Last updated: September 2026
The Genesis needs no BIOS. Plug a cartridge ROM into Genesis Plus GX or PicoDrive and it runs. The Sega CD is the add-on that changes that: its boot ROM lives in the CD unit, the disc games call into it, and every emulator refuses to load a CD image without it. So the first Sega CD game anyone tries fails, and the error says “unable to open CD BIOS”, and here you are.
Three files, one per region. This page has the hashes, including the two different US hashes that both count as correct, which emulator wants which name, the folder on PC, Steam Deck, Android and every handheld firmware, and the two ways to dump the ROM from a Sega CD you own. It is the Sega CD entry in my RetroArch BIOS reference.
The Three Files
Every Sega CD and Mega CD boot ROM is 131,072 bytes (128 KB).
| File | Region | MD5 | Version |
|---|---|---|---|
bios_CD_U.bin | North America | 854b9150240a198070150e4566ae1290 | Sega CD Model 2, v2.00W |
bios_CD_U.bin | North America | 2efd74e3232ff260e371b99f84024f7f | Sega CD Model 1, v1.10 |
bios_CD_E.bin | Europe | e66fa1dc5820d254611fdcdba0662372 | Mega CD Model 1, v1.00 |
bios_CD_J.bin | Japan | 278a9397d192149e84e820ac621a8edd | Mega CD Model 1, v1.00p |
Why there are two US hashes
Genesis Plus GX’s documentation lists the Model 2 dump (854b...) as the US file. PicoDrive’s documentation lists the Model 1 dump (2efd...). Both cores boot with either one; they read the ROM, not the hash. But every BIOS checker, from RetroArch’s core information screen to Batocera’s and Knulli’s missing-file reports, compares against one list, so one of the two real dumps gets flagged “wrong” depending on which core the checker follows. If your US file has either hash and is 131,072 bytes, it is correct. People search for that second hash by itself, which tells you how much confusion it causes.
The Model 1 and Model 2 ROMs differ in the boot animation and the CD player interface. Games do not care. I use the Model 2 file on PC because Genesis Plus GX is my core there, and whichever one the handheld firmware’s checker likes on the handhelds.
The optional Genesis file
bios_MD.bin is the Genesis TMSS boot ROM, the one that shows “Produced by or under license from Sega Enterprises” before a cartridge game. Genesis Plus GX lists it as optional, and it does nothing but draw that screen. Skip it.
Which Emulator Wants Which Name
| Emulator | BIOS | Filenames it accepts |
|---|---|---|
| Genesis Plus GX (RetroArch) | Required for CD games, ignored for cartridges | bios_CD_U.bin, bios_CD_E.bin, bios_CD_J.bin, picked by the game’s region |
| PicoDrive (RetroArch and standalone) | Required | The same three, plus its own names: us_scd1_9210.bin, us_scd2_9306.bin, eu_mcd1_9210.bin, eu_mcd2_9306.bin, jp_mcd1_9112.bin, jp_mcd2_921222.bin. It tries .bin then .zip. |
| Kega Fusion (Windows) | Required | Any name; you browse to each region’s file under Options > Set Config > Sega CD |
| ares | Required | Any name; it matches by hash from its Firmware folder |
| BlastEm | Required for CD | Set in its config file |
Use the bios_CD_*.bin names. Both RetroArch cores accept them, every firmware’s BIOS checker expects them, and PicoDrive’s longer names only matter if you want to keep several revisions side by side.
Where the Files Go
RetroArch
The system directory, at the root. Neither core searches subfolders.
| Platform | System directory |
|---|---|
| Windows | RetroArch\system\ |
| Linux | ~/.config/retroarch/system/ |
| macOS | ~/Library/Application Support/RetroArch/system/ |
| Android | /storage/emulated/0/RetroArch/system/ |
| Steam Deck (Flatpak) | ~/.var/app/org.libretro.RetroArch/config/retroarch/system/ |
Confirm under Settings > Directory > System/BIOS, then load Genesis Plus GX and check Quick Menu > Information > Core Information. Each region file shows Present or Missing with the hash the core expects, which for the US file is the Model 2 one.
EmuDeck on Steam Deck
Emulation/bios/, files at the root, no subfolder. EmuDeck’s rule is explicit about that.
Linux handhelds
| Firmware | BIOS folder | Sega CD core |
|---|---|---|
| OnionOS (Miyoo Mini) | BIOS/ at the SD card root | PicoDrive |
| muOS | MUOS/bios/ on the OS card | Genesis Plus GX or PicoDrive |
| Knulli / Batocera | /userdata/bios/, case sensitive | Genesis Plus GX |
| ROCKNIX / ArkOS | roms/bios/ on the games card | Genesis Plus GX, PicoDrive as the alternate |
| GarlicOS | BIOS/ at the root of the games card | PicoDrive |
| RetroPie | /home/pi/RetroPie/BIOS/ | Genesis Plus GX |
Knulli’s missing-BIOS check names the file and hash it wants, and it is case sensitive, so BIOS_CD_U.BIN fails. Watch for bios_CD_U.bin.bin from Windows hiding the extension.
Sega CD is a light target. Every device on this site runs it at full speed, down to the $35 R36S, and the CD audio is the only thing that separates a good experience from a great one.
Game Files
The BIOS is half the setup. Sega CD games are disc images, and the format matters:
.cue+.binis the standard. The.cuelists the data track and the audio tracks; load the.cue, never the.bin..chdis the compressed version and both RetroArch cores read it. One file per game, audio included. This is what I convert everything to; the CHD guide covers the tool..isoalone loses the audio tracks. Games boot silently and some hang at a point where they expect a CD audio track to play.
The BIOS region and the game region should match. Genesis Plus GX picks the BIOS by the game’s region automatically; PicoDrive does the same. A US BIOS with a Japanese disc gives you the Model 1 or Model 2 region-lock screen instead of the game.
How to Verify
Get-FileHash .\bios_CD_U.bin -Algorithm MD5
md5sum bios_CD_U.bin
Size 131,072 bytes, then one of the hashes in the table. A US file at either US hash is fine.
How to Dump the BIOS From Your Sega CD
The Sega CD’s boot ROM is mapped into the Genesis address space, so a flash cart on the cartridge slot can read it. Two routes:
- Mega Everdrive. The X3 and later models copy the Sega CD BIOS from the attached unit to
MEGA/CDBIOS.BINon the SD card the first time you use the Run CD function. That file is your dump; rename it by region. - mcdBiosDumper. An open-source dumper by fabiodl, on GitHub. It runs from a flash cart and sends the ROM out through controller port 2 to a serial adapter on your PC. More work than the Everdrive route, and the result is the same.
One caveat both projects mention: a software dump reads the ROM through the console’s memory map, and the hardware masks a couple of bytes in the vector table, so your dump’s hash will not match the reference sets exactly. It is a complete, working BIOS; it just fails a checksum by two bytes. The reference hashes came from reading the chip directly.
Common Problems
“Unable to open CD BIOS” (Genesis Plus GX). File missing, misnamed, or in a subfolder. The log names the exact file it looked for.
“No US BIOS files found, read docs” (PicoDrive). None of the accepted US names are in the system directory.
The checker says my US file is wrong but games run. The Model 1 versus Model 2 hash. Both are real. Ignore the checker or swap to the one it expects.
Game boots to the CD player screen. Region mismatch, or an .iso with no data track the BIOS recognises. Use the .cue or .chd.
No music. You loaded the .bin or an .iso with no audio tracks. Load the .cue.
Kega Fusion asks to “configure EUR Bios”. A PAL game with the European slot empty in its settings.
Why No Download Links
The boot ROM is Sega’s copyrighted code. A Mega Everdrive makes the dump as a side effect of playing a disc, which is the easiest legal BIOS dump on any system on this site, and the result is a file you can trust. The pack sites ship the wrong region, the wrong revision or an installer you did not want.
FAQ
Does the Genesis need a BIOS?
No. Cartridge games run with no BIOS on every emulator. Only Sega CD games need one.
Which bios_CD_U.bin hash is correct?
Both 854b9150240a198070150e4566ae1290 (Model 2) and 2efd74e3232ff260e371b99f84024f7f (Model 1). Each core’s documentation lists one; both cores boot either.
Where does bios_CD_U.bin go on OnionOS?
BIOS/ at the root of the SD card.
Can I play Sega CD on the Miyoo Mini Plus?
Yes, at full speed on PicoDrive with the three files in the BIOS folder.
Do I need bios_MD.bin?
No. It only adds the Sega licence screen before cartridge games.
Genesis Plus GX core docs | PicoDrive core docs | mcdBiosDumper
Last verified: September 2026