I noticed on a real 130XE that the VCOUNT register can report 131. This is completely undocumented because the internal VCOUNT goes from 0 to 261 (262 scanlines), so could never theoretically reach 131.
Note that the 131 value is extremely rare. I wrote a basic proram to read in vcount and compare against 130 and 131. It hits 130 1000x more often than it does 131. I'm hoping that's not an artifact of using basic to do the coding.
If internally the Antic increments VCOUNT, then a cycle later does a compare to reset it back to 0 then it's possible to read a 131 for one cycle or so.
Does anyone know what Antic is doing? I'd love a *clear* internal schematic of the chip but that's probably long gone. I'm trying to write a hardware description of the chip but I'm missing a lot of info. (The hardware description would be great for a perfectly accurate but quite slow emulator)
VCOUNT hits 131? 
