Posts by -trb-

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Don't Panic. Please wash hands.

    64 cycle (6567R56A) VICII was the first VICII that was released in the US (so its NTSC). This type of VICII is kind of rare, and it will cause problems with many C64 games, it has the "sparkle" problem, [...]

    It's not the VIC-II:


    Sparkle was widely attributed to bugs in the video chip that was the heart of the system, but in fact it was caused by a ROM chip of which 3 million were in service with no problems in other types of system, including the hit arcade video game Asteroids. Commodore engineers themselves first looked for the problem in the video chip. It took them three weeks to spot the ROM chip as the source of the defect, Charpentier said. "The problem was a random event - it didn't happen all the time. We thought the video chip was for some reason seeing wrong data. We didn't even suspect it could be the ROM. Finally we put the logic analyser on it and tracked it down." The ROM, which Charpentier and his group has designed years earlier, had a special precharging circuit to make it run faster, but the circuit made it sensitive to spurious signals. The video circuitry and the 6510 microprocessor alternated in controlling the system bus, and when control passed from one to the other, voltage spikes were sometimes generated.


    "It just happened that we hit the exact timing," Charpentier said. "If the spike had been a few nanoseconds shorter or longer, it wouldn't have been a problem. The spike was just wide enough that the ROM saw it as a valid address. It would ignore the next address request and give the video chip wrong data." Since the ROM contained the C64 character set, the screen display would be littered with random slices of characters.


    According to Nelson of Epyx, "This confetti interference-looking stuff on the screen, glowingly referred to as sparkle, has an extremely un-nice property: it causes hardware collisions - the sprites believe it really exists." Since the sparkle was causes by inappropriate data few to the video chip, it triggered the circuitry responsible for checking whether the movable display objects - sprites - were overlaying background objects on the screen. So software that depended on collision sensing to control the movement of objects on the screen would go berserk when confronted by sparkle.

    Input devices can be selected by pressing the Commodore key and 'i' together. Then use cursor keys to move the pointer and <RETURN> for confirming your choice and also for clicking the OK button. Of course the appropriate driver for 1351 has to be present on the current drive or on the system disk (can't remember in what way GEOS looks on disk and/or drives for drivers).