Barn Find MOS MCS6502 – A Restoration

A cool find. It’s so old, it has the ROR bug. :slight_smile:

In car collecting one of the ‘holy grail’ experiences is the ‘Barn Find’ finding and recovering a rare vehicle that has sat untouched, in some barn, or shed for some time. They are often in rough, but original condition and can evoke much excitement. As it turns out CPUs are not so different. I recently purchased a very rough and very old ATARI Arcade board.
[It was] in terrible condition, with lots of oxidation and ‘stuff’ on it. But it also had a white MOS 6502 processor. These are some of the very first CPUs made by MOS and are rather desirable, as in addition to their use by ATARI, they were used in the very first Apple computer, the Apple 1.
I stick it in the venerable 680x/650x Test Board, ensuring the board is configured right before applying power, and then…flip the switch…and the sight everyone wants to see Blinking LEDs! It passes a function check with flying colors, and when further tested reveals that it is indeed old enough to have the (in)famous ROR bug. The first MCS6502 did not support the ROR (Rotate Right) instruction. It was in fact present, but behaved incorrectly. Michael Steil over at has an excellent article on how the ROR instruction was broken. MOS had chips with working ROR available in June of 1976. That’s RIGHT after this particular 6502 was made, making it one of the very last ROR bug 6502s made.