Codemasters games for the Master System and Game Gear include an extra header and internal checksum routines. This is useful to check the ROM for errors because the normal BIOS checksum routine does not work due to the different mapper. It also stores the date and time the game was last compiled and displays this on the host system.


For most games, the checksum routine can be activated by holding a key combination at startup. Holding [Up]+[1]+[2] on controller 1 will work in most cases.


The header is found at offset $7fe0 in the file.

$7fe0ByteNumber of 16KB banks over which to calculate the checksum
$7fe4ByteHour (24 hour clock)
$7fe8Word$10000 - checksum
$7fea-$7fef are all zero.

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