Synchronizing code with the scan line

[quor@qedbbs.com (Matthew Ghio)]

philip@labtam.labtam.oz.au (Philip Stephens) writes:
 
>  I don't have the Apple //GS hardware reference, but I do have _Inside
>the Apple //GS_.  It listed $C02E as something akin to the scan line
>counter, but was coy about it's real function and said only the firmware
>uses it.  Is this what I'm looking for, and if so, what does it do
>exactly and what are the cavets in using it?

Yep, that's it!  The only caveat is that since more than 256 lines are 
scanned, the count won't fit in one byte.  So, $C02E is only incremented 
every two lines.  To get that last bit to tell you exactly what line 
you're on, you need ot read bit 7 of $C02F.  Bits 0-6 of C02F are the 
horizontal count, by the way.