User's Manual

DLT4000 SCSI Interface
DLT4000/DLT4500/DLT4700 Cartridge Tape Subsystem 8-95
Allocation Length
The Allocation Length specifies the maximum number of bytes that the initiator has
allocated for returned data.
8.4.14.1 Combined Header and Data Mode
In this mode, the drive returns a 4-byte header followed by the data bytes. The drive
terminates the DATA IN phase when 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 BUFFER header is followed by data bytes from
the target data buffer. The header has the following format:
Bit
7 6 5 4 3 2 1 0
0 Reserved
1 (MSB)
Byte 2 Available Length
3 (LSB)
Figure 8-62 READ BUFFER Data Head
Available Length
The Available Length field specifies the total number of data bytes available in the
target's buffer. This number is not reduced to reflect the allocation length, nor is it
reduced to reflect the actual number of bytes written using the WRITE BUFFER
command. Following the READ BUFFER header, the target will transfer data from its
data buffer.
8.4.14.2 Data Mode
In this mode, the DATA IN phase only contains buffer data.
8.4.14.3 Descriptor Mode
In this mode, a maximum of four bytes of READ BUFFER descriptor information are
returned. The drive returns the descriptor information for the buffer specified by the
Buffer ID. In this mode, the drive does not reject the valid Buffer IDs with a CHECK
CONDITION status, but returns all zeros in the READ BUFFER descriptor.
The Offset Boundary is 12 (0Ch), indicating buffer offsets should be integral multiples
of 4K.