Re: Hardware emulation of 6509 using 6502?

Den Tue, 06 Mar 2018 23:01:44 +0100 skrev Michał Pleban
<lists@michau.name>:
> Hello!
> 
> Mia Magnusson wrote:
> 
> > I really don't understand why this sram is there at all instead of
> > just map a part of the 64k dram in bank 0 as a 1k window in bank 15.
> 
> What I think personally is that the memory map pf the P500 is
> influenced heavily by the map of the B series.
> 
> And in the B series, you _cannot_ have DRAM mapped to bank 15, because
> you need a place for the 6509 to live when 8088 has access to DRAM.

The P500 has a totally separate adress and data bus for the DRAM in
bank 0, so the 8088 board (if there were any made for the P500, afaik
it only works in the B series?) should work. However the 8088 wouldn't
see memory in 6509 bank 0. Maybe that is what actually makes it
impossible to use the 8088 board? With no ram and no rom in the start
of memory the 8088 cannot use interrupts and can certainly never run
any ms-dos software.

Maybe the 8088 board might actually work (with different ROM) although
being rather useless in a P500? :)

> > Someone who were any good with software should had pulled some
> > emergency break before they actually sent the 6509 into
> > production :)
> 
> You are demanding common sense from Commodore? ;-)

:)

> > What they could had done is fill all non-rom non-i/o parts of bank
> > 15 with a mirror of bank 0.
> 
> But the problem is that bank 15 is almost entirely composed of ROM and
> I/O. There is 24 kB of ROM, another 28 kB for cartridges, 4 kB of
> I/O, 2 kB of screen memory (plus another 2 kB in the P500) and... not
> much space for anything else.

Well, software in a cartridge is the least likely to benefit from ram
in bank 15 compared to software that loads from disk, so a simple bank
switch between cartridge and mapping in DRAM might make sense.


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