Hitachi CinemaStar P7K500 Specifications page 99

Size of the Phy event counter as defined by bits 14:12 of Counter n Identifier.
The size of the Phy event counter shall be a multiple of 16-bits.
Data Structure Checksum
The data structure checksum is the 2's complement of the sum of the first 511 bytes in the data
structure. Each byte shall be added with unsigned arithmetic and overflow shall be ignored. The sum
of all 512 bytes of the data structure will be zero when the checksum is correct.
Reserved All reserved fields shall be cleared to zero
HITACHI Deskstar & CinemaStar P7K500 Hard Disk Drive specification (Rev 1.1)
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