An instruction set simulator (ISS) is a simulation model, usually coded in a high-level programming language, which mimics the behavior of a mainframe or microprocessor by "reading" instructions and maintaining internal variables which represent the processor's registers.

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

Instruction-set simulators can be implemented using three main techniques:

An ISS is often provided with (or is itself) a debugger in order for a software engineer/programmer to debug the program prior to obtaining target hardware. GDB is one debugger which has a compiled-in ISS. It is sometimes integrated with simulated peripheral circuits such as timers, interrupts, serial ports, general I/O ports, etc. to mimic the behavior of a microcontroller.

The basic instruction simulation technique is the same regardless of purpose: first execute the monitoring program passing the name of the target program as an additional input parameter.

The target program is then loaded into memory, but control is never passed to the code. Instead, the entry point within the loaded program is calculated, and a pseudo program status word (PSW) is set to this location. The Program Status Word (PSW) is composed of a status register and a program counter, the latter of which signifies the next instruction to be executed. Therefore, it is specifically the program counter that is assigned to this location. A set of pseudo registers are set to what they would have contained if the program had been given control directly.

It may be necessary to amend some of these to point to other pseudo "control blocks" depending on the hardware and operating system. It may also be necessary to reset the original parameter list to 'strip out' the previously added program name parameter.

Thereafter, execution proceeds as follows: