opcode

As nouns the difference between opcode and operand

is that opcode is a mnemonic used to refer to a microprocessor instruction in assembly language while operand is a quantity to which an operator is applied (in $3\; -\; x$, the operands of the subtraction operator are 3 and $x$).

As nouns the difference between opcode and mnemonics

is that opcode is (computing) a mnemonic used to refer to a microprocessor instruction in assembly language while mnemonics is .