VIDC1 / RO3.1 frame store emulation

I see plenty of comments there ;-)

; Calculate corresponding Y offset to use
LDR     r0, mbbase
SUB     r0, r1, r0
LDR     r1, mbpitch
DivRem  r2, r0, r1, lr

Get the desired byte offset from the base of the screen memory, then divide it by the row pitch to work out the row number to scroll to

; Clamp
CMP     r2, #0
MOVLT   r2, #0
LDR     r0, mbyvres
LDR     r1, mbyres
SUB     r0, r0, r1
CMP     r2, r0
MOVLE   r0, r2

Clamp row number such that 0 <= row < (buffer height – screen height)

; Same as current?
LDR     r2, mbyoff
CMP     r0, r2
EXIT    EQ

Don’t trigger an update if we’re already scrolled to that position

; Set flag requesting update
STR     r0, mbyoff
STR     pc, UpdateNeeded
EXIT

Store the position we want to scroll to and set UpdateNeeded to a non-zero value in order to trigger a message to be sent to the GPU the next time the update function runs (which is triggered by a repeating timer IRQ)

MergeUpdate is the function which sends the scroll message to the GPU. It generates it from a template message (mergetags). X offset is always 0, Y offset is set to mbyoff.

Could GraphicsV be extended so graphics driver restrictions can be discovered? The start alignment and width modulus differ across video drivers, but there’s no way of discovering what they are.

Likewise, minimum screen width and height would be useful.

And a GraphicsV call to return the logical DAG address would be useful, instead of having to re-map the memory via OS_Memory 13 to discover it.

That kind of stuff will probably be coming at some point, yeah.