Re: 6502 illegal opcodes questions

[Cameron Kaiser <ckaiser@floodgap.com>]

"Michael J. Mahon" <mjmahon@aol.com> writes:

>>>>The C64 mainly needed it for its text mode. Unlike the Apple, the 
>>>>character shapes reside within the Video chip's AND the CPUs normal 
>>>>memory map, are read over the normal address bus, and can be redefined 
>>>>by pointing a register to RAM instead of ROM.

>>>It was a conscious choice to make the Apple character set fixed.
>>>(And it's nice to have a character set fully defined at power-up.)

>>This is a marginal diversion, but I'm not sure what you mean by "fully
>>defined" since the C64's is, of course, also fully defined in the sense
>>that it's there. :)

>I meant that there was no SRAM to initialize to provide a character
>set--but perhaps I misunderstood the explanation of character generation
>in the C=64.

Ah, okay. In the C64's case, there isn't a need to copy to RAM either; the
VIC-II video chip can simply use Character ROM as it is positioned in the
default memory map. The sole difference is that the character set doesn't
*have* to come from the Character ROM; it can come from most other places
in RAM, too. But it is fully defined and ready to go on start-up without
loading it into RAM, yes.

--
      Cameron Kaiser * ckaiser@floodgap.com * posting with a Commodore 128
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