GBA ROMs are special chips that contain a standard ROM, address latches, and address counters all on one chip. Cart accesses can be either sequential or non-sequential. The first access to a random cart ROM location must be non-sequential. This type of access is done by putting the lower 16 bits of the ROM address on cart lines AD0-AD15 and setting /CS low to latch address lines A0-A15. Then /RD is strobed low to read 16 bits of data from that ROM location. (Data is valid on the rising edge of /RD.) The following sequential ROM location(s) can be read by again strobing /RD low. Sequential ROM access does not require doing another /CS high-to-low transitions because there are count up registers in the cart ROM chip that keep track of the next ROM location to read. Address increment occurs on the low-to-high edge of all /RD. In theory, you can read an entire GBA ROM with just one non-sequential read (address 0) and all of the other reads as sequential so address counters must be used on most address lines to exactly emulate a GBA ROM. However, you only need to use address latch / counters on A0-A15 in order to satisfy the GBA since A16-A23 are always accurate.


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