System BIOS, commonly known as BIOS, is an abbreviation for basic input/output system. It is the medium between the hardware and the operating system. The prime function of BIOS is to test computer hardware for its functionality every time the computer starts up. It conducts the POST (power on self test), which helps the computer to attain the ideal hardware settings by either manual or automatic input. Except in operating systems like Linux, system BIOS provides complete support for the computer.
Cylinder, Head and Sector Mode
Cylinder, head and sector mode is commonly abbreviated as CHS mode and also known as the normal mode among the three modes that relate to hard drives. This mode is used by hard drives that have fewer than 500 megabytes in disc space. Cylinder, head and sector stand for the three main parameters that are used in hard disk specifications.
The CHS mode is obsolete now because hard drives with capacities significantly greater than 500 mb are developed. Any hard drive using the CHS mode is limited to 1,024 cylinders,16 heads and 63 sectors, which translates into 500 megabytes of space. The major shortcoming of this mode is that it doesn't allow translation at the BIOS level. It was the mode used in previous BIOSes, especially before 1994, according to the PC Guide.
Extended Cylinder, Head and Sector Mode
The extended cylinder, head and sector (ECHS) mode is also known as the large mode. This mode is mainly used for hard drives with disk spaces between 504 megabytes and 8.4 gigabytes. It limits the hard drive to 1,024 cylinders, 256 heads and 63 sectors in the standard BIOS mode.
It is not frequently used, as hard drives with capacities greater than 8.4 gigabytes are commonly used in both domestic and industrial computers. Hard drives with capacity of up to 10 Gigabytes can run on this drive, but there is a prior requirement of disk partition, which may result in a barrier hit, causing the computer problem known as 'hanging.'
Logical Block Addressing
Logical block addressing or LBA mode is a unique type of mode that doesn't use the traditional idea of a specific numbered cylinder, head and sector address to locate data on a hard drive. Instead of the usual mode of addressing, it uses sectors to determine the data location on the hard drive. The sectors are numbered starting from zero to (n-1), in which 'n' is the number of sectors available on the hard drive.
A hard drive using the LBA mode is not subject to the 504 megabytes barrier, due to the unique translation it uses for the data. It is the most common mode in use today, as it can handle a huge amount of data on hard drives without any problem.
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