Re: Test suite for 6509 processor

Den Tue, 27 Feb 2018 11:31:53 +0100 skrev Michał Pleban
<lists@michau.name>:
> groepaz@gmx.net wrote:
> 
> > from my experience with hacking VICE for a decade or so... i'm
> > willing to bet there indeed exists at least one program that relies
> > on whatever odd behaviour. chances are its a program where you'd
> > least expect it :)
> 
> Well sure, when we speak about C64 where tens of thousands of programs
> are written doing all kinds of inconceivable things.
> 
> But on a CBM-II which has very little software, and when the
> differences in behaviour can only arise in a very specific
> interaction between 6509 and 8088 code, and nothing practical can be
> gained when exploiting it, I don't think that's an issue.

Agree. This is also why I'm thinking about some kind of simplified
hardware emulation of the 6525's for a (hypothetical) CBM-II replica.
One of the two 6525's seems to only be used as three simple 8-bit I/O
ports while the other seems to be used as an interrupt controller and
two 8-bit I/O ports. It's just the interrupt controller that needs some
work to emulate, the rest seems to be easy to emulate with a bunch of
6821's (the cheapest hardware that is similar to and the grandparent to
the MOS PIO series, i.e.6522, 6525, 6526). If it's ok to ignore the
possibility of switching data direction and hard code each port for the
actual use in an CBM-II it might even be possible to emulate using
simple logic IC's (and maybe shadow ram for reading back what's written
to an output, depending on what's easiest to construct).

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