What Is Instruction Set Architecture (ISA)? - Arm

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What is Instruction Set Architecture (ISA)?

An instruction set architecture (ISA) is the abstract interface between a processor’s hardware and the software it runs. It defines the supported instructions, data types, registers, memory access models, and I/O handling, enabling software compatibility across different implementations of a processor family.

Why is ISA Important?

ISAs are foundational to modern computing because they:

• Ensure software compatibility across different generations and configurations of processors within the same ISA family.
• Enable hardware innovation by allowing different microarchitectures to implement the same ISA.
• Support compiler optimization, performance tuning, and efficient system design.
• Facilitate software development across diverse markets like mobile, IoT, infrastructure, and AI.

For example, Arm's ISA supports a broad range of implementations—from power-efficient microcontrollers to high-performance compute platforms—driving innovation across edge and cloud workloads.

How does ISA Work?

An ISA defines the programmer-visible behavior of the processor. Software written in high-level languages is compiled into machine code that conforms to a specific ISA. The processor then executes this machine code by decoding and performing the defined operations. Despite differences in microarchitecture, processors that implement the same ISA can run the same binary software, enabling portability and hardware abstraction. ISAs can be extended to support new capabilities while maintaining backward compatibility. This is crucial for evolving processor designs without breaking existing software.

What are the Key ISA Components or Features?

An ISA typically includes the following elements:

• Instruction set: The full list of machine instructions supported by the processor (e.g., arithmetic, control flow, memory operations)
• Instruction formats: Bit-level layout defining opcodes, operands, and addressing modes
• Registers: Number, types, and roles (e.g., general purpose, floating point, special function)
• Data types: Supported types like integers, floating-point numbers, vectors, or packed data
• Memory architecture: Includes addressing modes, endianness, memory protection, and virtual memory support
• Privilege levels: Execution modes such as user mode and kernel mode to enable secure OS functionality
• Interrupts and exceptions: Mechanisms for handling asynchronous events and fault conditions

FAQs

What is the difference between an ISA and a microarchitecture?

The ISA defines what instructions a processor supports; the microarchitecture defines how those instructions are implemented in hardware.

Can the same software run on all processors with the same ISA?

Yes, as long as the processors conform to the same ISA version and the software was compiled for that ISA.

What are common ISA types?

Common classifications include RISC (e.g., Arm, RISC-V) and CISC (e.g., x86), which differ in complexity and instruction length.

Can ISAs be extended?

Yes, through optional extensions (e.g., SIMD, cryptographic instructions) that maintain backward compatibility.

Why do compilers care about ISAs?

Compilers generate machine code that must align with the ISA's rules for instructions, registers, and memory access.

Relevant Resources

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Arm Cortex CPUs

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• CPU: Executes instructions to perform computing tasks
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