> > >> 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.
> > 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.
> Yup, the 6502B I think it was. A nice system. - Jim
Not to forget the 65802/816, where Stack and Zero Page could
be located anywhere in the first 64k.
Gruss
H.
--
VCF Europa 4.0 am 03./04. Mai 2003 in Muenchen
http://www.vcfe.org/
Received on Mon Feb 10 2003 - 13:36:00 GMT