Specifications

Data Structures
1.19 Unit Control Block (UCB)
UCB$L_DX_BUF 228
UCB$L_DX_BFPNT 232
UCB$L_DX_RXDB 236
UCB$W_DX_BCRUCB$B_DX_SCTCNTUnused 240
Table 1–25 UCB Local Disk Extension
Field Name Contents
UCB$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 device has been placed on line since the system was last
bootstrapped.
UCB$L_MAXBLOCK Maximum number of logical blocks on random-access device. This field is
written by a disk driver during unit initialization and power recovery.
UCB$L_MAXBCNT Maximum number of bytes that can be transferred. A disk driver writes this
field during unit initialization and power recovery.
UCB$L_DCCB Pointer to cache control block.
UCB$L_QLENACC Queue length accumulator.
UCB$L_MEDIA Media address.
UCB$L_BCR Byte-count register. Some disk drivers use this field as an internal count
of the number of bytes left to be transferred in an I/O request. The symbol
UCB$W_BCR points to the low-order word of this field.
UCB$W_EC1 ECC position register. This field records the starting bit number of an error
burst. Disk driver register dumping routines copy the contents of this field
into an error message or diagnostic buffer.
The system correction routine IOC$APPLYECC reads the contents of this
field to locate the beginning of an error burst in a disk block.
UCB$W_EC2 ECC position register. Records the exclusive OR correction pattern. Disk
driver register dumping routines copy the contents of this field into an error
message or diagnostic buffer.
The system ECC correction routine IOC$APPLYECC reads the contents of
this field to correct disk data.
UCB$W_OFFSET Current offset register contents.
UCB$B_OFFNDX Current offset table index. When a disk driver transfer ends in an error, the
disk driver can retry the transfer a number of times with different offsets of
the disk head from the centerline. This field is an index into a driver table
of offset positions.
UCB$B_OFFRTC Current offset retry count. This field records the number of times to try a
particular offset setting in a disk transfer retry.
UCB$L_DX_BUF Address of sector buffer (used by floppy-disk drivers).
UCB$L_DX_BFPNT Pointer to current sector (used by floppy-disk drivers).
UCB$L_DX_RXDB Address of saved receiver-data buffer (used by floppy-disk drivers).
UCB$W_DX_BCR Current floppy byte count (used by floppy-disk drivers).
UCB$B_DX_SCTCNT Current sector byte count (used by floppy-disk drivers).
The UCB terminal extension is illustrated in Figure 1–27 and described in
Table 1–26.
1–98