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Quantum DLT 7000 Tape System 5-5
Table 5–2 Command Descriptor Block - Field Descriptions
Field Description
Operation Code The operation code specifies the command being requested. The list of
supported SCSI commands and their operation codes are contained in Table
5–1.
Logical Unit
Number
The Logical Unit Number contains the number of the device being
addressed. It must be set to 0. The Logical Unit Number is ignored if the
Command Descriptor Block is preceded by an IDENTIFY Message.
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.
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. Relative
Addressing indicates a technique used to determine the next Logical Block
Address to be operated on.
Transfer Length The transfer length field normally specifies the number of blocks to be
transferred between the initiator and the drive. For several commands, the
transfer length indicates the number of bytes (not blocks) to be sent. For
these commands, this field may be identified by a different name.
Parameter List
Length
The Parameter List Length is used to specify the number of bytes sent during
the DATA OUT phase. 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 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 phase 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, and it contains the
Flag and Link bits. Use of these bits is initiator-dependent. Setting the Link bit
= 1 provides an automatic link to the next command, bypassing the usual
ARBITRATION, SELECTION, and MESSAGE OUT phases that would normally
occur between commands. Other bits in the Control Field are considered to
be reserved.
RelAdr Relative Address must be 0 (not supported).