48 Bit Disk Addressing: In 2001 or 2008 at the latest, vendors will be shipping ATA hard drives larger than 137.2 Gigabytes. Using the 512 bytes sectors that are pretty much universally used by all PC Operating Systems, 137.2GB will exceed the 28 address bits available in the current ATA standard. There is a modification to the ATAPI standard called ATAPI-6 that modifies the standard to allow 48 bit addressing. In addition it expands maximum sectors per operation from 8 bits (256) to 16 bits (16536) thereby allowing reads or writes of blocks as large as 8Mb in one operation. DMA mode5 allowing 100MBps data transfer is also included. Disks of up to 144*10^15 bytes will be supported.

Drive IDs will be modified to indicate if they support 48 bit addressing. BIOSes will have to be modified (yet again) to support disks greater than 137.2GB. However, most current OSes including Mac OS9, many (all?) Linuxes, and all current Microsoft OSes are limited to 32 bits of addressing = 2.2 TB per disk -- a limit that will probably be approached within a few years,

Return To Index Copyright 1994-2008 by Donald Kenney.