4. History of BIOS and IDE limits
 
ATA Specification (for IDE disks) - the 137 GB limit
 
At most 65536 cylinders (numbered 0-65535), 16 heads (numbered 0-15), 255 sectors/track (numbered 1-255), for a maximum total capacity of 267386880 sectors (of 512 bytes each), that is, 136902082560 bytes (137 GB). In Sept 2001, the first drives larger than this (160 GB Maxtor Diamondmax) appeared.  
 
BIOS Int 13 - the 8.5 GB limit
 
At most 1024 cylinders (numbered 0-1023), 256 heads (numbered 0-255), 63 sectors/track (numbered 1-63) for a maximum total capacity of 8455716864 bytes (8.5 GB). This is a serious limitation today. It means that DOS cannot use present day large disks.  
 
The 528 MB limit
 
If the same values for c,h,s are used for the BIOS Int 13 call and for the IDE disk I/O, then both limitations combine, and one can use at most 1024 cylinders, 16 heads, 63 sectors/track, for a maximum total capacity of 528482304 bytes (528MB), the infamous 504 MiB limit for DOS with an old BIOS. This started being a problem around 1993, and people resorted to all kinds of trickery, both in hardware (LBA), in firmware (translating BIOS), and in software (disk managers). The concept of `translation' was invented (1994): a BIOS could use one geometry while talking to the drive, and another, fake, geometry while talking to DOS, and translate between the two.  
 
The 2.1 GB limit (April 1996)
 
Some older BIOSes only allocate 12 bits for the field in CMOS RAM that gives the number of cylinders. Consequently, this number can be at most 4095, and only 4095*16*63*512=2113413120 bytes are accessible. The effect of having a larger disk would be a hang at boot time. This made disks with geometry 4092/16/63 rather popular. And still today many large disk drives come with a jumper to make them appear 4092/16/63. See also over2gb.htm. Other BIOSes would not hang but just detect a much smaller disk, like 429 MB instead of 2.5 GB.  
 
The 3.2 GB limit
 
There was a bug in the Phoenix 4.03 and 4.04 BIOS firmware that would cause the system to lock up in the CMOS setup for drives with a capacity over 3277 MB. See over3gb.htm.  
 
The 4.2 GB limit (Feb 1997)
 
Simple BIOS translation (ECHS=Extended CHS, sometimes called `Large disk support' or just `Large') works by repeatedly doubling the number of heads and halving the number of cylinders shown to DOS, until the number of cylinders is at most 1024. Now DOS and Windows 95 cannot handle 256 heads, and in the common case that the disk reports 16 heads, this means that this simple mechanism only works up to 8192*16*63*512=4227858432 bytes (with a fake geometry with 1024 cylinders, 128 heads, 63 sectors/track). Note that ECHS does not change the number of sectors per track, so if that is not 63, the limit will be lower. See over4gb.htm.  
 
The 7.9 GB limit
 
Slightly smarter BIOSes avoid the previous problem by first adjusting the number of heads to 15 (`revised ECHS'), so that a fake geometry with 240 heads can be obtained, good for 1024*240*63*512=7927234560 bytes.  
 
The 8.4 GB limit
 
Finally, if the BIOS does all it can to make this translation a success, and uses 255 heads and 63 sectors/track (`assisted LBA' or just `LBA') it may reach 1024*255*63*512=8422686720 bytes, slightly less than the earlier 8.5 GB limit because the geometries with 256 heads must be avoided. (This translation will use for the number of heads the first value H in the sequence 16, 32, 64, 128, 255 for which the total disk capacity fits in 1024*H*63*512, and then computes the number of cylinders C as total capacity divided by (H*63*512).)  
 
The 33.8 GB limit (August 1999)
 
The next hurdle comes with a size over 33.8 GB. The problem is that with the default 16 heads and 63 sectors/track this corresponds to a number of cylinders of more than 65535, which does not fit into a short. Many BIOSes couldn't handle such disks. (See, e.g., Asus upgrades for new flash images that work.) Linux kernels older than 2.2.14 / 2.3.21 need a patch. See IDE problems with 34+ GB disks below.  
 
The 137 GB limit (Sept 2001)
 
As mentioned above, the old ATA protocol uses 16+4+8 = 28 bits to specify the sector number, and hence cannot address more than 2^28 sectors. ATA-6 describes an extension that allows the addressing of 2^48 sectors, a million times as much. There is support in very recent kernels.  
 
The 2 TiB limit
 
With 32-bit sector numbers, one can address 2 TiB. A lot of software will have to be rewritten once disks get larger.