Instruction set

An instruction set, or instruction set architecture (ISA), describes the aspects of a computer architecture visible to a programmer, including the native datatypes, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O (if any).

Related Topics:
Register - Addressing mode - Interrupt - Exception

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An ISA is a specification of the set of all binary codes (opcodes) that are the native form of commands implemented by a particular CPU design. The set of opcodes for a particular ISA is also known as the machine language for the ISA.

Related Topics:
Opcode - CPU design - Machine language

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"Instruction set architecture" is sometimes used to distinguish this set of characteristics from the microarchitecture, which is the set of processor design techniques used to implement the instruction set (including microcode, pipelining, cache systems, and so forth). Computers with different microarchitectures can share a common instruction set. For example, the Intel Pentium and the AMD Athlon implement nearly identical versions of the x86 instruction set, but have radically different internal designs.

Related Topics:
Microarchitecture - Microcode - Pipelining - Cache - Intel - Pentium - AMD - Athlon - X86

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This concept can be extended to unique ISAs like TIMI (Technology-Independent Machine Interface) present in the IBM System/38 and IBM AS/400. TIMI is an ISA that is implemented as low-level software and functionally resembles what is now referred to as a virtual machine. It was designed to increase the longevity of the platform and applications written for it, allowing the entire platform to be moved to very different hardware without having to modify any software except that which comprises TIMI itself. This allowed IBM to move the AS/400 platform from an older CISC architecture to the newer POWER architecture without having to rewrite any parts of the OS or software associated with it.

Related Topics:
IBM System/38 - IBM AS/400 - Virtual machine - CISC - POWER

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When designing microarchitectures, engineers use Register Transfer Language (RTL) to define the operation of each instruction of an ISA.

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An ISA can also be emulated in software by a interpreter. Due to the additional translation needed for the emulation, this is usually slower than directly running programs on the hardware implementing that ISA. Today, it is common practice for vendors of new ISAs or microarchitectures to make software emulators available to software developers before the hardware implementation is ready.

Related Topics:
Emulate - Interpreter

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