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.
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.
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.