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 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 > fried. > > 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? 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. 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. Bill - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail cbm-hackers-request@dot.tcm.hut.fi.
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