Input/Output Supervisor Block

An Input/Output Supervisor Block (IOSB) is a data structure employed exclusively by the Start Input/Output interface of MVS/370 and successor IBM mainframe operating systems. The IOSB describes the I/O request and passes parameters to and receives responses from the Input/Output Supervisor (IOS).

An IOSB is always paired with a Service Request Block (SRB). The SRB schedules processor activity, as required, on any available processor in connection with the I/O request.