Bios
BIOS, the Basic Input/Output System, is the firmware that a traditional IBM-compatible PC runs first when powered on: it tests and initializes the hardware, offers a small set of low-level services, and then finds and starts the operating system's boot loader. It is stored on a chip on the motherboard, and its design dates to the original IBM PC of 1981. The term is often used loosely to mean the setup screen reached with a key such as Delete or F2, or even to mean any PC startup firmware, but strictly it names the legacy design that UEFI has since replaced on almost all new machines.
The term BIOS was introduced by Gary Kildall in his CP/M operating system in the mid-1970s, and the concept was to isolate the hardware-specific code from the rest of the system. CP/M's BIOS held the routines for the console, disk and printer of a particular machine, so the main operating system could be moved to new hardware by rewriting only that layer. The IBM PC copied the idea in a read-only chip on its motherboard, and its services were used by DOS programs through software interrupts, for example the interrupt for video and the one for disk access. The BIOS was copyrighted by IBM, so rivals had to reproduce its behavior without copying the code: Compaq did this with its Compaq Portable of 1982 using a clean-room reverse-engineering process, creating the IBM-compatible clone industry, and firms such as Phoenix Technologies, Award and AMI later sold BIOS code to many makers.
At power-on, the BIOS runs the power-on self-test (POST), then configures the hardware and looks for something to boot. POST checks the processor, memory and essential devices, and failures are reported by beep codes or error messages, since a graphics output may not yet exist. The BIOS then initializes the chipset, keyboard, display and storage controllers, reads its settings from a small battery-backed memory traditionally called CMOS, and follows the configured boot order through disks, optical drives, USB devices or the network. For a disk it reads the first 512-byte sector, the master boot record (MBR), checks for the signature bytes 0x55 and 0xAA at its end and, if valid, loads and jumps to the boot code stored there, which usually chains to a boot loader such as GRUB or the Windows Boot Manager. If the battery that powers the settings fails, the clock and configuration reset, which is a common source of date errors on old machines.
The design of the BIOS imposed limits that eventually led to its replacement. It ran in 16-bit real mode with about a megabyte of addressable memory, so its drivers and setup screens were cramped and slow. The MBR partition scheme stores sector addresses in 32 bits, which with 512-byte sectors caps usable disk size at 2 tebibytes, and allows only four primary partitions without workarounds. Updating the BIOS, done by flashing a new image to the chip, carried the risk of bricking the board if power failed, and the BIOS had no built-in way to verify that the boot code it ran was trustworthy. These constraints, along with the rise of large disks and security requirements, led the industry to UEFI, which keeps the role but changes the design, and many modern machines still offer a compatibility mode to boot old operating systems.
Some common beliefs about BIOS should be corrected. The BIOS is not the operating system, and it does not load every program; it hands control to the boot loader and mostly steps aside, although legacy operating systems such as DOS kept calling its services. Clearing the CMOS resets settings but does not erase disks, and a BIOS update does not usually speed up a computer, though it can add support for new processors or fix security flaws. Today the word BIOS survives in everyday speech even on UEFI machines, so someone told to enter the BIOS is most likely being sent to a UEFI setup screen.
Read the BIOS as the first small program a legacy PC runs, whose job is to test the machine, offer basic hardware services and hand control to a boot loader found in the first sector of a disk, a design that made the PC compatible and cloneable in 1981 and whose limits are the reason UEFI replaced it.