Re: CBM 1541 (fwd)

From: Nicolas Welte (
Date: 2000-11-20 10:33:45

"COPLIN, Nicholas." wrote:
> [COPLIN, Nicholas.]  Wow... all a lot more subtle than I first expected...
> >From testing over the weekend it seems that simply using the 80column mode
> on the C128 doesn't disable the VIC-II access either, you need to manually
> poke the blanking codes...

On the C128 it is even MUCH more sophisticated. Since the 80 column chip
is completely independant from the rest of the system, its use does not
affect any memory accesses there. The VIC-IIe has two new registers, one
of which has a TEST bit and the 2MHz bit. The 2MHz bit disables most of
the VIC-IIe's memory accesses (probably it simply tristates the bus),
but it does not blank the screen. You have to do that manually. Also,
the VIC-IIe will keep doing the memory refresh accesses on each scanline
and this causes a slowdown to 1 MHz for I think five cycles on each
line. The effects of the TEST bit on the memory accesses is a mostly
unexplored story, only recently it was found that its use has effects on
some internal counters, which makes the manipulation of video timings

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