Video Line Interrupt Control Register
From SpecNext official Wiki
| 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 display 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 when horizontal position is somewhere between 256-319 standard pixels and previous line drawing is finished). So the code in interrupt handler will start execution when the display beam is still in H-BLANK area and just preparing to start beaming the desired line.
The LSB part of desired interrupt line is in Video Line Interrupt Value Register ($23).