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

From: Jim Keohane <jimkeo_at_multi-platforms.com>
Date: Sat Feb 8 21:57:58 2003

Yup, the 6502B I think it was. A nice system. - Jim

Jim Keohane, Multi-Platforms, Inc.

   "It's not whether you win or lose. It's whether you win!"
----- Original Message -----
From: "Eric Smith" <eric_at_brouhaha.com>
To: <cctalk_at_classiccmp.org>
Sent: Saturday, February 08, 2003 19:45
Subject: Re: Designing around a 6502 (was Re: Assembly on a Apple IIc+)


> 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 - 21:57:58 GMT