An Instruction Set Architecture (ISA) is part of the abstract model of a computer that defines how the CPU is controlled by the software. The ISA acts as an interface between the hardware and the software, specifying both what the processor is capable of doing as well as how it gets done.
The ISA provides the only way through which a user is able to interact with the hardware. It can be viewed as a programmer’s manual because it’s the portion of the machine that’s visible to the assembly language programmer, the compiler writer, and the application programmer.
The ISA defines the supported data types, the registers, how the hardware manages main memory, key features (such as virtual memory), which instructions a microprocessor can execute, and the input/output model of multiple ISA implementations. The ISA can be extended by adding instructions or other capabilities, or by adding support for larger addresses and data values.
