Re: IIgs RAM card Question

[dempson@actrix.gen.nz (David Empson)]

Roy and/or Janet Miller <millers@inetnebr.com> wrote:

> David Empson wrote:
> 
> > The IIgs imposes further bank restrictions, due to its memory
> > organisation (and the legacy of emulating an 8-bit Apple II).
> >
> > > The 8086/8 can address 16 banks of 64K.
> >
> > Not exactly.  It can address 1 MB, but uses a segmented architecture
> > which only allows access to up to four 64 KB segments at a time.
> 
> What you have shown David, is that both the 8088 and the 65816 have
> restrictions due to their compatibility with their predecessors: the 8080
> and the 6502.

I wasn't aware of any compatibility between the 8080 and the 8088,
except in a rather coarse manner (similar processor model).  It isn't
even compatible at the assembly source level, though the 8088 is
generally a functional superset and an automatic translation tool can
probably do a source level (or even object level) conversion.

(The NEC V-series family has an actual 8080 emulation mode, but I'm not
aware of any Intel 8086/88 derivative with this feature.)

I think that the 8086/8 limitations were more due to the fact that it
was really only a 16-bit processor with an extended addressing range,
and this was the best solution that Intel could come up with to deal
with the discrepancy between a 16-bit pointer and a 20-bit address bit.
It also had practical benefits, such as making a multitasking
environment reasonably easy to manage (without individual processes
needing to be deal with the extended address range).


The 65816, on the other hand, has a very compatible emulation of the
6502/65C02, and uses a superset of the same instruction set when running
in both emulation and native mode.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P O Box 27-103, Wellington, New Zealand