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SCSI Commands

Quantum DLT 8000 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

This field specifies the maximum number of bytes that the initiator

has allocated for returning data. The host uses this field to limit the

size of data transfers to its own internal buffer size.

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Quantum DLT 8000 Manual Página 149