The Unit Control Block (UCB) maintains the status of an I\/O Device. There is one UCB for each I\/O device managed by the OS.<\/p>\n
@UCB DSECT ,\r\nUCBNEXT DS A NEXT UCB\r\nUCBCUA DS XL2 DEVICE ADDRESS\r\n*\r\nUCBDEVT DS X DEVICE TYPE\r\nUCBTRDR EQU 1 - CARD READER\r\nUCBTPUN EQU 2 - CARD PUNCH\r\nUCBTPRT EQU 3 - PRINTER\r\nUCBTDASD EQU 4 - DASD\r\nUCBTTAPE EQU 5 - TAPE\r\nUCBTTERM EQU 6 - TERMINAL\r\n*\r\nUCBSTAT DS X DEVICE STATUS\r\nUCBSONLN EQU X'80' DEVICE ONLINE\r\nUCBSMNT EQU X'08' MOUNTED\r\n*\r\nUCBDEVNM DS CL4 DEVICE TYPE NAME\r\nUCBIOB DS A PTR TO IOB QUEUE\r\nUCBINTR DS A INTERRUPT ROUTINE\r\nUCBINTRP DS F INTERRUPT PARAMETER\r\nUCBDVEXT DS A DEVICE EXTENSION\r\nUCBCSW DS XL8 CHANNEL STATUS WORD<\/pre>\nThe UCB’s are chained together and must be arranged in ascending sequence based on the Device Address. The UCBDEVT field indicates the class of device. The actual device type is specified in the UCBDEVNM field as a four-byte character field.<\/p>\n
The UCBIOB field points to the IORB queue. The first IORB is the currently active request. \u00a0If additional requests are scheduled for the device they are chained off the active request.<\/p>\n
The UCBINTR field contains a pointer to the interrupt routine to receive control if no active IORB is queued when an I\/O Interrupt occurs. \u00a0If this field is zero then no action is taken and the interrupt is ignored. \u00a0The UCBINTRP is used to maintain a parameter value to be used the the interrupt routine.<\/p>\n
UCBDVEXT is a pointer to a device specific extension. \u00a0If there is no device specific extension associated with a device this field is zero.<\/p>\n
The UCBCSW field contains a copy of the Channel Status Word at the time the interrupt occurred.<\/p>\n
@UCBDASD DSECT ,\r\nUCBDVOL DS CL6 VOLUME SERIAL\r\nUCBDTRKC DS H TRACKS\/CYLINDER\r\nUCBDTRKZ DS H TRACK SIZE\r\nUCBDVTOC DS 0XL8 VTOC EXTENT\r\nUCBDVTOB DS XL4 VTOC BEGIN CCHH\r\nUCBDVTOE DS XL4 VTOC END CCHH<\/pre>\nThis is the initial description of the DASD Device Specific Extension. \u00a0I say initial because it may change as the DASD specific I\/O code is developed.<\/p>\n
It contains the Volume Serial, the number of tracks per cylinder, the track size and the VTOC extent description. \u00a0This information will be filled in when the DASD Volume is mounted.<\/p>\n
[Next – I\/O Request Block – @IORB<\/a>]<\/p>\n","protected":false},"excerpt":{"rendered":"
The Unit Control Block (UCB) maintains the status of an I\/O Device. There is one UCB for each I\/O device managed by the OS. @UCB DSECT , UCBNEXT DS A NEXT UCB UCBCUA DS XL2 DEVICE ADDRESS * UCBDEVT DS … Continue reading