Re: 6561 (PAL VIC-I) interlace mode bit

From: Richard Atkinson <>
Date: Fri, 26 Aug 2011 19:59:23 +0100
Message-ID: <BDD4F016BA2D466BB43A6119E2367332@abion>
Looks like they've intentionally disabled the function of the INT 
(interlace) bit. Maybe it interfered with the PAL encoding system in some 

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" <>
Sent: Friday, August 26, 2011 4:21 PM
To: <>
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
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Received on 2011-08-26 20:00:02

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