IEE459I 09.16.42 DEVSERV QDASD UNIT VOLSER SCUTYPE DEVTYPE CYL SSID SCU-SERIAL DEV-SERIAL EFC 0D010 Z1ARA1 2107922 2107900 10017 D000 0175-BALB1 0175-BALB1 *OK **** 1 DEVICE(S) MET THE SELECTION CRITERIA **** 0 DEVICE(S) FAILED EXTENDED FUNCTION CHECKING Figure 4-2 Output of DS QDASD command Because traditional devices do not allow more than one I/O at a time, IOS uses the UCB as a queue anchor block. If the device is already executing a previous I/O operation initiated by this system, a new I/O request is queued in the UCB. 4.3 Channel subsystem The System z channel subsystem contains the following items: One or more special processor units (PU), which are called system-assist processors (SAPs) An SAP runs special I/O Licensed Internal Code. The SAP takes responsibility for some of the processing during the execution of an I/O operation, freeing up the operating system CPs to do other work. It schedules an I/O operation, checks for the full availability of the I/O path, and provides a queue mechanism if an I/O path is not available. Channels, which are able to communicate with I/O control units (CUs) and manage the movement of data between processor storage and these control units They are located in I/O cards in the zSeries I/O cage. Maximum number of addressible