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Logical block addressing (LBA) is a method for identifying storage blocks with a single, sequential address. Address 0 identifies the first logical block, address 1 the next, and numbering continues through the available range. An LBA is an address—not a universal byte size or a description of where data sits physically on a disk.
What logical block addressing means
The ATA-3 glossary defines LBA as addressing a device through “the linear mapping of sectors” (ATA-3 working draft, section 2.1.11). In practical terms, software can refer to a logical block by its number rather than by a combination of geometry-style coordinates.
In the SCSI-2 model, addresses start at block zero and proceed contiguously to the last logical block on a logical unit or within a partition (SCSI-2 specification). The device or protocol defines the relevant range; LBA itself does not specify how much storage is available.
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How LBA differs from CHS addressing
CHS stands for cylinder, head, and sector. It identifies a location through three geometry-style values. LBA instead uses one linear block number, making the address representation simpler to work with.
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A legacy Seagate ATA manual illustrates the mapping by showing LBA 0 corresponding to cylinder 0, head 0, sector 1 (Seagate ATA manual). That is a historical representation, not a reason for contemporary software users to reason about a disk’s physical geometry.
Does one LBA always equal 512 bytes?
No. An LBA identifies a logical block; the logical block length determines how many bytes that block represents. To convert an LBA to a byte offset, use:
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byte offset = LBA × logical block size
For example, the ATA/ATAPI Command Set-3 draft describes both a layout with 512-byte logical sectors grouped over 4,096-byte physical sectors and a native arrangement with 4,096-byte logical and physical sectors (ATA/ATAPI Command Set-3 draft, Revision 5 (2013)). These are examples of possible layouts, not a description of every drive. Consult the device’s reported logical block size rather than assuming every address represents 512 bytes.
Why sector alignment can matter
Logical and physical sector boundaries do not always match. The ATA/ATAPI Command Set-3 draft notes that a write beginning or ending at an LBA misaligned with a larger physical sector may require a read-modify-write operation: the device reads the affected physical sector, changes the relevant data, and writes the sector back. This is a documented possibility, not a guarantee that every misaligned write will have a particular performance impact.
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When diagnosing alignment-related behavior, the useful facts are the logical block size, physical sector size, and starting and ending LBAs of the operation. LBA alone does not reveal those physical details.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Are LBA numbers and their limits the same everywhere?
No single maximum LBA applies to every storage interface or command. Address-field width depends on the protocol and command format. For example, the cited SCSI-2 specification gives a 21-bit LBA field in a six-byte command descriptor block and 32-bit fields in ten- and twelve-byte command descriptor blocks. These are examples from that specification, not universal limits for current storage systems.
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For a specific device or software operation, check the applicable protocol command and the device’s reported block size and address range. LBA provides a way to name logical blocks; it does not itself increase capacity or promise a particular speed.
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