[Coco] GIME behavior

[Boisy G. Pitre]

I believe Mark's assumption came from some code that I wrote, which is obviously incorrect.  $FF90 does indeed appear to be write only.

I'm not a hardware guy, but couldn't you do your own decoding of the address, clock and R/W lines to track writes to $FF90, storing that value somewhere as well as writing it to the "true" $FF90 address, then intercepting the read call and returning the value then?  Is there a hardware scheme to mask an address to do this?

On Apr 13, 2011, at 11:16 PM, Darren A wrote:

> On 4/13/11, Mark Marlette wrote:
>> 
>> Lucky, no way.....Luck and magic, not here, designed, engineered. Boisy is
>> the software guy. There is a method to this as I recall but has at least
>> been a year if not more since I looked at this. I will have to go look at my
>> notes from my test code. Not going to happen for at least another ~20hrs
>> when playtime comes around again.....
>> 
>> This a REALLY important address for the games we play at C-9 with the
>> hardware.
>> 
>> So if you can't mode change then you might want to explain a bit more on
>> your project as I don't quite follow the design. If this is going to be a
>> product and you don't want to be detailed. I understand as I hope you will
>> as well.
>> 
> 
> ---
> 
> I am tracking $FF90 in my hardware so that I can make the mode change
> and restore it later.  The problem was that I wanted to use the same
> address ($FF90) to read the tracked state.  Since this does not appear
> to be possible I will have to proceed by using an address outside the
> GIME range.
> 
> If you can describe some other method to obtain the value in $FF90
> without having to go to this extra effort then that would be much
> appreciated.
> 
> Darren
> 
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Boisy G. Pitre
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