Bit 5, VERTB, causes an interrupt at line 0 (start of vertical blank) of the video display frame. The system is often required to perform many different tasks during the vertical blanking interval. Among these tasks are the updating of various pointer registers, rewriting lists of Copper tasks when necessary, and other system-control operations. The minimum time of vertical blanking is 20 horizontal scan lines for an NTSC system and 25 horizontal scan lines for a PAL system. The range starts at line 0 and ends at line 20 for NTSC or line 25 for PAL. After the minimum vertical blanking range, you can control where the display actually starts by using the DIWSTRT (display window start) register to extend the effective vertical blanking time. See Chapter 3, "Playfield Hardware," for more information on DIWSTRT . If you find that you still require additional time during vertical blanking, you can use the Copper to create a level 3 interrupt . This Copper interrupt would be timed to occur just after the last line of display on the screen (after the display window stop which you have defined by using the DIWSTOP register).
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