] in Char_ROM.h. Used by VIC-II for text/bitmap rendering.
Source

1541 ROM optional in repo HLE fallback

Commodore 1541 floppy drive ROM

  • System: Commodore C64
  • Embedded as builtin_drive_rom[] in 1541_ROM.h. Patched at runtime to disable checksum and trap disk I/O.
Source

Next steps

Section titled “Next steps”
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Maintained by Abdess on GitHub. Report a problem.

Packs from release v2026.10.10, .

MIT for the tooling and metadata. BIOS and firmware files are third-party system software, preserved for personal backup, archival and interoperability.