Editing Parallel ATA (section)

=== x86 BIOS size limitations ===
{{See also|Logical block addressing#Enhanced BIOS|l1=Enhanced BIOS}}
<!-- Also, do we need to mention the earlier 504MB limit in earlier BIOSes, and make clear that it was not the interface's problem but rather the BIOS... or was it? Or is that so old that we don't need to bother? [_maybe the words "such as" already allow for the 504MB, 32MB, and other random limits_]-->

Initially, the size of an ATA drive was stored in the system [[x86]] BIOS using a type number (1 through 45) that predefined the C/H/S parameters<ref name=pdef>{{cite web| url = http://www.kva.kursk.ru/bios1/HTML1/standard.html| title = kursk.ru – Standard CMOS Setup| access-date = 2011-05-27| archive-date = 2018-10-04| archive-url = https://web.archive.org/web/20181004103524/http://www.kva.kursk.ru/bios1/HTML1/standard.html| url-status = dead}}</ref> and also often the landing zone, in which the drive heads are parked while not in use. Later, a "user definable" format<ref name=pdef/> called C/H/S or cylinders, heads, sectors was 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 on, an "identify drive" command was implemented that can be sent and which will return all drive parameters.

Owing 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 particular numerical maximum.

The first of these BIOS limits occurred when ATA drives reached sizes in excess of 504 [[Mebibyte|MiB]], because some motherboard BIOSes would not allow C/H/S values above 1024 cylinders, 16 heads, and 63 sectors. Multiplied by 512 bytes per sector, this totals {{gaps|528|482|304}} bytes which, divided by {{gaps|1|048|576}} bytes per [[Mebibyte|MiB]], equals 504 [[Mebibyte|MiB]] (528 [[Megabyte|MB]]).

The second of these BIOS limitations occurred at 1024 [[Cylinder-head-sector#Cylinders|cylinders]], 256 [[Cylinder-head-sector#Heads|heads]], and 63 [[Disk sector|sectors]], and a problem in [[MS-DOS]] limited the number of heads to 255. This totals to {{gaps|8|422|686|720}} bytes (8032.5 [[Mebibyte|MiB]]), commonly referred to as the 8.4 gigabyte barrier. This is again 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 small 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 large hard drives to help overcome these problems. However, if the computer was 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 appear to the operating system to be damaged.<!-- INT 13/48, EDD compatible !? -->

Later, an extension to the x86 BIOS [[INT 13H|disk services]] called the "[[Enhanced Disk Drive]]" (EDD) was made available, which makes it possible to address drives as large as 2<sup>64</sup> sectors.<ref>{{cite web |url = http://home.teleport.com/~brainy/interrupts.htm |title = teleport.com – Interrupts Page |archive-url=https://web.archive.org/web/20011102202657/http://home.teleport.com/~brainy/interrupts.htm |archive-date=2 November 2001 |url-status=dead}}</ref>