Fujitsu M2488 - page 154

TAPE UNIT SCSI COMMANDS M2488 PRODUCT GUIDE
4-72 CG00000-011503 REV. A April 1997
Descriptor Mode (011b):
In this mode, a maximum of four bytes of READ BUFFER descriptor information are returned.
The descriptor information for the LUN receiving the command is returned. If there is no buffer
associated with the specified buffer ID, the target returns all zeroes in the READ BUFFER
descriptor. The Buffer Offset field in the CDB is ignored in this mode. The allocation length
should be set to four or greater. The target transfers the lesser of the allocation length or four bytes
of READ BUFFER descriptor. The READ BUFFER descriptor is defined in Table 4-66.
** NOTE **
This controller operates on 32-byt e data buffer boundaries, and
returns a value of 5 for the offset boundary. The NVRAM oper ates
on a one-byte boundary and returns a value of zer o for the offset
boundary.
** NOTE **
When performing Write Buffer operations to the Data Buffer, t he max-
imum Transfer Length that c an be written is the Buffer Ca pacity of the
Data Buffer minus two. The two remaining bytes in the Data Buffer
are needed to store the two by te CRC which is automatically
appended to the data w hen it is stored in t he buffer.
Table 4-66. Read/Write Data Buffer Descriptor (buffer ID 0)
BITS
BYTES 7 6 5 4 3 2 1 0
0Offset Boundary (05h)
1MSB
Buffer Capacity
2
3LSB
Table 4-67. Read/Write NVRAM Descriptor (buffer ID 1)
BITS
BYTES 7 6 5 4 3 2 1 0
0Offset Boundary (00h)
1MSB
Buffer Capacity
2
3LSB
Table 4-68. Descriptor Mode Field Description
BYTE BIT VALUE DESCRIPTION
0 0-7
The Offset Boundary field returns the boundary alignment within the selected buffer
for subsequent WRITE BUFFER and READ BUFFER commands. The value con-
tained in the offset boundary field is interpreted as a power of two.
The value contained in the buffer offset field of subsequent WRITE BUFFER and
READ BUFFER commands must be a multiple of the offset boundary raised to a
power of 2 (i.e., a multiple of 32). Refer to Table 4-69
1-3 The Buffer Capacity field returns the size of the selected buffer in bytes.