4 Mar
2008
4 Mar
'08
1:35 p.m.
woodelf said:
[...]
CP/M card's , do they count as a OS. :)
I suspect any OS using the 6502 can't have multi-tasking OS
because you can't change page 0 or the stack page.
With additional hardware, you can. Just add a small address decoding logic
which remaps pages 0 and 1, either by swapping to elsewhere in the address
space or to a different RAM (additional address pins come from some
D-register). I know this has been done in the past in some embedded
systems. With only 64K of address space, you'll have to implement some
sort of MMU anyway; the address mapping PLA of the C64 is already a
primitive version of a MMU.
Even without such hardware, you can write software to swap pages 0/1 to
other locations - it is just not extremely fast to do task switching. this
way. Hey, you can even refresh DRAM by periodically calling a subroutine
that just contains 256 NOPs; already done in the past.
--
Holger