[Coco] GIME behavior

[Mark Marlette]

Darren,

As you are finding out all CoCos are not created equally.

Can you talk about your project a bit more? If not I understand.

I have some guesses as to why one would be tracking $ff90. :)

Regards,

Mark
Cloud-9

----- Original Message -----
From: "Darren A" <mechacoco at gmail.com>
To: "CoCoList for Color Computer Enthusiasts" <coco at maltedmedia.com>
Sent: Monday, April 11, 2011 4:39:18 PM
Subject: [Coco] GIME behavior

Does anyone know if the GIME drives the CPU data bus when you read
from a write-only register?

I have a hardware project which stores writes to $FF90 in a separate
latch. I had figured I could also use address $FF90 to read the latch
since this is a write-only register in the GIME. No such luck.

I know my hardware is working because when connected to a CoCo 1 or 2
I can read and write $FF90 as if it were RAM.  On a CoCo 3 the latch
does record writes (verified with LEDs), but trying to read it back
does not work.  The only explanation I can think of is that the GIME
does drive the data bus on a read of $FF90 even though it provides no
useful data.

Darren

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