[Coco] What do you make of this non-approved HSCREEN mode?

Robert Gault robert.gault at worldnet.att.net
Sun Jun 7 22:13:19 EDT 2009

Paul Fitch wrote:
> <snip>
>> The control byte at $FF99 changes what the video circuitry shows in
>> Coco3 mode. The MMU bytes control what part of memory you 
>> have actual access to for reading or making changes.
>> In light of this, care to modify what you said in your last 
>> paragraph? :)
> Not particularly<g>.  I'm not an expert, but I'll assume that the paragraph
> above comes from the "official" documentation.  Yet you have evidence that
> "undocumented" things are happening.  How do you reconcile the differences?
> You can't until you fully understand whats happening.
> You obviously have more experience with the coco's hardware and software
> than I do.  But part of my day job is trouble shooting problems.  And I've
> learned that you can't fix a problem, you can't even accurately explain it,
> until you know everything that's going on.  
> That program you wrote shouldn't do what it is doing.  Ie, we don't have the
> full picture.    

I completely agree with that last comment. :) The program elicits some 
very strange and seemingly impossible behavior from the Coco3.

Personally, I've no idea what could cause this short of a poorly 
designed GIME.

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