Please read Jim Butterfield's reply below to this thread.
Enjoy.
-Todd Elliott
--------- Forwarded message ----------
From: Jim Butterfield <fjb@freenet.toronto.on.ca>
To: Todd S Elliott <eyethian@juno.com>
Date: Tue, 20 Apr 1999 09:46:21 -0400 (EDT)
Subject: Re: Fw: Re: Dating a C-64 board
Message-ID: <Pine.3.89.9904200943.C28699-0100000@queen>
> On Mon, 19 Apr 1999, Ethan Dicks wrote:
> > --- William Levak <wlevak@cyberspace.org> wrote:
> > > Read and write to PET video memory is crontrolled by an interrupt
> > > generated by the clock on one of the interface chips.
It actually comes from the CRT's retrace circuitry. But I'm being picky.
> > > It is possible to disable this interrupt and speed up the PET.
Yup. See details below.
> > 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.
To be exact, it's the system I/O instructions that do this check. If a
machine language program did its output through CHAROUT ($FFD2), it would
wait for retrace before writing to screen memory. But direct stores to
screen - whether from Basic POKE comands or M/L - would trigger the
"snow" effect. It's most noticeable with M/L, or course, because the
information is delivered so much faster.
It might be interesting to note that the snow is controllable. Long long
ago, Brad Templeton devised a "pixel cursor" that could precisely place
one pixel of "snow" at any selected point on the screen. (Brad authored
the PAL assembler, and is currently operator of the Clarinet news
system).
> > This problem was improved in later models, but BASIC continued to
> > respect the retrace bit.
Nope .. Commodore cleaned up the video ram circuitry and, by the time
BASIC 2 was implemented, didn't need to pay any attention to retrace.
They made noises about how much faster their new operating system was;
but it was mostly the redesign of video that did it.
> I've set this bit on many PET's without burning out anythig. It merely
> turns off the vertical output. This is one of those modern legends.
Sorry to disagree. To recap the tech aspect: in early PET/CBM machines,
Basic waited until "retrace time" before writing to the screen, so that
there would not be a screen-snow problem. It did this by watching the
retrace bit in the VIA chip (bit 5 at address $E840/59424). On earlier
PETs you could defeat this wait by changing the directional register of
this I/O bit; when it became an output, rather than an input, the system
would think that the screen was in retrace all the time, and would
deliver the screen data immediately.
This was quite safe on the earlier machines; but somewhere along the way,
Commodore changed the design so that this could cause failure; after the
design change, there were two TTL-level devices connected to each other,
both trying to output different levels. There were indeed failures, and
early users who rejoiced in the speedup now found that they were
endangering their machines.
> There have been many reports of pokes that will damage the PET. NONE
of
> them are true. When I was servicing PET's I checked out a number of
> these, and never found one that did any damage. I also talked to the
> engineer at Commodore that was in charge of checking out these reports.
> He assured me that he had checked out every report and NONE of them
were
> true. Perhaps this is a manifestation of people's subconscious desire
to
> destroy computers.
There was another one that came in when Commodore introduced the CRT
controller chip to later models of the CBM line ("Fat 40" and 80xx
units). Users playing POKE games with this chip (at $E880/1, decimal
59520/2) could vary the frequency and sizing of the screen raster. Keep
in mind that the business end of this was transistor circuitry driving a
flyback yoke; take the frequency too far out of line and the yoke's
inductance could start to cause damage. I heard a number of reports of
damaged yokes (surprisingly .. I would have thought that the driving
circuitry would have been more susceptible to damage).
In principle, computers should be designed so that software can't damage
hardware. But there were indeed some exceptions in the early PET/CBM
days.
Ya wanna munge the retrace,
Ya'll end up getting smoke;
Ya try to make the screen go faster,
Ya'll wreck the screen and munge the raster,
Be careful: that's no yoke!
--Jim
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