TAPE UNIT SCSI COMMANDS M2488 PRODUCT GUIDE
4-30 CG00000-011503 REV. A April 1997
If the Format code field is set to 11b then the LOCATE command is rejected. The Format code
field is not checked against the actual format of the tape. This field is ignored except when it con-
tains the reserved value 11b.
Values for the Wrap, Physical reference and Format mode fields are reported by the READ POSI-
TION command (with its BT bit set to 1.) These values are also reported in Sense data when the
Sense data contains a Block ID.
a. A 36 track tape consists of two interleaved groups of 18 tracks; each group is called a wrap. The first wrap is
written first and runs from Physical BOT towards Physical EOT. The second wrap is written second and runs
from Physical EOT towards Physical BOT. The tape unit hides the transition from the first wrap to the second
wrap so that the user sees an [abstract] continuous length of tape running from Logical BOT (the beginning of
the first wrap) to Logical EOT (the end of the second wrap).
Table 4-27. Block ID Format Field Description
BYTE BIT VALUE DESCRIPTION
0 0-6 0
1 to 95
The Physical reference field can either be 0 or in the range 1 to 95.
The value 0 indicates that a valid physical reference value is not known for the
target position.
The values in the range 1 to 95 indicate an approximate physical location on tape
close to the target position. When the Physical reference value is not 0, the tape
unit can use the value to perform a high speed locate to get close to the target
position.
0 7
0
1
The Wrap field indicates whether the target position is in the first wrap or second
wrap a of tape.
If Wrap equals 0 then the target position is in the first wrap.
If Wrap equals 1 then the target position is in the second wrap.
If the wrap of the target position is unknown then the Wrap field should be set to
0.
1 6-7 The Format code field indicates the format of the tape cartridge. The meaning of
the possible values are described in Table4-28.
1
2-3 0-5
0-7 A unique Logical block position is associated with each data block and filemark
written on a tape. The first filemark or data block on tape is assigned Logical
block position 0; the Logical block position increases by 1 for each data block
and filemark thereafter. The maximum Logical block position is 3FFFEFh.
Table 4-28. Format Mode Values
FORMAT CODE VALUE FORMAT
00b 18 track, non-packeted
01b 36 track, packeted
10b 18 track, packeted
11b reserved