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According to this page, the Apple 1 monitor program was located in addresses $F000-$FFFF, a ROM program to use the optional cassette interface card was at addresses $C100-$C1FF, and the peripheral address for writing to the cassette was at $C028.

The most likely explanation is that the address range $C000-$CFFF was reserved for the interface card slot, and that the cassette program was in its own ROM on the interface card. However, it is also possible that there was only one ROM, on the motherboard with both the monitor and cassette programs. Is there evidence documenting how the cassette program was stored?

This picture of the cassette interface card may help: Apple 1 cassette interface card

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  • The address decoding on that board is interesting. It looks as though odd addresses within the range from 0xC080-0xC0FF get converted into even addresses, while addresses in the range 0xC100-0xC1FF have their bottom 8 bits passed through as-is.
    – supercat
    Mar 17 at 21:21
  • @supercat - It's also dependent upon the output from the audio comparator. That is the way the signal is read. At some point in the code it goes one of two ways depending upon the output of the comparator. Mar 18 at 14:26
  • @KevinWhite: I meant to say that, but the comment-edit timeout expired. Given the lack of synchronization I would expect that the software probably just uses something like a BIT instruction to test what's there, but if the software had synchronized the output it could have done something creative like have a loop which runs "DEY / BNE back" until the input switches, but changes in a way that would make it possible to identify three different transition times every five cycles.
    – supercat
    Mar 18 at 14:48
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    @supercat - I'm surprised there isn't a problem with the audio signal not being synchronized with the clock. I would expect the PROM to give intermediate outputs if the transitions did not meet setup time. Mar 18 at 15:44
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The Card featured 256 bytes of ROM.

Is there evidence documenting how the cassette program was stored?

The evidence is right there in the PCB photo you added. The two MMI 6301 chips, labled APPLE A3 and APPLE A4, at position 3 and 4, are 256 by 4 PROMs. The same type, produced by Monolithic Memories, as used on the Apple 1 motherboard for the monitor program. Those are marked APPLE A1 and APPLE A2 as well as located on position A1/A2.

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