SCSI Commands

READ Command (08h)

Table 5–61 READ Command Descriptor Block - Field Descriptions (continued)

Field Name

Description

 

 

Fixed

This bit specifies whether fixed-length or variable-length blocks are to be transferred,

 

and gives meaning to the Transfer Length field of the READ command.

 

When set to 0, variable-block mode is requested. A single block is transferred with

 

the Transfer Length specifying the maximum number of bytes the initiator has

 

allocated for the returned data.

 

When the Fixed bit is set to 1, the Transfer Length specifies the number of blocks to

 

be transferred to the initiator. This is valid only if the logical unit is currently

 

operating in Fixed Block mode.

 

When the Transfer Length is 0, no data is transferred and the current position on the

 

logical unit does not change. This is not an error condition.

 

A successful READ with Fixed bit set to 1 transfers (current block length) x (# of

 

blocks x block size) bytes of data to the host. Upon termination of READ, the

 

medium is logically positioned after the last block of data transferred (EOM).

 

Note that a READ command in fixed mode with an odd number of blocks returns a

 

CHECK CONDITION: the DLT 7000 tape drive does not support odd block number

 

transfers.

 

 

Filemark, End-of-Data, and End-of-Medium/Partition Handling

If the tape drive reads a Filemark, it returns a CHECK CONDITION status. Within the sense data, the Filemark and Valid bits are set and the Sense Key field is set to NO SENSE. The information fields contain the residue count. The Additional Sense Code and Additional Sense Code Qualifier fields are set to FILEMARK DETECTED. Upon termination, the medium is logically positioned after the Filemark.

If the drive detects End-of-Data (EOD) during a READ, the drive returns a CHECK CONDITION status. Within the sense data, the Valid bit is set and the Sense Key field is set to BLANK CHECK. The End-of-Medium (EOM) bit may be set if the drive determines that the tape is positioned past the PSEN marker. The information fields contain the residue count. The Additional Sense Code Qualifier fields are set. Upon termination, the medium is physically positioned before EOD and after the last block on tape.

Quantum DLT 7000 Tape System

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Quantum Instruments DLT 7000 manual Read Command 08h, Read Command Descriptor Block Field Descriptions
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