Video Line Interrupt Control Register

Number	$22
Readable	Yes
Writable	Yes
Short Description	Controls the timing of raster interrupts and the ULA frame interrupt.

Bit	Function
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)

** Aliases of interrupt enable bits in nextreg 0xC4.

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 Video Line Interrupt Value Register ($23).

Since core 3.1.5 the numbering of lines can be offset by Vertical Video Line Offset Register ($64).