Cartridge ROM mapping

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Cartridge ROM mapping

Post by @username@ » Sun Feb 14, 2021 10:58 pm

From looking at the BASIC 12KB ROMs, they appear to use $A000-$BFFF(ROM1) and $7000-$7FFF(ROM2). An anomoly in BASIC82A and B, is that the first 256 bytes of ROM2 are also available at $4000.

In the emulator - this is easily handled with a software AND mask.

However, I wondered if perhaps the original cartridge had another means of mapping the address lines.

Reading the "Circuit 1" PDF, it's eay to follow ROM 0/1/2 through the 74LS139 decoder B. So is it safe to assume that any 4K ROM on the other side of ROM2 enable can be read from $4000/$5000/$6000/$7000? Is there a mapping connection on the physical cartridge to facilitate this?

If you look at Planet Defender 6K against Planet Defender 8K - you will see that the difference is just the lower 2K at $A000 is repeated at $A800. Otherwise these dumps are binary identical - which would indicate the 8K dump is unnecessary - as the main routines read from $A200.

So is this the cartridge itself doing this translation to mirror the same 2K block when read?

Look forward to the hw guys letting me know the answers!
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Re: Cartridge ROM mapping

Post by MADrigal » Thu Feb 25, 2021 8:29 pm

Mate, unfortunately I do not own the BASIC cartridge with the BASIC 82 roms anymore, so I cannot help you with that.
But one thing I can tell you for sure: all dumps were done on actual EPROMs not from the cartridge w/connectors to an EPROM programmer.
So whatever you see in the dumps, is the actual content of the EPROM chips.
Some 6K ROMs saved in a 8K EPROMs have mirrored 2K, but from memory, a few 8K EPROMs had the 2K blanked (full of 00 or FF). Again I think this is quite random and there are probably different dump types around.
And also there is a 16K version of BASIC83 (or was it VTL V1 BASIC, I dont remember) - which I found on cartridge once only, hence extremely rare. I think that also has some mirroring and blank spaces
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