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In logical block addressing, only one number is used to address data, and each linear base address describes a single block. The LBA scheme replaces earlier schemes which exposed the physical details of the storage device to the software of the operating system. Chief among these was the cylinder-head-sector (CHS) scheme, where blocks were addressed by means
[4] [5] It defines new functions within the INT 13h service, all having function numbers greater than 40h, that use 64-bit logical block addressing (LBA), which allows addressing up to 8 ZiB. (An ATA drive can also support 28-bit or 48-bit LBA which allows up to 128 GiB or 128 PiB respectively, assuming a 512-byte sector/block size).
The term fixed-block architecture was created by IBM in 1979 [3] to distinguish this format from its variable-length record format. Each track is divided into fixed-length blocks, consisting of an ID field and a data field. Application programs refer to blocks by relative block number, and cannot address them by cylinder, head and record.
Devices may encompass multiple logical units, which are addressed by logical unit number (LUN). Simple devices have just one LUN, more complex devices may have multiple LUNs. A "direct access" (i.e. disk type) storage device consists of a number of logical blocks, addressed by Logical Block Address . A typical LBA equates to 512 bytes of storage.
The layout of a disk with the GUID Partition Table. In this example, each logical block is 512 bytes in size and each entry has 128 bytes. The corresponding partition entries are assumed to be located in LBA 2–33. Negative LBA addresses indicate a position from the end of the volume, with −1 being the last addressable block.
In 1996, support for logical block addressing (LBA) was introduced in Windows 95B and MS-DOS 7.10 (Not to be confused with IBM PC-DOS 7.1) in order to support disks larger than 8 GB. Disk timestamps were also introduced. [2] This also reflected the idea that the MBR is meant to be operating system and file system independent.
Later, the first formalized ATA specification used a 28-bit addressing mode through LBA28, allowing for the addressing of 2 28 (268 435 456) sectors (blocks) of 512 bytes each, resulting in a maximum capacity of 128 GiB (137 GB). ATA-6 introduced 48-bit addressing, increasing the limit to 128 PiB (144 PB). As a consequence, any ATA drive of ...
Logical block addressing (LBA) is supported in MS-DOS 7 for accessing larger hard disks, unlike earlier versions which only supported cylinder-head-sector (CHS)-based addressing. Unlike MS-DOS 7.0, MS-DOS 7.1 recognizes a hard disk beyond the first 8.4GB. [14] Year 2000 support was added to DIR command via the new /4 option.