inspect and repair rom header checksums · snes · game boy · gba · genesis · runs locally
[drop rom]
Drop a ROM file
.sfc .smc .gb .gbc .gba .md .sms .gg — SNES · Game Boy · GBC · GBA · Sega Genesis · Master System · Game Gear
[status]
drop a rom to inspect its header checksums
[how checksums work]
SNESSum of all 16-bit words in the ROM (checksum bytes zeroed). Stored as a 16-bit value and its complement (XOR 0xFFFF).
Game Boy / GBCHeader checksum at 0x14D: -(sum(0x134–0x14C)+1) & 0xFF. Global checksum at 0x14E/0x14F: sum of all bytes except itself.
GBAHeader checksum at 0x00BD: -(sum(0xA0–0xBC)+0x19) & 0xFF. Only one byte — checked at boot.
Sega GenesisSum of all 16-bit words from 0x200 onward, stored big-endian at 0x18E. Must match after any ROM edit.
Master System / Game GearSum of all bytes except the 2-byte checksum field in the TMR SEGA tag. Little-endian.
[notice]
you supply your own files. fatcousin does not host, link to, or recommend sources for rom files, bios files, or copyrighted game data. this tool is for preservation, modding, research, and education with files you lawfully possess. all processing runs in your browser — files never leave your device.