The ATA-5 Specification barrier

    According to the ATA specification, any drive over 7.88GB (8.46 billion Bytes) is supposed to report its "geometry" as 16383/16/63 (C/H/S). Which enables old software to access the first 7.84 to 7.88GB of such a drive, using CHS addressing and the BIOS Int 13h system call. And while a P-CHS over 16383/16/63 can be used to access all the drive capacity-- up to the ATA-5 128GB limit-- that is not the recommended way to do so. Rather, LBA addressing should be used to access those larger capacity drives. (Or at least any partition that goes over the 7.88GB BIOS Int 13h CHS barrier.)

    So, LBA uses a 28 bit binary number to specify the sector number on a drive.  Thus:
 
 228 = 268435455 Sectors
 
 268435455 Sectors x 512 Bytes/Sector = 137438952960 Bytes
 
 Resulting in the 128GB (137.4 billion Bytes) ATA-5 size limit.

    The current ATA-6 specification provides a new (separate) 48-bit binary field, to specify the sector number on drives over 128GB. This in turn requires new controller hardware, and new drivers that support this additional 48-bit addressing.

 

Related Information:

[WWW Link]My computer doesn't recognize my new larger hard drive
http://www.md4pc.com/questions/58.htm    [New Window]
 
[WWW Link]The operating system only recognizes 128 GB or 137 GB
http://maxtor.custhelp.com/cgi-bin/maxtor.cfg/php/enduser/olh_adp.php?p_faqid=960    [New Window]
This information, from Maxtor Technical Support, applies to all makes of ATA-6 drives.
 
Back UP To: Specific Drive Capacity Limits
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Page Content Updated: 24 January 2004
   

Hard drives over 7.88GB are supposed to report their geometry as 16383/16/63 (C/H/S). This in effect means that "geometry" cannot be used to reliably calculate the size of a hard drive. Instead, the drive size is found in the LBA capacity field, returned by the ATA IDENTIFY command. Under the new ATA-6 specification, drives over 128GB are supposed to report an LBA capacity of #FFFFFFF (all 28 bits are 1's). Which comes to 268435455 sectors or 137,438,952,960 Bytes. And the actual drive size will now be reported by the new 48 bit capacity field.