Specifications

Data Structures
1.19 Unit Control Block (UCB)
Table 1–24 UCB Local Tape Extension
Field Name Contents
UCB$W_DIRSEQ Directory sequence number. If the high-order bit of this word, UCB$V_
AST_ARMED, is set, it indicates that the requesting process is blocking
ASTs.
UCB$B_ONLCNT Number of times the device has been placed on line since the system was
last bootstrapped.
UCB$B_PREV_RECORD Tape position prior to the start of the last I/O operation.
UCB$L_RECORD Current tape position or frame counter.
UCB$L_TMV_RECORD Position following last guaranteed successful I/O operation.
UCB$W_TMV_CRC1 First CRC for mount verification’s media validation.
UCB$W_TMV_CRC2 Second CRC for mount verification’s media validation.
UCB$W_TMV_CRC3 Third CRC for mount verification’s media validation.
UCB$W_TMV_CRC4 Fourth CRC for mount verification’s media validation.
The UCB local disk extension is illustrated in Figure 1–26 and described in
Table 1–25.
Figure 1–26 UCB Local Disk Extension
Base UCB (164 bytes) 0
Error Log UCB Extension (16 bytes) 164
Dual Path UCB Extension (12 bytes) 180
UCB$W_DIRSEQUCB$B_ONLCNTReserved 192
UCB$L_MAXBLOCK 196
UCB$L_MAXBCNT 200
UCB$L_DCCB 204
UCB$L_QLENACC 208
UCB$L_MEDIA 212
UCB$L_BCR 216
UCB$W_EC1UCB$W_EC2 220
UCB$W_OFFSETUCB$B_OFFNDXUCB$B_OFFRTC 224
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