Logical Block Addressing - Enhanced BIOS

Enhanced BIOS

The earlier IDE standard from Western Digital introduced 22 bit LBA; in 1994, the ATA-1 standard allowed for 28 bit addresses in both LBA and CHS modes. The CHS scheme used 16 bits for cylinder, 4 bits for head and 8 bits for sector, counting sectors from 1 to 255. This means the reported number of heads never exceeds 16 (0-15), the number of sectors can be 255 (1-255; though 63 is often the largest used) and the number of cylinders can be as large as 65,536 (0-65535), limiting disk size to 128 GiB (≈137.4 GB), assuming 512 byte sectors. These values can be accessed by issuing the ATA command "Identify Device" (ECh) to the drive.

However IBM BIOS implementation defined in the INT 13H disk access routines used quite a different 24-bit scheme for CHS addressing, with 10 bits for cylinder, 8 bits for head, and 6 bits for sector, or 1024 cylinders, 256 heads, and 63 sectors. This INT 13H implementation had pre-dated the ATA standard, as it was introduced when the IBM PC had only floppy disk storage, and when hard disk drives were introduced on the IBM PC/XT, INT 13H interface could not be practically redesigned due to backward compatibility issues. Overlapping ATA CHS mapping with BIOS CHS mapping produced the lowest common denominator of 10:4:6 bits, or 1024 cylinders, 16 heads, and 63 sectors, which gave the practical limit of 1024×16×63 sectors and 528 Mbytes (504 MiB), assuming 512 byte sectors.

In order for BIOS to overcome this limit and successfully work with large hard drives, a CHS translation scheme had to be implemented in BIOS disk I/O routines which would convert between 24-bit CHS used by INT 13H and 28-bit CHS numbering used by ATA. The translation scheme was called Large or Bit Shift Translation. This method would remap 16:4:8 bit ATA cylinders and heads to 10:8:6 bit scheme used by INT 13H, generating much more "virtual" drive heads than the physical disk reported. This increased the practical limit to 1024×256×63 sectors, or 8.4 Gbytes (7.8 GiB).

To further overcome this limit, INT 13H Extensions were introduced with BIOS Enhanced Disk Drive Services specification, which removed practical limits on disk size for operating systems which are aware of this new interface, such as DOS 7.0 component in Windows 95. This Enhanced BIOS subsystem supports LBA addressing with LBA or LBA-Assist method, which uses native 28-bit LBA for addressing ATA disks and performs CHS conversion as needed.

The Normal or None method reverts to the earlier 10:4:6 bit CHS mode which does not support addressing more than 528 Mbytes.

Until the release of ATA-2 standard in 1996, there were a handful of large hard drives which did not support LBA addressing, so only Large or Normal methods could be used. However using the Large method also introduced portability problems, as different BIOSes often used different and incompatible translation methods, and hard drives partitioned on a computer with BIOS from a particular vendor often could not be read on a computer with a different make of BIOS. The solution was to use conversion software such as OnTrack Disk Manager, EZ-Drive, etc., which installed to the disk's OS loader and replaced INT 13H routines at boot time with custom code. This software could also enable LBA and INT 13H Extensions support for older computers with non LBA-compliant BIOSes.

The current 48-bit LBA scheme, introduced in 2003 with ATA-6 standard, allows addressing up to 128 PiB. Current PC-Compatible computers support INT 13H Extensions, which use 64-bit structures for LBA addressing and should encompass any future extension of LBA addressing, though modern operating systems implement direct disk access and do not use the BIOS subsystems, except at boot load time. However, the common DOS style Master boot record partition table only supports disk partitions up to 2 TiB in size. For large partitions this needs to be replaced by another scheme for instance the GUID Partition Table which has the same 64-bit limit as the current INT 13H Extensions.