Quantum 10K II - Manual (Page 74)

SCSI Description
Quantum Atlas 10K II Ultra160/m SCSI Hard Disk Drives 5–7
Table 5–2 Command Descriptor Block — Field Descriptions (continued)
Field Description
Logical Block Address The 6-byte READ, SEEK, and WRITE Command Descriptor Blocks contain
a 21-bit Logical Block Address. The 10-, 12-, and 16- Command
Descriptor Blocks contain a 32-bit Logical Block Address.
Commands that require additional parameter data specify the length of
the Logical Block Address that is needed. See the specific command
descriptions for more detailed information.
Relative Addressing indicates a technique used to determine the next
Logical Block Address to be operated on. The drive does not support
Relative Addressing, it defaults to a value of 0, which specifies that the
Logical Block Address specifies the first logical block of a range of logical
blocks to be operated on by the command.
Transfer Length The transfer length field normally specifies the number of sectors to be
transferred between the initiator and the drive. For several commands,
the transfer length indicates the number of bytes (not sectors) to be sent.
For these commands, this field may be identified by a different name
Commands that use one byte for the transfer length value allow up to
256 sectors of data to be transferred by one command. A transfer length
value of 0 indicates that 256 sectors are to be sent. Transfer length
values of 1 through 255 indicate the number of sectors to be transferred.
Commands that use multiple bytes for the transfer length value function
differently. A transfer length value of 0 indicates that no data transfer is
to occur. Transfer length values of 1 or greater indicate the number of
sectors to be transferred.
Parameter List Length The Parameter List Length is used to specify the number of bytes sent
during the data-out buffer transfer. This field is typically used for
parameters that are sent to a drive (for example, mode, diagnostic, and
log parameters). A parameter list length of 0 indicates that no data is to
be transferred.
Allocation Length The Allocation Length field specifies the maximum number of bytes that
the initiator has allocated for returned data. The Allocation Length is
used to limit the amount of data returned to the initiator.
An Allocation Length of 0 indicates that no data is to be transferred from
the drive to the initiator. The drive terminates the data-in buffer transfer
when the specified number of bytes have been transferred to the
initiator or when all available data has been transferred, whichever is
less.
Control Field The Control Field is the last byte of every Command Descriptor Block; its
format is shown in Figure 5-2 and described in Table 5-3.