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M2488 PRODUCT GUIDE TAPE UNIT SCSI COMMANDS
April 1997 CG00000-011503 REV. A 4-53
Block Descriptor:
A Block Descriptor Length of 0 indicates no block descriptors are included in the parameter list.
This condition is not considered an error. The block descriptor length does not include the length
of the pages.
The block descriptor specifies the medium characteristics for all of a logical unit. The block
descriptor contains the Density Code, number of blocks, and block length fields.
Table 4-46. Buffered Mode Values
BUFFERED
MODE
DESCRIPTION
0h Target does not report a GOOD status on WRITE commands until the data blocks are actually written on
the medium.
1h The Target may report GOOD status on WRITE commands as soon as all of the data specified in the
WRITE command has been transferred to the buffer. Write data from multiple initiators can reside in the
buffer prior to writing the data to the medium (default).
2h The Target may report GOOD status on WRITE commands as soon as all of the write data has successfully
transferred to the target’s buffer from any one initiator prior to writing the block(s) to the medium. If an ini-
tiator issued a WRITE command while the buffer contains write data from a different initiator, the target
writes any buffered data to the medium prior to accepting any data from the new initiator.
3h -7h Reserved
Table 4-47. Block Descriptor
BITS
BYTES 7 6 5 4 3 2 1 0
0
Density Code
1-3
Number of Blocks = 000000h
4
Reserved
5-7
Block Length
Table 4-48. Block Descriptor Field Description
BYTE BIT VALUE DESCRIPTION
0 0-3 Density Codes of 00h and 09h write tapes in the format described by ANSI
standard X3B5/94-043 (36-track), and read tapes in the formats described
by ANSI standard X3B5/94-043 (36-track), X3.180-1990 (18-track) and
X3.224.1992 (18-track extended). A Density Code of 7Fh indicates the
Density Code is not changed. Any other code specification is rejected with
a CHECK CONDITION status with the sense key set to ILLEGAL
REQUEST, and the additional sense key set to INVALID FIELD IN
PARAMETER LIST.
NOTE: Since the ANSI SCSI-2 Standard does not define a 36-track for-
mat, we use Density Code 09h to represent the 36-track format.
Density Code 28h is described in Chapter 5.
1-3 0 The Number of Blocks field must contain 0. This indicates that all of the
remaining logical blocks of the logical unit have the medium characteris-
tics specified by the block descriptor unless a subsequent MODE SELECT
command changes those parameters.