Re: Dating a C-64 board

From: Ethan Dicks (
Date: 1999-04-19 21:09:58

--- William Levak <> wrote:
> Read and write to PET video memory is crontrolled by an interrupt
> generated by the clock on one of the interface chips. 
> It is possible to disable this interrupt and speed up the PET.  However,
> when this is done, it results in "snow" on the screen.  Simply put,  the
> cirtuitry is not quite as fast as the numbers say it should be, and it is
> necessary to resort to this scheme to produce an acceptable display.

IIRC, that interrupt is not an interrupt per se, but a bit that can be
read (at 59460?)  It's the origin/cause of the killer POKE.  BASIC checks
to see if it is safe to alter screen memory.  M/L does not.  I have several
animations that perform the "speed poke" to turn the input on that retrace
bit to an output.  The software effect of this is to cause the BASIC print
routine to always print *now* and not wait for retrace.  The hardware effect
on pre-6845 PETs is nil.  Nothing minds if you turn this input bit around.
Later PETs got most unhappy if you toggled this signal - I was told that the
effect was to short-circuit a totem-pole transistor arrangement, and one of
the transistors in the video circuit couldn't pass that much current, and

BASIC 1 on the static PETs had snow problems during screen scrolling in BASIC.
This problem was improved in later models, but BASIC continued to respect the
retrace bit.

Anyone else case to correct/expound on this?


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