Explain line IRQ differences between VGA and composite/RGB #201
Description
In interlaced modes, the least significant bit of IRQLINE_L is ignored, so that raster interrupts end up happening on both fields.
On the VERA, the raster interrupt is delivered at the very end of the line that you call for, so your code will get control as the next line is scanning out. If you were to change the scroll of a layer, it would take effect on the next line.
| Output type | IRQLINE set to | IRQ delivered | Control gained on line | Scroll offset changed on line |
|---|---|---|---|---|
| VGA | 14 | end of 13 | 14 | 15 |
| VGA | 15 | end of 14 | 15 | 16 |
| RGB/NTSC even field | 12 or 13 | end of 12 | 14 | 16 |
| RGB/NTSC odd field | 12 or 13 | end of 13 | 15 | 17 |
| RGB/NTSC even field | 14 or 15 | end of 14 | 16 | 18 |
| RGB/NTSC odd field | 14 or 15 | end of 15 | 17 | 19 |
This means that for split-screen parallax, NTSC and RGB outputs need their raster interrupts set one line earlier than for VGA. For instance, 14 on VGA requires 12 (or 13) on NTSC.