MD.emu - RetroBIOS¶
Technical notes
Android and Linux member of the EX Emulator series by Robert Broglia, published as com.explusalpha.MdEmu (MD.emu/metadata/conf.mk:2,4,6,7, README.md:3-4) and reached by ES-DE through its MD-EMU find rule. Genesis Plus GX is vendored under MD.emu/src/genplus-gx and driven by the app's own system, input and audio layers (MD.emu/src/CMakeLists.txt:6-55), while the Sega CD hardware comes from the Gens code PicoDrive carried (MD.emu/src/scd/LC89510.cc:1-7) and disc images are read through Mednafen's CD layer (MD.emu/CMakeLists.txt:12). The build defines LSB_FIRST, NO_SYSTEM_PICO and MDFN_CD_NO_CCD (MD.emu/CMakeLists.txt:25), leaving Pico out and CD support in. Content is read from .bin, .smd, .md, .gen and .sms cartridge images and from .cue, .iso and .chd discs (MD.emu/metadata/conf.mk:5, MD.emu/src/main/system.ccm:91-101). A .sms name switches the machine to Power Base Converter mode (MD.emu/src/genplus-gx/loadrom.cc:383-387); Game Gear and SG-1000 sit inside #if 0 blocks in the renderer with no SYSTEM_GG or SYSTEM_SG defined anywhere in the tree (MD.emu/src/genplus-gx/vdp_render.cc:934-962,974-989, MD.emu/src/genplus-gx/system.h:35-40).
The CD boot ROM is the one file the user supplies and it carries no name of its own. Three pickers store arbitrary URIs under USA CD BIOS, Japan CD BIOS and Europe CD BIOS, filtered on the cartridge extensions (MD.emu/src/main/EmuMenuViews.cc:289-346, MD.emu/src/main/system.ccm:84-86,136, MD.emu/src/main/options.cc:57-61,86-90). Loading a disc picks one of the three by region: the Game Region setting when it is not Auto, otherwise byte 0x20b of the boot sector, 0x7a for USA, 0x64 for Europe, anything else Japan (MD.emu/src/main/Main.cc:254-264,311-332, MD.emu/src/main/EmuMenuViews.cc:132-144,167-170, MD.emu/src/main/options.cc:31). An empty path ends the load with "Set a {} BIOS in the Options" and a failed read with "Error loading BIOS" (MD.emu/src/main/Main.cc:333-339). The file goes into the cartridge buffer, and a path ending in .zip, .7z or .rar is scanned instead, the first entry with a cartridge extension being taken (MD.emu/src/main/Main.cc:337, imagine/src/base/common/ApplicationContext.cc:542-562, imagine/src/fs/ArchiveFS.cc:75-78). init_rom accepts the image only at exactly 131072 bytes with "BOOT" at 0x124 or 0x128 of the raw bytes, which is what raises sCD.isActive; anything else ends the load with "Invalid BIOS" (MD.emu/src/genplus-gx/loadrom.cc:426-432, MD.emu/src/main/Main.cc:340-344). Cartridge content never reads it.
No other boot ROM exists here. t_config has no bios field (MD.emu/src/genplus-config.h:46-54), the Master System mapper table names BIOS only in two game titles (MD.emu/src/genplus-gx/cart_hw/sms_cart.cc:119,126), and the TMSS OS ROM is copied out of the loaded image rather than read from a file, behind a config.tmss bit nothing turns on (MD.emu/src/genplus-gx/genesis.cc:313-320, MD.emu/src/genplus-gx/mem68k.cc:471-476). Upstream's load_bios and the bios_CD_U.bin, bios_CD_E.bin and bios_CD_J.bin names it opens have no counterpart in this tree (upstream core/loadrom.c:394-415, upstream libretro/libretro.c:3416-3418).
The lock-on ROMs read nothing. osd.h keeps upstream's ggenie.bin, areplay.bin, sk.bin and sk2chip.bin paths and all three loaders are compiled (MD.emu/src/osd.h:13-16, MD.emu/src/genplus-gx/cart_hw/ggenie.cc:43-49, MD.emu/src/genplus-gx/cart_hw/areplay.cc:43-49, MD.emu/src/genplus-gx/cart_hw/md_cart.cc:499-531), but every one of them hangs off config.lock_on, which the value initialised config object leaves at zero and no code in the tree ever writes (MD.emu/src/main/AppMeta.cc:26, MD.emu/src/genplus-gx/cart_hw/md_cart.cc:499,702,850). There is no menu item and no config key for it (MD.emu/src/main/system.ccm:81-89), where the libretro port drives the same switch from a core option (upstream libretro/libretro.c:1715-1721).
Per content the app writes and reads files named after the ROM: .srm cartridge backup memory, .brm CD backup memory it formats itself when absent (MD.emu/src/main/Main.cc:93-101,124-233), .gp states (MD.emu/src/main/system.ccm:162) and a .pat cheat list under the cheats path (MD.emu/src/main/Cheats.cc:388-391,438-441). Settings live in MdEmu.config (MD.emu/src/main/AppMeta.cc:34). Mednafen's image reader picks up an .sbi beside a cue sheet when one is present (EmuFramework/src/shared/mednafen/cdrom/CDAccess_Image.cpp:928-943), and gpOverlay.png and the shader sources are drawn by the framework from the application bundle (EmuFramework/include/emuframework/AssetManager.hh:61-66, EmuFramework/src/AssetManager.cc:58-73, MD.emu/src/main/AppMeta.cc:188-199).
3 files | 3 required, 0 optional | 3 in repo, 0 missing
bios_CD_U.bin required in repo north-america
Sega CD boot ROM read for the USA region
- System: sega-megacd
- Size: 128.0 KB
- Validation: size, signature
- Source:
MD.emu/src/main/Main.cc:311-344,MD.emu/src/genplus-gx/loadrom.cc:426-432,MD.emu/src/main/EmuMenuViews.cc:290-295,MD.emu/src/main/EmuMenuViews.cc:317-345 - Platforms: batocera, emudeck, lakka, recalbox, retroarch, retrobat, retrodeck, retropie, rocknix, romm
- Config key:
CFGKEY_MD_CD_BIOS_USA_PATH - Picked by the user from any path, under any name ending in .bin, .smd, .md, .gen or .sms, or inside a .zip, .7z or .rar holding one. The tests are the length and the "BOOT" text at 0x124 or 0x128; a file failing either aborts the load rather than falling back.
bios_CD_J.bin required in repo japan
Mega CD boot ROM read for the Japan region
- System: sega-megacd
- Size: 128.0 KB
- Validation: size, signature
- Source:
MD.emu/src/main/Main.cc:311-344,MD.emu/src/genplus-gx/loadrom.cc:426-432,MD.emu/src/main/EmuMenuViews.cc:290-295,MD.emu/src/main/EmuMenuViews.cc:317-345 - Platforms: batocera, emudeck, lakka, recalbox, retroarch, retrobat, retrodeck, retropie, rocknix, romm
- Config key:
CFGKEY_MD_CD_BIOS_JPN_PATH - Taken by the default branch: USA and Europe go to the other two paths and every other region code lands here, both Japanese ones included.
bios_CD_E.bin required in repo europe
Mega CD boot ROM read for the Europe region
- System: sega-megacd
- Size: 128.0 KB
- Validation: size, signature
- Source:
MD.emu/src/main/Main.cc:311-344,MD.emu/src/genplus-gx/loadrom.cc:426-432,MD.emu/src/main/EmuMenuViews.cc:290-295,MD.emu/src/main/EmuMenuViews.cc:317-345 - Platforms: batocera, emudeck, lakka, recalbox, retroarch, retrobat, retrodeck, retropie, rocknix, romm
- Config key:
CFGKEY_MD_CD_BIOS_EUR_PATH - Reached when the boot sector byte at 0x20b is 0x64 or the Game Region setting is Europe.
Generated on 2026-08-23T16:12:06Z