Raster Interrupt Control Register
|Short Description||Controls the timing of raster interrupts and the ULA frame interrupt.|
|7||(R) INT signal (even when Z80N has interrupts disabled) (1 = interrupt is requested)
(W) Reserved, must be 0
|6-3||Reserved, must be 0|
|2||If 1 disables original ULA interrupt (Reset to 0 after a reset)|
|1||If 1 enables Line Interrupt (Reset to 0 after a reset)|
|0||MSB of Line Interrupt line value (Reset to 0 after a reset)|
The line interrupt value uses coordinate system of Copper coprocessor, i.e. line 0 is the first line of pixels. But the line-interrupt happens already when the previous line's pixel area is finished (i.e. the raster-line counter still reads "previous line" and not the one programmed for interrupt). The INT signal is raised while display beam horizontal position is between 256-319 standard pixels, precise timing of interrupt handler execution then depends on how-quickly/if the Z80 will process the INT signal.
The LSB part of desired interrupt line is in Raster Interrupt Value Register ($23).