On 2014-07-06 at 15:23:15, smf (smf@null.net) wrote: > >I was also interested how they did it internally. Like if they changed the > >6502 core (doubtful) > > or for example wired the two upper address nybbles together or something > > like that... > > Only 2kb of rom is available because that is all they put inside, don't > forget they need ram in there as well as IO so they can't can't just > duplicate 2kb of rom throughout 64kb address space. RAM and I/O are in the lower half of the 4KiB. It is quite possible that all 4KiB is “duplicated” throughout the whole address space. > I would imagine that because of certain 6502 features the rom is mapped from > F800 to FFFF, with ram at least from 0000-0200 (however much they included). I guess the most important “certain feature” are the H/W vectors. > The rom and ram might be mirrored at other addresses, you achieve this by > looking at less address lines not by wiring lines together. They might just > use A15 for the rom chip select, so the rom is duplicated every 2k from > 8000-FFFF. If they wanted to make it only appear at F800 then they'd use A15 > AND A14 AND A13 AND A12 AND A11, but it would cost them more and the only > benefit is the rom isn't mirrored. I think you misunderstood. Since 2KiB of ROM starts at $800 (NOT $f800) it most probably /has to/ be mirrored if you don’t want to change the CPU core. And I concur with Jim and Gerrit - the cheapest way must have been to just make the last four address bits “don’t care”. Message was sent through the cbm-hackers mailing listReceived on 2014-07-06 15:00:02
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