What is instruction ? Describe the instruction formats

Instruction: In computer science, an instruction is a single operation of a processor defined by the processor instruction set.

Instruction format:

Label field	Op-code field	Operand field	Comment field
NEXT	ADD	AL 07H	ADD CORRECTION FACTOR

Figure: Instruction format

Instruction format has four fields, as shown in figure.

The first field in an assembly language statement in the label field, a label is a symbol or group of symbols used to represent an address which is not specifically known at the time the statement is written. Labels are usually followed by a colon. Labels are not required in a statement; they are just inserted where they are needed.

The op-code field of the instruction contains the mnemonic for the instruction to be performed. Instruction mnemonics are sometimes called operation codes or op-codes. The ADD mnemonic in the example statement in the figure indicates that we want the instruction to do an addition.

The operand field of the statement contains the data, the memory address, the port address, or the name of the register on which the instruction is to be performed. In the example instruction in figure there are two operands, AL and 07H. Specified in the operand field; AL represents the number 07H. This assembly language statement thus says, “ADD the number 07H to the contents of the AL register. ” The final field in an assembly language statement such as that in figure is the comment field, which starts with a semicolon. Comments do not become part of the machine language program, but they are very important.