Explain line IRQ differences between VGA and composite/RGB

[mooinglemur]

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.