User's Guide

Page 48 DDS-4 SCSI Interface Manual, Rev. B
LOCATE and the BT Bit
Assume that the data was written on the tape as shown in the following table.
Block 0 1 2 3 4 5 6 7 8 9 10 11 12 13
BOT D S S S S S D D D F D D D EOD
Position A B C D
If the Block Address Type (BT) bit is set in the CDB when the LOCATE command is
issued, all the setmarks and/or filemarks are logically grouped with its nearest Data
Block on its BOT side to form one logical block. The following table shows the
logical blocks when the BT bit is set. In the table, the data block (block number 0)
and five setmarks (block numbers 1-5) are grouped together to form one logical
block. the filemark (in block number 9) would be grouped with the data block (block
number 8) to form another logical block.
Block 0 0 0 0 0 0 1 2 3 3 4 5 6 7
BOT D S S S S S D D D F D D D EOD
If the BT bit is 0 in the CDB when the LOCATE command is issued, then each
filemark and setmark on the tape is counted like the logical block while searching
the target. See the table that follows.
Block 0 1 2 3 4 5 6 7 8 9 10 11 12 13
BOT D S S S S S D D D F D D D EOD
Referring to the first table, the table below shows the tape position after each
LOCATE command is complete.
LOCATE Block Address From BT bit Tape Position
1 BOT ON C
1 EOD ON C
1 BOT OFF A
1 EPD OFF A
4 BOT ON D
4 EOD ON D
4 BOT OFF B
4 EOD OFF B