Video Line Interrupt Control Register

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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)

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).