I've heard the 6502 was originally supposed to have an additional opcode, but when they made it the silicon was bugged, so they advertised the chip as having one less opcode. Can anyone identify what this opcode is or what it does/was supposed to do?

The ROR instructions didn't work properly on the initial silicon, and were left out of the earliest data sheets (how can it be broken if it's undocumented?). Within a few months, ROR was fixed and officially added to the repertoire, and the rest is history.

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Got a kilobyte lying fallow in your 65xx's memory map? Sprinkle some VTL02C on it and see how it grows on you!

Mike B. (about me) (learning how to github)

Ok thanks, so all the opcodes that were designed in function correctly; that's what I was trying to figure out.

LOL! Well, no! The ROR issue was simply the most serious problem on the NMOS chip, and it got fixed promptly. But the NMOS '02 retained several other idiosyncracies, some of which can be considered bugs. Probably the most notable is the jump indirect instruction (opcode $6C), which in some circumstances behaves in a way that's unexpected and/or undesired by most people.

When the CMOS chip (65C02) was released, there were numerous changes, most of them minor. But even the 'C02 has a subtle quirk or two remaining.

-- Jeff

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
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