== Programmable raster interrupt ==
The PRI register (at address 6800h) specifies the scan line where the interrupt occurs. The interrupt will occur at the end of that scan line. The raster interrupt will start exactly 10us following the HSYNC start. Unlike the R52 raster interrupt system, it is not dependant on the width of the HSYNC Setting this the PRI register to 0 (the default value at power-up) reverts to the classic [[Gate Array]] R52 raster interrupt system instead.
PRI can be reprogrammed as required to produce multiple interrupts per frame.