Abstract: A computer system comprises a CPU, a main memory, and plurality of I/O Processors (IOPs), coupled to each other by a system I/O bus. The IOPs perform slave processing functions relating to I/O devices. A simulation protocol is defined for the IOPs, whereby the host CPU can command an IOP to execute a simulation script. The simulation script defines one or more I/O devices to be simulated, and specifies a simulated workload associated with the devices. The IOP executes the simulation script by sending simulated input streams to the host and receiving output destined for the simulated I/O devices from the host on the system I/O bus. An IOP may simulate multiple I/O devices, and may simulate I/O devices concurrently with servicing real I/O devices. In typical use, one or more applications programs will execute on the CPU concurrently with the execution of one or more simulation scripts in the IOPs attacked to the system I/O bus.