Instruction Set Simulator - AbsoluteAstronomy.com

An instruction set simulator (ISS) is a simulation

Simulation

Simulation is the imitation of some real thing available, state of affairs, or process. The act of simulating something generally entails representing certain key characteristics or behaviours of a selected physical or abstract system....

model, usually coded in a high-level programming language

High-level programming language

A high-level programming language is a programming language with strong abstraction from the details of the computer. In comparison to low-level programming languages, it may use natural language elements, be easier to use, or be from the specification of the program, making the process of...

, which mimics the behavior of a mainframe or microprocessor

Microprocessor

A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...

by "reading" instructions and maintaining internal variables which represent the processor's registers

Processor register

In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...

.

Instruction simulation is a methodology employed for one of several possible reasons:

To simulate the machine code
Machine code
Machine code or machine language is a system of impartible instructions executed directly by a computer's central processing unit. Each instruction performs a very specific task, typically either an operation on a unit of data Machine code or machine language is a system of impartible instructions...

of another hardware device or entire computer for upward compatibility
Compatibility
Compatibility may refer to:* Astrological compatibility* Compatibilism – a philosophical position* Compatibility * Compatibility * Compatibility * Electromagnetic compatibility* Interpersonal compatibility-Computing:...

-- a full system simulator typically includes an instruction set simulator.

For example, the IBM 1401

IBM 1401

The IBM 1401 was a variable wordlength decimal computer that was announced by IBM on October 5, 1959. The first member of the highly successful IBM 1400 series, it was aimed at replacing electromechanical unit record equipment for processing data stored on punched cards...

was simulated on the later IBM/360 through use of microcode

Microcode

Microcode is a layer of hardware-level instructions and/or data structures involved in the implementation of higher level machine code instructions in many computers and other processors; it resides in special high-speed memory and translates machine instructions into sequences of detailed...

emulation.

To monitor and execute the machine code instructions (but treated as an input stream) on the same hardware for test and debugging purposes, e.g.