User's Guide

Chapter 2 Media Changer Commands
Read Buffer Command (3Ch)
Quantum SuperLoader 3 Software Interface Guide 96
Table 44 Read Buffer
Command Descriptor Block
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 device 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 Data Mode (0010b)) can
be used to transfer the data across the SCSI Bus.
Combined Header and
Data Mode (000b) 2
In this mode, the device returns a 4-byte header followed by data bytes.
The device terminates the
DATA IN phase when the Allocation Length
bytes of header and data have been transferred or when all available data
has been transferred to the initiator, whichever is less. The 4-byte
READ
Field Name Description
Mode The SuperLoader 3 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
0000b Combined Header and Data Mode (000b)
0010b Data Mode (0010b)
0011b Descriptor Mode (0011b)
1010b Read Data from Echo Buffer (1010b)
1011b “Echo Buffer Descriptor Mode (1011b)” on page 98
Buffer ID This field is ignored in some modes and reserved in others. Refer to the
specific mode description for more detail.
Buffer Offset This field is ignored in some modes, reserved in others, and valid in mode
0010b. Refer to the specific mode description for more detail.
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.