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ON GUID patition table - GTP


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Link: http://developer.apple.com/technotes/tn2006/tn2166.html



Technical Note TN2166 Secrets of the GPT

Apple has switched to a new disk partitioning scheme known as the GUID partition table, or GPT. This new scheme offers a number of advantages over the previous scheme, but it also presents some new challenges.



The Road to GPT

Apple's previous disk partitioning scheme, known as Apple partition map (APM), was introduced with the Macintosh II in 1987. The scheme was very well designed, and it has survived, with very few changes, for almost twenty years.


However, in recent years APM's limitations have been looming on the horizon. Specifically, APM is restricted to 32-bits worth of blocks. With a standard block size of 512 bytes, this translates to a maximum disk size of 2 TB. With the rate that hard disks are growing, it's easy to imagine a typical desktop computer shipping with more than 2 TB of storage in the next few years.


Apple did consider extending APM to support larger disks. However, as such a change would break all existing partitioning tools, it was just as convenient to switch to an entirely new partitioning scheme. After some serious thought, Apple decided to adopt GPT.


The GUID partition table (GPT) partitioning scheme was introduced by Intel as part of an effort to introduce more modern firmware to generic PC hardware. Traditional PC hardware uses BIOS firmware, which uses a partitioning scheme known as master boot record (MBR). MBR has lots of severe limitations, and is not appropriate for a modern computer. Intel's modern firmware, known as the Extensible Firmware Interface (EFI), includes a new partitioning scheme, GPT.






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