[Coco] Writing a CPU simulator for the 6809

Tony Cappellini cappy2112 at gmail.com
Tue Aug 27 15:08:44 EDT 2013


Message: 7
Date: Mon, 26 Aug 2013 23:30:43 -0400
From: Mark McDougall <msmcdoug at iinet.net.au>
Subject: Re: [Coco] Writing a CPU simulator for the 6809
To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
Message-ID: <gxhfgcemixjhhya46nilemkq.1377574128590 at email.android.com>
Content-Type: text/plain; charset=utf-8

>>Emulating the other hardware is no different to emulating the CPU. I'm
having some difficulty >>understanding how you're comfortable with the idea
of software emulation of a CPU but not a >>VDG!?!

1. Partly because my background is in software, although in the last few
years I have spent a lot of time working with microcontrollers, lower level
code and understanding more about hardware.

2. Because I have a reasonably good understanding of how to emulate a CPU
having looked at the source code for the Apple 2 simulator. Executing the
CPU instruction set in software makes sense to me. What I haven't figured
out how to do is to mimic the instruction cycles, so that my software
version appears to run at the same speed as a real 6809. The Apple 2
simulator has code for this, but I don't understand how the execution speed
and isntruction cycle timing is actually achieved. However, running a
software version of any CPU on Windows, Linux or OSX is bound to be
innacurate (instruction cycle timing wise that is), because of all the
background OS activity going on. I would expect these innacuracies to play
hell with video timing, although VCC, MAME & MESS seem to be working very
well.

3. I never really understood the details of the VDG and never really needed
to understand what it does, back when I actually had a color computer. I
think it is solely responsible for generating the correct timing of the
video signal so that the user can see the coco displayed on a monitor.

I really would like to see the sources for VCC.



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