Quantum 10K II - Manual (Page 220)

SCSI Description
Quantum Atlas 10K II Ultra160/m SCSI Hard Disk Drives 5–153
5.27 READ SKIP MASK Command (E8h)
The READ SKIP MASK command allows the initiator to condition the function of
the READ commands to allow only selected data to be transferred over the SCSI
interface. A READ SKIP MASK command preceeds a READ (10) command; the
READ (10) command must be linked to the READ SKIP MASK command or the
skip mask is lost and a CHECK CONDITION status is returned.
Bit
Byte
7 6 5 4 3 2 1 0
0 Operation Code (E8h)
1 Reserved
2 - 5
(MSB)
Logical Block Address
(LSB)
6 Skip Mask Length
7 - 8
(MSB)
Transfer Length
(LSB)
9 VU Reserved Flag Link
Figure 5–71 READ SKIP MASK Command — Data Format
Table 5–72 READ SKIP MASK Command — Field Descriptions
Field Description
Logical Block Address This field specifies the first logical block of the corresponding READ (10)
COMMAND. This allows a SEEK to begin immediately. The LBA of the
following READ (10) command must the same as the LBA of the READ
SKIP MASK command.
Skip Mask Length This field indicates the length in bytes of the skip mask. The mask is up to
2048 bits, giving a data address capability of 1 MB in 512-byte block
length. The mask consists of a sequence of bits where a “1” bit is for the
blocks of data to be transferred, and a “0” for the blocks of data that are
to be skipped. The first “1” bit of the mask corresponds to the first LBA to
be transferred. Any unused bits at the end of the last byte in the mask
must be set to 0. The mask is transferred in the data-out buffer transfer of
the skip mask command.
Transfer Length This field must be equal to the transfer length in the linked READ (10)
command. The transfer length field must be equal to the number of “1”
bits in the skip mask. If a READ (10) is used with 0 as the transfer length, a
transfer length of 256 data blocks is implied.