An opcode table (also called an opcode matrix) is a visual representation of all
opcode
In computing, an opcode (abbreviated from operation code) is an enumerated value that specifies the operation to be performed. Opcodes are employed in hardware devices such as arithmetic logic units (ALUs), central processing units (CPUs), and ...
s in an instruction set. It is arranged such that each axis of the table represents an upper or lower
nibble
In computing, a nibble, or spelled nybble to match byte, is a unit of information that is an aggregation of four- bits; half of a byte/ octet. The unit is alternatively called nyble, nybl, half-byte or tetrade. In networking or telecommuni ...
, which combined form the full byte of the opcode. Additional opcode tables can exist for additional instructions created using an
opcode prefix
In computing, an opcode prefix is an numeric value that alters the function of a following opcode. On some instruction set architectures multiple opcode prefixes are allowed sequentially, with all combining to alter the subsequent opcode. The opc ...
.
Table values
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.
Example opcode table
This is the opcode table for the
MOS Technology 6502
The MOS Technology 6502 (typically pronounced "sixty-five-oh-two" or "six-five-oh-two") William Mensch and the moderator both pronounce the 6502 microprocessor as ''"sixty-five-oh-two"''. is an 8-bit computing, 8-bit microprocessor that was desi ...
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 mode
Addressing modes are an aspect of the instruction set architecture in most central processing unit (CPU) designs. The various addressing modes that are defined in a given instruction set architecture define how the machine language instructions ...
s.
Because not all the 256 opcodes are not used, some opcode spaces are blank and the low nibble columns 3, 7, B, and F are missing from the table.
References
{{Reflist
External links
Game Boy LR35902 opcode tableZ80 opcode table
Machine code