For this machine there can be two more possible addressing modes in addition to direct andimmediate.
Opcode field of an instruction is a group of bits which define various processor operations like LOAD, STORE, ADD and SHIFT to be performed on some data stored in registers or memory.
The operand address field can be data or can refer to data- which is address of data or can be labels which may be the address of an instruction you want to execute next, these kind of labels are generally used in Subroutine call instructions. An operand address may be:
- CPU register address
- Memory address
- I/O device address
The mode field of an instruction specifies a range of alternatives for referring to operands using the given address. Please note that if operands are placed in processor registers then an instruction executes faster than that of operands placed in memory as the registers are very high-speed memory used by CPU. Though to put the value of a memory operand to a register you will need a register LOAD instruction.