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

Hi There was some software multitasking done on 6502's, as well as other machines. This was done in Forth ( both preemptive

>From: "Patrick Finnegan" <pat(a)purdueriots.com> > >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 >mode'). > >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. > >Pat >-- >Purdue Universtiy ITAP/RCS >Information Technology at Purdue >Research Computing and Storage >http://www-rcd.cc.purdue.edu