Logical Block Addressing (LBA) is a method of accessing hard disk drives. This allows IDE disks larger than 504 megabytes (1024 cylinders) in size to be fully partitioned using the MS-DOS fdisk command. Prior to this technology, ROM BIOS limitations prevented the fdisk command from using an entire hard disk that was larger than 504 megabytes (MB) in size, except by using other controller ROM routines that effectively translate the hard disk's native geometry.

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LBA is a run-time function of the system BIOS. The BIOS uses LBA for the following commands: read (with and without retries), read verify, read long, write (with and without retries), write verify, write long, read multiple, write multiple, read DMA, write DMA, seek, and format track.

As with non-LBA systems, information about the hard disk's true geometry is stored in the system CMOS. When reporting information about the hard disk to MS-DOS, however, a system employing LBA essentially depicts a hard disk with fewer than 1024 cylinders and the LBA BIOS performs a translation from the MS-DOS track, head, and sector to logical block numbers used by the drive, allowing the entire disk to be used without special drivers or third- party disk utilities.

The LBA standard specifies the following two types of drive parameter tables:

•	Automatic (recommended for Novell NetWare and Unix).
•	Translation (recommended for MS-DOS, Windows, OS/2 2.x, and Windows NT).

The following sections provide information about the use of LBA with various BIOS chip sets, including the specific versions of these BIOS chip sets that support LBA.

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American Megatrends Inc.

American Megatrends, Inc. has confirmed that versions of its BIOS dated 4-25-94 and later are compatible with LBA. For more information, contact American Megatrends.

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Phoenix

Micro Firmware, the sole provider of BIOS upgrades for Phoenix BIOS chip sets, has confirmed that BIOS versions 4.03 and later are compatible with LBA. However, various computer and system board manufacturers may be using specific revisions of the version 4.03 BIOS that do not support LBA. In these cases, the hardware manufacturer should be contacted for an appropriate BIOS upgrade. For more information, contact Micro Firmware.

This article refers to products manufactured by vendors independent of Microsoft; we make no warranty, implied or otherwise, regarding these products' performance or reliability.

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