Posted By
IstvanV
on 2008-10-29
09:40:59
|  Re: plus/4 raster interrupt
> The only reason I can think of for this value to be #$A1 and not something else is that #$A1 is the raster line number that is the division between top and bottom parts of a split screen.
Yes. The bottom part actually begins at line $A4 with the default vertical scroll setting, but it takes some time until the kernal actually sets the TED registers, so the interrupt is set slightly earlier. For a clean division, $FF14 should be set before line $A3, and $FF06, $FF07, and $FF12 should be set between lines $A3 and $A4.
There is also another interrupt after the end of the 40x25 text area (which is between lines 4 and $CC). This is the main raster interrupt where things like scanning the keyboard matrix are done.
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