The Basic Input-Output System, known simply as the BIOS, is a computer program written in flash memory electronics on the motherboard.
The Bios controls the operation of the motherboard or motherboard and its components. It is responsible for performing the basic functions of computer management and configuration
After a reset or power-up, the processor executes the instruction found in the so-called reset vector (16 bytes before the maximum addressable instruction in the case of x86 processors), there is the first line of BIOS code: is an unconditional jump instruction, which refers to a lower address in the BIOS.
In older PCs the processor continued to read the instructions directly in RAM (since that memory was of the same speed of the RAM), executing the POST routines to verify the operation of the system and later loading an operating system (of 16 bits) into RAM, which would share BIOS functionality.
According to each manufacturer of the BIOS, it will perform different procedures, but in general a copy of the firmware is loaded towards the RAM, since the latter is faster. From there the detection and configuration of the various devices that may contain an operating system is performed. While performing the search process for an OS, the BIOS program offers the option of accessing the RAM-CMOS system where the user can configure various system features, such as the real-time clock. The information contained in RAM-CMOS is used during BIOS execution to configure devices such as fans, buses and controllers.
The BIOS hardware drivers are written in 16 bits being incompatible with 32 and 64 bit OSs, they load their own versions during startup, replacing those used in the early stages.
For a motherboard reference, the manufacturer can publish several BIOS revisions, which solve problems detected in the first batches, code better drivers, or support new processors.
The update of this firmware can be done with a program to burn a new version directly from the operating system, the programs are owned by each company that develops the firmware and can
usually be obtained on the internet next to the BIOS itself.
The BIOS update is perceived as not risk free, since a failure in the procedure leads to the motherboard not starting. Because of this some manufacturers use systems such as bootblock, which is a portion of BIOS that is protected and not upgradeable like the rest of the firmware.
BIOS Firmware on Adapter Cards
A system can contain various chips with BIOS firmware in addition to the one on the motherboard: video, network and other cards carry bits of code in memory (with the help of the main BIOS) that allows the operation of those devices.
The vast majority of x86 Architecture Motherboard vendors delegate to third-party BIOS production. Manufacturers often write and publish firmware updates that fix problems or provide compatibility with new products.
The main BIOS vendors are American Megatrends (AMI) and Phoenix Technologies (which purchased Award Software International in 1998). There are BIOS projects under the free software scheme like Coreboot that offer alternate firmware for a few motherboard references.
Unlike other system components, the video card should run from initial startup, long before any operating system is being loaded into RAM: on systems with embedded video, the BIOS on the motherboard contains routines necessary to operate the video of the board.
The first computers (which did not have integrated video) had BIOS capable of controlling any MDA and CGA adapter cards. In 1984 when new systems like the EGA appeared, it was necessary to add a video BIOS to maintain compatibility with those systems that did not have the management routines for the new standard; since that time the video cards have their own firmware.
The BIOS of these adapters provides basic tools to handle the video hardware offered by the card. When the computer starts, some of these cards display the brand of the card, model and firmware version in addition to the size of the video memory.
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