An instruction set architecture or ISA is one of the three major components of a computer architecture alongside microarchitecture and systems design. It specifically corresponds to a conceptual or abstract model of a computer. Furthermore, as its name implies, it defines the set of instructions a computer processor must support.
Note that computer architecture is a set of rules and methods that describe the organization, implementation, and functionality of a computer system. A microarchitecture, on the other hand, describes how a particular microprocessor will implement the ISA while systems design represents all other require hardware other than a processor to run a computer system.
What is an Instruction Set Architecture
Understanding the Purpose of an ISA
The general purpose of an instruction set architecture centers on defining how the processor such as a central processing unit is controlled by the software. It fundamentally serves as a bridge, as well as a boundary between software and hardware.
More specifically, however, an ISA specifies the set of basic operations a computer must support, including the functional definition of operations, as well as the detailed descriptions of how to invoke and access these operations.
It essentially pertains to the programming built in a processor or emulated via a software. it can also be considered as a machine language because it provides a set of rules to the processor, thereby telling it what it needs to do to run a computer system.
A typical ISA includes instructions for handling data and data transfer, memory operations, arithmetic and logic operations, addressing modes, control flow operations and floating point, the type and size of operands, and co-processor instructions.
Importance of an Instruction Set
The importance of an instruction set architecture revolves around enabling software developers to write more efficient codes or, more specifically, write a software program or application that will efficiently work with a particular computer system.
Hence, it is essential for these developers to understand what a particular ISA can do and how a compiler makes use of the specified set of instructions. Another importance is that it can be useful for understanding the output of the compiler, thus allowing easier debugging.
An ISA also determines the capabilities of a computer. Of course, the desired capabilities of a particular system would only be possible by using a suitable instruction set. In a way, an ISA can determine and affect the performance of a computer system.
Major Instruction Set Designs
The design of an instruction set architecture is a relatively hotly debated issue. There are two major designs: complex instruction set computer or CISC and reduced instruction set computer or RISC. CISC was the first widely used design.
However, American technology company IBM analyzed CISC in the 1970s and concluded that there is a large volume of instructions in the set that could be eliminated while still maintaining operability. This resulted in the introduction of RISC.
Both CISC and RISC have their respective advantages and disadvantages. The x86 instruction set from Intel is derived from CISC. Most Windows-based computer systems, as well as relevant applications, are designed based on this instruction set.
On the other hand, ARM architecture from Arm Ltd. is derived from RISC architectures. It is the common architecture found in the implementation of Android and iOS computer systems. Apple began using ARM-based processors with the launch of its M1 series of chips.
It is important to note there are other types and examples of instruction set designs. These designs differ in terms of the intended use cases. Some are very simple and straightforward, enough to operate simpler computers such as scientific calculators.