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A tricky Commodore PET repair: tracking down 6 1/2 bad chips
In 1977, Commodore released the PET computer, a quirky home computer that combined the processor, a tiny keyboard, a cassette drive for st...
This was actually reassuring since it showed that most of the computer was working: not just the monitor, but the video RAM, character ROM, system clock, and power supply were all operational. With an oscilloscope, I examined signals on the system bus and found that the clock, address, and data lines were full of activity, so the 6502 CPU seemed to be operating. Specifically, it grabbed the STX instruction (86) and used that as part of the address, writing FF (a checkerboard character) to screen memory at 865E 3 instead of to the BASIC data structure at 005E.
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