emulator: Genesis Plus GX
type: libretro
core_classification: community_fork
source: "https://github.com/libretro/Genesis-Plus-GX"
upstream: "https://github.com/ekeeke/Genesis-Plus-GX"
profiled_date: "2026-03-23"
core_version: "v1.7.4"
display_name: "Sega - MS/GG/MD/CD (Genesis Plus GX)"
cores:
  - genesis_plus_gx
  - genesisplusgx
  - genesisplusgx-expanded
systems:
  - sega-megadrive
  - sega-megacd
  - sega-mastersystem
  - sega-gamegear
  - sega-sg1000
  - sega-pico

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.

files:
  # -------------------------------------------------------
  # Sega Mega CD / Sega CD - boot ROMs (required for CD)
  # -------------------------------------------------------
  - name: bios_CD_U.bin
    system: sega-megacd
    required: true
    note: "Sega CD (NTSC-U) boot ROM. Loaded for REGION_USA."
    source_ref: "core/loadrom.c:409"

  - name: bios_CD_E.bin
    system: sega-megacd
    required: true
    note: "Mega CD (PAL) boot ROM. Loaded for REGION_EUROPE."
    source_ref: "core/loadrom.c:412"

  - name: bios_CD_J.bin
    system: sega-megacd
    required: true
    note: "Mega CD (NTSC-J) boot ROM. Loaded for default/Japan region."
    source_ref: "core/loadrom.c:415"

  # -------------------------------------------------------
  # Sega Master System - boot ROMs (optional, behind config.bios)
  # -------------------------------------------------------
  - name: bios_U.sms
    system: sega-mastersystem
    required: false
    note: "Master System (NTSC-U) boot ROM. Snail maze game built in."
    source_ref: "core/loadrom.c:511"

  - name: bios_E.sms
    system: sega-mastersystem
    required: false
    note: "Master System (PAL) boot ROM. Snail maze / Sonic built in."
    source_ref: "core/loadrom.c:514"

  - name: bios_J.sms
    system: sega-mastersystem
    required: false
    note: "Master System (NTSC-J) boot ROM."
    source_ref: "core/loadrom.c:517"

  # -------------------------------------------------------
  # Sega Game Gear - boot ROM (optional, behind config.bios)
  # -------------------------------------------------------
  - name: bios.gg
    system: sega-gamegear
    required: false
    note: "Game Gear boot ROM (Majesco 1KB or standard). Shows startup logo."
    source_ref: "core/loadrom.c:480"

  # -------------------------------------------------------
  # Sega Mega Drive / Genesis - TMSS boot ROM (optional)
  # -------------------------------------------------------
  - name: bios_MD.bin
    system: sega-megadrive
    required: false
    note: "Mega Drive TMSS boot ROM. Shows 'Produced by or under license' screen."
    source_ref: "libretro/libretro.c:3651"

  # -------------------------------------------------------
  # Lock-on ROMs (optional, feature-gated)
  # -------------------------------------------------------
  - name: ggenie.bin
    system: sega-megadrive
    required: false
    note: "Game Genie ROM. Loaded when Game Genie lock-on is enabled."
    source_ref: "core/cart_hw/ggenie.c:64"

  - name: areplay.bin
    system: sega-megadrive
    required: false
    note: "Action Replay (Pro) ROM. Loaded when Action Replay lock-on is enabled."
    source_ref: "core/cart_hw/areplay.c:64"

  - name: sk.bin
    system: sega-megadrive
    required: false
    size: 2097152
    validation: [size]
    note: "Sonic & Knuckles ROM. Required for S&K lock-on feature."
    source_ref: "core/cart_hw/md_cart.c:867"

  - name: sk2chip.bin
    system: sega-megadrive
    required: false
    size: 262144
    validation: [size]
    note: "Sonic & Knuckles UPMEM ROM. Used by Sonic 2 & Knuckles lock-on."
    source_ref: "core/cart_hw/md_cart.c:873"

platform_details:
  megacd:
    bootrom_size: 131072      # 128 KB
    source_ref: "core/cd_hw/scd.h:73"
    note: |
      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       # 2 KB
    source_ref: "libretro/libretro.c:3648-3666"
    note: |
      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    # 4 MB (stored above cart ROM)
    source_ref: "core/loadrom.c:504-527"
    note: |
      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    # 4 MB (stored above cart ROM)
    source_ref: "core/loadrom.c:467-489"
    note: |
      Handles 1KB Majesco GG BIOS (mapped to $0000-$03FF) and
      standard larger BIOS ROMs.