Accepted by...............................................

Simulators are useful in allowing user to develop or invest-igate operating systemis programs which theyr would normally 'e unable to run due to lack of available equipmnent or privileged instruction restrictions, Through simulation of I/0 devices and I/0 instructions the user programs can include channel programs to the devices as if the equipment were actually attached to the system he is using. Having the privileged I/0 related instructions, which are more than one third of all priviledged instructions, at his disposal, gives the user much greater flexibility in the study of operating systems programs. This document describes simulation of the input/output sys-tem for an IBMv/370 type machine. It is part of a simulator of a complete machine now being used for running student programs. The I/0 simulator is software simulating S/370 hardware done on a S/370 machine