[Coco] Writing a CPU simulator for the 6809

[Steve Bjork]

On 8/27/2013 5:50 AM, fodavis wrote:
> Now, here's the fun part. You can implement a timing accurate 
> emulation without actually emulating in real time! As long as it 
> doesn't need to interface accurately with external timing sensitive 
> stuff like sound hardware, you can run slower than real time as long 
> as all the relative timings are correct. The machine being emulated 
> won't know the difference. If the hardware running the emulator is 
> fast enough, the simulation will be able to run in real time, of course.

fodavis is hitting the main point about the speed and timing of the 
emulator.  Its not the overall speed nor the timing of the overall 
clocks cycles, But how well the states of each hardware component syncs 
to each other.

As long as you have the VDG, CPU, I/O firing their data at the right 
time then emulation should work with most software.  (Even programs that 
use hardware tricks.)  The key is to know what each bit of hardware is 
doing on each master clock of the system.

You already got a good start with understanding the cycles of the CPU, 
now its time to start dive into the VDG.  Start with the 6846/SAM system 
before diving into the GIME of the CoCo 3.  Remeber, the SAM is memory 
controller and $FF22 VDG mode control the information that feeds into 
the VDG.  Flipping the bits on the SAM and $FF22 is how us video game 
programmers got our little special effect to work.  As side note, the 
VDG reads $FF22 mode control on every scan line.  So, you can change the 
video mode from time and time again on every screen.  I'm sure you did 
not think about what in your emulation of the VDG.

You going to know a lot hardware be the time you done with this project.

Steve