Re: Hardware emulation of 6509 using 6502?

From: Mia Magnusson <mia_at_plea.se>
Date: Wed, 14 Mar 2018 01:10:45 +0100
Message-ID: <20180314011045.00003ec2@plea.se>
Den Tue, 13 Mar 2018 23:29:38 +0000 (UTC) skrev Steve Gray
<sjgray@rogers.com>:
> Very intriguing. That plus an internal 1MB ram expansion for the
> B-series would make an interesting machine. Steve 

Or a new machine? :)

With 1MB ram, the logic required to both emulate a 6509 and also run a
65816, you have about half a B machine. Might aswell make a completely
new machine, compatible with the B machines.

I'm (extremely slowly) working on making a copy of the B schematics in
KiCad. (That's btw why I sent an update of the KiCad 6509 definition).
So far I've drawn the first two pages, with the CPU and all the timing
generation circuits.

To fully understand how the B works, I think I need some kind of
software to make timing diagrams. I don't know if any such software
exist, I've actually been tinkering with the idea to use project
planning software as the min/max delay time in each gate could be
treated as min/max time for a task in a project, and how signals depend
on previous delays can be treated like how a task in a project depends
on earlier tasks to finish. That would only display one full CPU cycle
though, but that is probably no problem.

The timing generation on the B is IMHO an example of bad engineering.
The basic idea is sound, to divide the 18MHz dot clock by 9 with a shift
register wired up as a ring counter and use those outputs to control
stuff. However some stuff looks horrible like they use 74*74 flip flops
hooked up with one output connected to it's set or reset input to
create a short spike on the outputs each time a pulse reaches the clock
input. This makes the function really depending on gate times not only
being fast enough but also slow enough (!).

The P has far less of this stuff as the VIC chip generates a lot of the
timing and also as it runs at half the clock frequency compared with
the B.

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Received on 2018-03-14 22:45:13

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