HGST Ultrastar 7K4000 (SAS) Hard Disk Drive Specification
196
18.21.3 Descriptor (Mode 00011b)
In this mode, a maximum of four bytes of READ BUFFER descriptor information are returned. The drive returns the descrip-
tor information for the buffer specified by the Buffer ID.
• Buffer ID field should normally be set to zero, indicating the drive data transfer buffer. If any other value is specified,
the results may be unpredictable.
• Buffer Offset field is reserved.
• Allocation Length should be set to four or greater. The drive transfers the allocation length or four bytes of READ
BUFFER descriptor, whichever is less. The allocation length of zero indicates no data is transfered.The allocation
length of greater than zero and less than four (size of the Descriptor) is an invalid request and will cause the com-
mand to be terminated with Check Condition status. The drive shall set sense key to Illegal Request and additional
sense code to Illegal Field in CDB.
The READ BUFFER descriptor is defined in the figure below.
Table 149: Read Buffer Description
The value contained in the Buffer Offset field of subsequent WRITE BUFFER and READ BUFFER commands should be a
multiple of two to the power of the offset boundary. The offset boundary is always set to nine, which indicates Sector Bound-
aries.
Byte
Bit
7654321
0 Offset Boundary = 0x09
1-3
(MSB) Buffer Capacity
(LSB)
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