[Coco] Another GIME quirk - looks important!

[Robert Gault]

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.