[atari7800] Re: [personal] Re: AW: 7800 cartridge detection

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The BIOS compares FE00-FE7F with FE80-FF00 and if they are the same, it assumes there no cartridge and jumps to internal game.
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I think you misundersood that part of the code. Actually it
compares each byte in $FE00-$FE7F with itself. It uses indexed
addressing modes for both accesses, but the second access is
always crossing a page boundary while the first one isn't.

If there is no cartridge to drive the data bus, then the 6502
will usually read the last data that was on the bus. This is
different when crossing a page boundary, so if the compare
doesn't match, then the 7800 assumes that there is no cartridge
and starts the internal game.


Ciao, Eckhard Stolberg


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