Re: How does the ZIP chip work?


"Eric Smith" <eric@xxxxxxxxxxxx> wrote in message
news:qh64oqnxwe.fsf@xxxxxxxxxxxxxxxxxxxxxx
> Bryan Parkoff wrote:
>> Do you mean that 6502 / 65C02 CPU chip are limited to 1MHz according
>> to
>> the CPU design?
>
> No, they're limited to 1 MHz due to the Apple II design.
>
>> If you rewire 6502 CPU to 14M crystal osc by modifying
>> timing logic, it might work without accelerator card.
>
> Not if you want your Apple II to actually work. The other logic and
> memory chips in the Apple and on peripheral cards are not fast enough,
> so nothing would work correctly.
>
> But even if the electronics otherwise worked, you wouldn't get proper
> video, because all the timings would be approximately fourteen times too
> fast. Your horizontal and vertical sync would be around 220 KHz and
> 840 Hz, rather than around the required frequencies of 15.75 KHz and 60
> Hz.
>
> And even if all that worked, you'd have to rewrite RWTS and any sound
> routines.
>
Eric,

Well, I mean that timing remains unchanged for video and RAM which they
run at 1MHz for video and 2MHz for RAM. TV will always be the same as
horizontial 15KHz and vertical 60Hz. The fact is that first cycle of 14M
responds to video scanner and 6502 CPU before it begins to suspend or delay
until 15th cycle of 14M. If you redesign the timing which it allows 6502
CPU to run each cycle of 14M without delay, 6502 CPU will run at 14MHz while
video scanner and RAM stay at the same speed of 1MHz and 2MHz.
Michael Mahon said that it is impossible because data will be wrong when
it is read or writen to the RAM.

Bryan Parkoff


.

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