An opcode table (also called an opcode matrix) is a visual representation of all opcodes in an instruction set. It is arranged such that each axis of the table represents an upper or lower nibble, which combined form the full byte of the opcode. Additional opcode tables can exist for additional instructions created using an opcode prefix.

The structure and arrangement of an opcode table appears as follows:

Each cell from 00–FF contains information about the operation such as the equivalent assembly instruction corresponding to the opcode, parameters, and CPU cycle counts.

This is the opcode table for the MOS Technology 6502 microprocessor from 1975. The 6502 uses 8-bit opcodes. Of the 256 possible opcodes available using an 8-bit pattern, the original 6502 uses only 151 of them, organized into 56 instructions with (possibly) multiple addressing modes. Because not all 256 opcodes are used, some opcode spaces are blank and the low nibble columns 3, 7, B, and F are missing from the table.