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# Genesis Plus GX - RetroBIOS

Type | libretro
Classification | community_fork
Source | https://github.com/libretro/Genesis-Plus-GX
Upstream | https://github.com/ekeeke/Genesis-Plus-GX
Version | v1.7.4
Profiled | 2026-03-23
Cores | genesis_plus_gx
Systems | sega-megadrive, sega-megacd, sega-mastersystem, sega-gamegear, sega-sg1000, sega-pico

Platform details

megacd: - bootrom_size: 131072 - source_ref: core/cd_hw/scd.h:73 - notes: CD boot ROM region is auto-detected from disc image header.

Hardware model detected from boot ROM string at offset 0x120: WONDER-MEGA BOOT -> Wondermega WONDERMEGA2 BOOT -> Wondermega M2 / X'Eye CDX BOOT ROM -> CDX / Multi-Mega

    **megadrive:**
    - tmss_rom_size: 2048
    - source_ref: libretro/libretro.c:3648-3666
    - notes: TMSS boot ROM validated by checking "GENESIS OS" at offset 0x120.

Only loaded when genesis_plus_gx_bios core option is enabled.

    **mastersystem:**
    - bios_max_size: 4194304
    - source_ref: core/loadrom.c:504-527
    - notes: BIOS ROM loaded into cart.rom + 0x400000. SEGA mapper assumed

for ROMs larger than 48KB, otherwise no mapper. Region selected based on current region_code setting.

    **gamegear:**
    - bios_max_size: 4194304
    - source_ref: core/loadrom.c:467-489
    - notes: Handles 1KB Majesco GG BIOS (mapped to $0000-$03FF) and

standard larger BIOS ROMs.

Technical notes

Genesis Plus GX is a Sega 8/16-bit emulator covering SG-1000, Master System, Game Gear, Mega Drive/Genesis, and Mega CD/Sega CD.

BIOS loading is handled by load_bios() in core/loadrom.c. The libretro port resolves filenames from the system directory in libretro/libretro.c:3516-3527.

Mega CD BIOS is required to boot CD images (load fails without it). Master System, Game Gear, and Mega Drive BIOS files are optional boot ROMs gated behind the "genesis_plus_gx_bios" core option (config.bios). Lock-on ROMs (Game Genie, Action Replay, Sonic & Knuckles) are optional and loaded only when the corresponding lock-on feature is enabled.

12 files | 3 required, 9 optional | 12 in repo, 0 missing

`bios_CD_U.bin` -required

- System: sega-megacd
- Source: `core/loadrom.c:409`
- Platforms: batocera, emudeck, lakka, recalbox, retroarch, retrobat, retrodeck, retropie
- Sega CD (NTSC-U) boot ROM. Loaded for REGION_USA.

`bios_CD_E.bin` -required

- System: sega-megacd
- Source: `core/loadrom.c:412`
- Platforms: batocera, emudeck, lakka, recalbox, retroarch, retrobat, retrodeck, retropie
- Mega CD (PAL) boot ROM. Loaded for REGION_EUROPE.

`bios_CD_J.bin` -required

- System: sega-megacd
- Source: `core/loadrom.c:415`
- Platforms: batocera, emudeck, lakka, recalbox, retroarch, retrobat, retrodeck, retropie
- Mega CD (NTSC-J) boot ROM. Loaded for default/Japan region.

`bios_U.sms` -optional

- System: sega-mastersystem
- Source: `core/loadrom.c:511`
- Platforms: lakka, recalbox, retroarch, retropie
- Master System (NTSC-U) boot ROM. Snail maze game built in.

`bios_E.sms` -optional

- System: sega-mastersystem
- Source: `core/loadrom.c:514`
- Platforms: lakka, recalbox, retroarch, retropie
- Master System (PAL) boot ROM. Snail maze / Sonic built in.

`bios_J.sms` -optional

- System: sega-mastersystem
- Source: `core/loadrom.c:517`
- Platforms: lakka, recalbox, retroarch, retrodeck, retropie
- Master System (NTSC-J) boot ROM.

`bios.gg` -optional

- System: sega-gamegear
- Source: `core/loadrom.c:480`
- Platforms: lakka, recalbox, retroarch, retrodeck, retropie
- Game Gear boot ROM (Majesco 1KB or standard). Shows startup logo.

`bios_MD.bin` -optional

- System: sega-megadrive
- Source: `libretro/libretro.c:3651`
- Platforms: lakka, recalbox, retroarch, retrodeck, retropie
- Mega Drive TMSS boot ROM. Shows 'Produced by or under license' screen.

`ggenie.bin` -optional

- System: sega-megadrive
- Source: `core/cart_hw/ggenie.c:64`
- Platforms: lakka, retroarch, retrodeck, retropie
- Game Genie ROM. Loaded when Game Genie lock-on is enabled.

`areplay.bin` -optional

- System: sega-megadrive
- Source: `core/cart_hw/areplay.c:64`
- Platforms: lakka, retroarch, retrodeck, retropie
- Action Replay (Pro) ROM. Loaded when Action Replay lock-on is enabled.

`sk.bin` -optional

- System: sega-megadrive
- Size: 2.0 MB
- Validation: size
- Source: `core/cart_hw/md_cart.c:867`
- Platforms: lakka, retroarch, retrodeck, retropie
- Sonic & Knuckles ROM. Required for S&K lock-on feature.

`sk2chip.bin` -optional

- System: sega-megadrive
- Size: 256.0 KB
- Validation: size
- Source: `core/cart_hw/md_cart.c:873`
- Platforms: lakka, retroarch, retrodeck, retropie
- Sonic & Knuckles UPMEM ROM. Used by Sonic 2 & Knuckles lock-on.

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