IOS

 I/O Supervisor is responsible to start I/O operation and monitor events from channel, control unit and devices. 

To start an I/O, IOS store the address of a channel program in caw (channel address word) follow by start I/O instruction. When I/O is done. IOS performs termination processing. IOS also responds to event, purging or restoring an I/O operation. 

Program and access method interact with IOS via Drivers. Most of access method uses EXCP driver. The EXCP and EXCPVR macro invokes the EXCP driver. The driver convert the virtual addresses in the channel command words into real address. It issues STARTIO macro to starts the I/O. IOS takes over and issue the STARTIO instruction or queue the request for later execution