The Instruction Set Architecture (ISA) is the part of the processor that is visible to the programmer or compiler designer. They are the parts of a processor design that need to be understood in order to write assembly language, such as the machine language instructions and registers. Parts of the architecture that are left to the implementation ate not part of ISA. The ISA serves as the boundary between software and hardware.  

The term instruction will be used in this unit more often. What is an instruction? What are its components? What are different types of instructions? What are the various addressing schemes and their importance? This unit is an attempt to answer these questions. In addition, the unit also discusses the design issues relating to instruction format. We have presented here the instruction set of MIPS (Microprocessor without Interlocked Pipeline Stages) processor (very briefly) as an example. 

Objectives

After going through this unit you should be able to:

• describe the characteristics of instruction set;
• discuss various elements of an instruction;
• differentiate various types of operands;
• distinguish various types of instructions and various operations performed by the instructions;
• identify different types of ISAs on the basis of addresses in instruction sets;
• identify various addressing schemes; and
• discuss the instruction format design issues.