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

From: Patrick Finnegan <>
Date: Sat Feb 8 15:39:00 2003

On Sat, 8 Feb 2003, Jim Keohane wrote:

> =====excerpt=2======================
> 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.

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. You
could even implement kernel and user mode into the MMU, making it swap
pages automatically on an interrupt or 'memory write' to signal a syscall
(and a swapping of pages, interrupt to the CPU and transition to 'kernel

I think I'm going to need to start playing with designing a 6502-based
machine now... Or maybe I should just get back to working on putting
machines into racks so I have some floorspace around here to work in.

Purdue Universtiy ITAP/RCS
Information Technology at Purdue
Research Computing and Storage
Received on Sat Feb 08 2003 - 15:39:00 GMT

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