Address Offset Feature; Enable/Disable Address Offset Mode - Hitachi T7K250 - Deskstar - Hard Drive Specifications

10.16 Address Offset Feature

Computer systems perform initial code loading (booting) by reading from a predefined address on a drive. To allow
an alternate bootable operating system to exist in a system reserved area on a drive, this feature provides a Set
Features function to temporarily offset the drive address space. The offset address space wraps around so that the
entire drive address space remains addressable in offset mode. Max LBA in offset mode is set to the end of the
system reserved area to protect the data in the user area when operating in offset mode. The Max LBA can be
changed by a Set Max Address command to access the user area. If the native MAX LBA is set, the whole user area
can be accessed. But any commands which access sectors across the original native maximum LBA are rejected with
error, even if this protection is removed by an Set Max Address command.

10.16.1 Enable/Disable Address Offset Mode

Subcommand code 09h Enable Address Offset Mode offsets address Cylinder 0, Head 0, Sector 1, LBA 0, to the
start of the nonvolatile protected area established using the Set Max Address command. The offset condition is
cleared by Subcommand 89h Disable Address Offset Mode, Hardware reset or Power on Reset. If Reverting to
Power on Defaults has been enabled by Set Features command, it is cleared by Soft reset as well. Upon entering
offset mode the capacity of the drive returned in the Identify Device data is the size of the former protected area. A
subsequent Set Max Address command with the address returned by the Read Max Address command allows access
to the entire drive. Addresses wrap so the entire drive remains addressable.
If a nonvolatile protected area has not been established before the device receives a Set Features Enable Address
Offset Mode command, the command fails with Abort error status.
Disable Address Offset Feature removes the address offset and sets the size of the drive reported by the Identify
Device command back to the size specified in the last nonvolatile Set Max Address command.
Figure 1 : Device address map before and after Set Feature
Deskstar T7K250 Hard Disk Drive Specification
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