No operation. An assembly language instruction in several architectures, used in timing loops, to "comment out" binary code, or to leave space for future code expansion.

The official NOP ($EA) in the 6502 architecture is one byte and two cycles, but there are plenty of other NOPs in the original 6502 opcode matrix. The one that produces the fastest path through the binary code (one cycle per byte) is STA immediate.

See also: 6502 instructions | 6502 addressing modes

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