Looks like they've intentionally disabled the function of the INT (interlace) bit. Maybe it interfered with the PAL encoding system in some way. On the C128 VIC-IIe, you can really mess up the PAL colour encoding by stealing (IIRC) odd numbers of lines using the TEST bit. On the TV I tested this on several years ago, the colour disappeared and would occasionally flicker back on but mostly stayed off. In PAL the alternation of the R-Y component of the colour burst and colour information must be maintained each line. On the PAL C128 the number of lines is even (312) and so a complete sequence of 156 PAL switch alternations occurs every frame. Possibly the PAL switch is tied directly to the LSB of the Y counter. If this were the case, it wouldn't work well with interlace mode. -------------------------------------------------- From: "Segher Boessenkool" <email@example.com> Sent: Friday, August 26, 2011 4:21 PM To: <firstname.lastname@example.org> Subject: Re: 6561 (PAL VIC-I) interlace mode bit > I've looked at the Y decoder on the 6561 (PAL) chip. It decodes: > > Y=1 start of vblank > Y=10 end of vblank > Y=4 start of vsync > Y=7 end of vsync > Y=311 last line of (even?) frame > Y=312 and INT and not INT last line of (odd?) frame > > so interlace mode does nothing in effect: 311 is enabled on > every frame, 312 is never enabled. > > Would be fun to compare with a 6560 :-) > > > Segher > > > Message was sent through the cbm-hackers mailing list Message was sent through the cbm-hackers mailing listReceived on 2011-08-26 20:00:02
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