[Coco] Another GIME quirk - looks important!
Robert Gault
robert.gault at worldnet.att.net
Sun Jun 21 09:42:44 EDT 2009
Here is some quite strange behavior in an '87 GIME. Could be a bug but
it also indicates there are still some hidden "features" existing in all
retail Coco3s.
The recent thread on oddball Coco3 graphics modes was an attempt to
obtain information on the internal workings of the GIME. Further
exploration in this vein discovered a VSYNC circuit interaction with
oddball video settings.
$FF98 bits 2 1 0
1 1 1 reserved - seems to repeat the first horizontal row
for all other rows
$FF99 bits 6 5
1 0 reserved - some speculation on 210 lines per field,
actually something else
$FF93 bit 5 FIRQ Timer
3 FIRQ VSYNC
If you set $FF98 Lines Per Row = 111 and $FF99 Line Per Field = 10, FIRQ
VSYNC no longer functions! This is the first hard evidence for why you
lose the upper and lower borders with LPF=10 and/or get screen scrolling.
This is not likely to be of any use in programming but it does reflect
the inner workings of the GIME.
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