Hitachi ULTRASTAR 15K450 HUS154530VLF400 - Manual (Page 268)

Hitachi Ultrastar 15K450 (FC-AL) Hard Disk Drive Specification
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Allocation Length specifies the number of bytes that have been allocated for the returned parameter data. If the length is not
sufficient to contain all the parameter data, the first portion of the data shall be returned. The actual length of the parameter
data may be determined from the Additional Length field in the parameter data.