For an instruction, the operation field contains a symbolic operation code(opcode). The assembler translates a symbolic opcode into a machine language opcode. Opcode symbols often describe the operation's function;
For Example, MOV, ADD, SUB.
In an assembler directive, the operation field contains a pseudo-operation code(pseudo-op). Pseudo-ops are not translated into machine code;rather, they simply tell the assembler to do something.