x86 BIOS size limitations

Initially the size of an ATA drive, then called "IDE" was stored in the system x86 BIOS using a a type number 1 - 45 that predefined the C/H/S parameters.[14] and often the landing zone as well. Which decides where to park the drive heads. Later a "user definable" format[14] called C/H/S or cylinders, heads, sectors were made available. These numbers were important for the earlier ST-506 interface, but were generally meaningless for ATA -- the CHS parameters for later ATA large drives often specified impossibly high numbers of heads or sectors that did not actually define the internal physical layout of the drive at all. From the start and up to ATA-2 every user had to specify explicitly how large every attached drive was. From ATA-2 an "identify drive" command were implemented that can be sent and which will return all drive parameters.

Due to a lack of foresight by motherboard manufacturers, the system BIOS was often hobbled by artificial C/H/S size limitations due to the manufacturer assuming certain values would never exceed a certain numerical maximum.

The first of these BIOS limits occurred when ATA drives reached sizes in excess of 504 megabytes. Because some motherboard BIOS would not allow C/H/S values above 1024 cylinders, 16 heads, and 63 sectors. Multiplied by 512 bytes per sector, this totals 528 482 304 bytes which divided by 1 048 576 bytes per megabyte, equals 504 megabytes.

The second of these BIOS limitations occurred at 1024 cylinders, 256 heads, and 63 sectors, but a bug in MS-DOS and MS-Windows 95 limit the number heads to 255. This totals to 8 422 686 720 bytes, commonly referred to as the 8.4 gigabyte barrier. This also a limit imposed by x86 BIOSes, and not a limit imposed by the ATA interface.

It was eventually determined that these size limitations could be overridden with a tiny program loaded at startup from a hard drive's boot sector. Some hard drive manufacturers such as Western Digital started including these override utilities with new large hard drives to help overcome these problems. However, if the computer were booted in some other manner without loading the special utility, the invalid BIOS settings would be used, and the drive could either be inaccessible or could appear to be damaged to the operating system.

Later an extension to the x86 BIOS disk services called the "Extended Disk Drive" (EDD) were made available which makes it possible to address drives as large as 264 bytes.

July 7, 2011

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