48bitLBA.com - Overview

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48-bit LBA Technology Overview

Basic Overview

Hard Disk capacities have encountered various barriers ever since they were designed. There have been at least 10 which have mostly been a result of BIOS or operating system issues. The most recent barrier of 137 gigabytes is due to the fact that the original interface design spec with only 28 bits to address data on a disk simply does not have enough room for anything beyond 137 gigabytes. 48-bit Logical Block Addressing (LBA) is a technology which extends the capacity of IDE ATA/ATAPI devices beyond this limit of 137.4 GB. This limit applies to IDE ATA/ATAPI devices only and not to SCSI interface devices. The actual limit with 28-bits is a maximum of 268,435,456 sectors of 512 bytes of data or 137.4 gigabytes. With 48-bit addressing the limit is 144 petabytes (144,000,000 gigabytes).

In order to move beyond the 137 GB limit, a proposal from Maxtor was submitted to the ANSI NCITS T13 Technical Committee and adopted. The new ATA standard, ATA/ATAPI-6, resolves the 137 GB limit by increasing the maximum number of address bits from 28 to 48. The new standard also increases the number of sectors that can be transferred by a single command from 256 to 65,536. The standard implements 48-bit LBA support through a 48-bit address feature set. Commands in this feature set are implemented in such a way so that drives can support previous commands along with the new commands for 48-bit support.

Are there any major issues with 48-bit LBA?

With 48-bit LBA disk drives could now be built with capacities greater than 137 GB. The 48-bit LBA enhancement to the ATA interface meant system software could now access data beyond the 137 GB limit. Because 48-bit LBA is an enhancement to the ATA interface specification, any system software which needs to access these 48-bit LBA hard drives must be modified to support 48-bit LBA enhancements to the ATA interface specification. Two major classes of system software which are affected by 48-bit LBA are the BIOS and operating system. The BIOS (Basic Input Output System) is firmware on the system's motherboard which tests and initializes basic chipset hardware and boots the operating system such as Windows XP. Basically, the major issues with 48-bit LBA are that depending upon how the drive is being installed on the system, a BIOS upgrade may need to be installed or the operating system upgraded or Service Pack installed. For further details, consult Major Issues under Contents on the left.

What is the next limit?

With 48-bit LBA there is a limit of 144 petabytes (144,000,000 gigabytes). The next capacity barrier will come sooner than that at 2.2 terabytes (2,200 gigabytes) because many of today's operating systems are based on 32-bit addressing. With hard disk capacities growing at the current rate it is estimated that hard drives at this limit could be available as early as 2004.

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