[Coco] (very very slightly OT) 6800 emulator

[jdaggett at gate.net]

On 29 Apr 2007 at 21:40, Manney wrote:

> Joel Ewy wrote:
> 
> >>> Interesting. Did you know the first 6809 emulator was written as a
> >>> 6800 program? It was made during the design of the 6809 at
> >>> Motorola.
> >> Isn't that a bit backwards? :) But this question is coming from
> >> someone who isn't quite as in depth about 680x processors as he'd
> >> like to be. :)
> >>
> > It isn't backwards so much as forward, or ahead of its time.  :) How
> > better to test a microprocessor that doesn't exist yet but to
> > emulate it on one that does?
> 
> Well, I thought that normally the 'host' processor had to be more
> powerful than the emulated processor to handle the emulation?
> 
> -M.
****************

Considering back in the mid 70's there were not the programs to simulate an HDL design 
like today. There was a board with logic that simulated the internal structure of the 6809 
that used a 6800 processor as the core. You also have to remember that the 6809 is built 
on the 6800 processor. The actual ASIC is very similar internally than one would tend to 
believe. Motorola did not do a complete redesign of the 6800 to achieve the 6809. Instead 
it used what was alerady done in the 6800 and altered what was needed to achieve the 
end results. 

The 6809 is not object code upward compatible but many of the pnuemonics do the same 
function with different opcodes. That is why some 6800 code need only be reassembled in 
order to work on a 6809. 

james