Hardware Hobbyists vs. Emulator Enthusiasts vs Replica Recreators

Josh Dersch derschjo at mail.msu.edu
Sat Jun 20 14:45:37 CDT 2009

Andy Holt wrote:
> I've spent time writing an emulator for a machine I don't own and for
> which I have a certain amount of documentation.
> <<<<
> I've written a couple of emulators a long time ago.
> The more memorable one being for the F8 microprocessor that ran on the ICL
> 1900, was written in
> assembler (probably GIN5, might have been PLAN) and whose execution speed on
> the mainframe was
> almost the same as for a real hardware F8.
> The difficult decision was whether to follow the documentation or the real
> hardware in a very
> common situation.

I ended up hitting the same sort of decision while writing my PERQ 
emulator (or rather, the decision was forced on me at a later point) -- 
the PERQ's CPU contains an "expression stack" which is 16 levels deep.  
According to the official microcode documentation, the behavior of this 
stack on overflow/underflow is "undefined."  I coded the emulator to log 
a warning when this occurred and to clip the value into the range 
[0,16], under the (naive) thinking that the only times this clipping 
code would be hit would be when an emulation error occurred, and I'd 
want to know about it.

Well, that caused a very subtle bug that took me a long time to track 
down.  There is some microcode (notably, a RNG loaded by the game 
PERQMan) that accidentally "leaks" a pop on every invocation and it's 
only the fact that the _real_ hardware wraps the eStack pointer around 
that allows it to run on the real hardware without destroying the 
stack.  (It's still dumb luck that it manages to work at all.)  This was 
crashing horribly on my emulator with really odd results.  After doing a 
lot of debugging, I tracked it to the RNG microcode and made a very 
simple change to the eStack code.  Now I can play Pac-Man on PERQemu, 
and all is well with the world...


> Andy

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