Designing around a 6502 (was Re: Assembly on a Apple IIc+)

From: Eric Smith <>
Date: Sat Feb 8 18:47:00 2003

Jim Keohane wrote:
>> A 6502 task context
>> would therefore require moving about 1KB, which would take about 4,500
>> instructions (at one instruction per cycle.) On a circa-1980's
>> machine, with a 1MHz clock, that would take about 4.5 msec.

Pat Finnegan wrote:
> This gives me awfully devious ideas... First, were there any
> 'multitasking machines' designed around the 6502? If you wanted to do
> multitasking, it seems like you could design a fairly simple MMU that
> would swap out the zero-page and stack (or all of the memory pages) for
> different ones, depending on the running task. Leaving only a few
> registers that need to be saved, it would leave a very small overhead
> for task swapping.

The Apple /// hardware supports this. It allows you to select an alternate
pair of pages to replace page zero and page one. AFAIK, it was never used
for user-level multitasking, but was used to provide separate user and
OS contexts.
Received on Sat Feb 08 2003 - 18:47:00 GMT

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