The Basic Input Output System or BIOS is a firmware that runs when you start the computer, which locates and recognizes all the devices needed to load the operating system’s memory.
It is a very basic software installed on the motherboard that allows it to accomplish its mission. And provides low-level communication, performance and hardware configuration of the system that at least handles the keyboard and provides a basic output (standard beeping from the speaker of the computer in the event that an error occurs) during startup. The BIOS is usually written in assembly language.
The first use of the term “BIOS” occurred in the operating system CP / M, and describes the part of CP / M that run at startup, attached directly to the hardware. CP / M usually had a simple bootable boot read-only memory, and nothing more. Most versions of MS-DOS have a file called “IBMBIO.COM” or “IO.SYS” that is analogous to the BIOS of CP / M.
The BIOS is a basic input / output that normally goes unnoticed to the end user. Essentially, it loads the operating system into RAM. And has a hardware component and other software, the latter provides a text interface. In addition to several options for configuring the hardware installed on the PC, such as clock, or devices which start the operating system (Microsoft Windows, GNU / Linux, Mac OS X, etc.).
The Power-On Self-Test (POST, self-test at startup) is the first step in the wider process called priming. At this stage, the BIOS tests the presence of various devices and attempts to assign the necessary resources to operate without conflict. When POST is complete, control is transferred to the operating system.
Since the introduction of the IBM-compatible PC in August 1981, the BIOS first issues orders to the system during the startup phase, for example to indicate where to find it the boot loader.
Some BIOS settings can be adjusted by the user (for devices to detect a boot box, type and frequency of the processor, etc). All these parameters are permanently stored by a memory of reduced size (a few hundred bytes) at low power (CMOS) battery-powered (usually lithium) present on the motherboard. This memory is commonly called, by abuse, “CMOS”.
The source code of the first PC and AT BIOS was included with the IBM Technical Reference Manual. Today, most motherboards are shipped without the source code of the BIOS. The user must download updates provided by the manufacturer. These updates are sometimes needed to support certain devices.
The BIOS also contains diagnostic tools to check integrity of critical components such as memory, keyboard, hard drive, ports of entry / exit, etc. A system can contain several BIOS firmware chips. In addition to the BIOS boot located on the secondary storage unit and the motherboard.