Elements of machine Instruction

Each instruction must have elements that contain the information required by the CPU for execution. These elements are as follows

• Operation code: Specifies the operation to be performed (e.g.. ADD, I/O). The operation is specified by a binary code, known as the operation code, or opcode.
• Source operand reference: The operation may involve one or more source operands, that is, operands that are inputs for the operation.
• Result operand reference: The operation may produce a result.
• Next instruction reference: This tells the CPU where to fetch the next instruc­tion after the execution of this instruction is complete.

The next instruction to be fetched is located in main memory or, in the case of a virtual memory system, in either main memory or secondary memory (disk). In most cases, the next instruction to be fetched immediately follows the current instruction. In those cases, there is no explicit reference to the next instruction. Source and result operands can be in one of three areas:

• Main or virtual memory: As with next instruction references, the main or vir­tual memory address must be supplied.
• CPU register: With rare exceptions, a CPU contains one or more registers that may be referenced by machine instructions. If only one register exists, reference to it may be implicit. If more than one register exists, then each register is assigned a unique number, and the instruction must contain the number of the desired register.
• I/O device: The instruction must specify (he I/O module and device for the operation. If memory-mapped I/O is used, this is just another main or virtual memory address.