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SCSI Commands

5-166 Quantum DLT 8000 Tape System

READ BUFFER Command (3Ch) (continued)

Table 5–74 READ BUFFER Command Descriptor Block — Field Descriptions

Field Name Description

Mode The tape drive supports the following values within this field. If any

non-supported value is set, the drive terminates the command with a

CHECK CONDITION status, ILLEGAL REQUEST sense key set.

Mode Description

000b Combined Header and Data (see 5.16.1)

010b Data (see 5.16.2)

011b Descriptor (see 5.16.3)

Buffer ID Must be 0, 1, or 2.

Buffer 0: This 1100 KB buffer is intended to be used in conjunction

with the WRITE BUFFER command to provide a diagnostic

capability for testing the SCSI bus and/or hardware integrity.

Buffer 1: Choosing Buffer 1 results in the tape system transferring the

contents of controlled scratchpad RAM and EEPROM over

the SCSI bus. For DLT 8000 tape systems, a total of 264K is

transferred (256K for RAM, 8K for EEPROM).

Buffer 2: Choosing Buffer 2 results in the tape system transferring the

contents of data cache RAM over the SCSI bus. For DLT

8000 tape systems, a total of 8 MB is transferred.

Buffer Offset The Buffer Offset field allows the host to specify where the start of

the data is within the buffer.

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.

The host should first send a READ BUFFER command, in Descriptor mode, to

determine the size of the buffer being returned. In response to the READ BUFFER

command, the tape system returns four bytes of data, three of which contain the size of

the buffer. The host can then use this data to establish the Buffer Offset/Allocation

Length fields of the CDB. Once the size of the buffer is known, Mode 2 (Data Only, see

Section 5.16.2) can be used to transfer the data across the SCSI Bus.

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Quantum DLT 8000 Manual Página 310