Address Offset Feature (Vendor Specific); Enable/Disable Address Offset Mode; Figure 70. Device Address Map Before And After Set Feature - Hitachi IC25N040ATCS04-0 - Travelstar 40GN 40GB UDMA/100 4200RPM 2MB 2.5" IDE Hard Drive Specifications

11.11 Address Offset Feature (vendor specific)

Computer systems perform initial code loading (booting) by reading from a predefined address on a disk
drive. To allow an alternate bootable operating system to exist in a reserved area on a disk 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 disk drive address space remains addressable in offset mode. The
Set Max pointer is set to the end of the reserved area to protect the data in the user area when operating
in offset mode. This protection can be removed by a Set Max Address command to move the Set Max
pointer to the end of the drive. But any commands which access sectors across the original native maxi-
mum LBA are rejected with error, even if this protection is removed by a Set Max Address command.

11.11.1 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.
Before Enable Address Offset Mode
A reserved area has been created using a non-volatile Set Max command.
User Accessible Area
LBA 0
After Enable Address Offset Mode
The former reserved area is now the user accessible area.
The former user accessible area is now the reserved area.
After Set Max Address Command useing the Value Returned by Read Max Address

Figure 70. Device address map before and after Set Feature

Reserved Area
LBA R
User Accessible Area
(former Reserved Area)
LBA 0
LBA 0
Hitachi Travelstar 60GH & 40GN hard disk drive specifications
LBA M
Reserved Area
(former User Accessible Area)
LBA M-R
User Accessible Area
95
LBA M
LBA M