Re: Dating a C-64 board

From: Andre Fachat (
Date: 1999-04-19 11:16:04

William Levak wrote:
> The VIC20, on the other hand, uses the old scheme from the PET.  The
> screen is updated every thirtieth of a second, using a system interrupt.
> This is fast enough for the disk drive, but not for the cassette, hence
> the screen is blanked while accessing the cassette.

I don't know about the VIC20, but the PET does not refresh the screen
every thirtieth of a second. Instead video memory is always read
during Phi1, the clock phase where the CPU does not need the bus.
I.e. video memory read is interleaved with CPU access and does not
interfere in any way in the PET.

(The 80 columns PET even read two bytes during Phi1, which is
possible due to parallel RAM chips (8032) or faster DRAMs (8296))


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Andre Fachat, Institute of physics, Technische Universität Chemnitz, FRG
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