![]() ![]() ![]() At this point, the majority of the BIOS’ work has completed and it’s almost ready to move on to the next stage of the boot process. It’s the BIOS that’s responsible for a variety of error messages such as “ keyboard error or no keyboard present” or warnings about mismatched/unrecognized memory. The BIOS on newer motherboards is smart enough to even recognize and identify USB devices, such as external drives and USB mice, letting you boot from USB sticks and use your mouse in legacy software.ĭuring the POST procedure, quick tests are conducted where possible, and errors caused by incompatible hardware, disconnected devices, or failing components are often caught. It scans the IO buses for attached hardware, and identifies and maps access to the hard disks you have connected to your PC. Once your PC has been powered on, the BIOS begins its work as part of the POST (Power-On Self Test) process. It bridges all the various parts of your PC together, and interfaces between them as required, setting up your video display to accept basic VGA and show it on the screen, initializing the memory banks and giving your CPU access to all the hardware. The startup routines and overall functionality of the simpler components like the RAM and PSU is hardwired into them as a series of logic circuits (AND/NAND and OR/NOR gates), while more complicated parts such as the video card have their own microcontrollers that act as mini-CPUs, controlling the hardware and interfacing with the rest of your PC to delegate and oversee the work. ![]() As each individual component receives life-giving electricity, it is powered up and brought online to its initial state. Electrical components of the PC are initially responsible for bringing your computer to life, as debouncing circuits take your push of the power button and trigger a switch that activates the power supply and directs current from the PSU to the motherboard and, mainly through it, to all the various components of your PC. When your PC is first powered up, a lot happens. Through standardized calls to the BIOS (“interrupts” in computer parlance), the operating system can trigger the BIOS to read and write to the disk and interface with other hardware components. The BIOS is the lowest level of software that interfaces with the hardware as a whole, 2 and is the interface by means of which the bootloader and operating system kernel can communicate with and control the hardware. ![]() The BIOS code is baked into the motherboard of your PC, usually stored on what is called an EEPROM 1 and is considerably hardware-specific. The BIOS is where hardware meets software for the first time, and where all the boot magic begins. The implementations of each component can differ greatly depending on your hardware and operating system, but the rules they follow and the process by which they work are always the same. In the diagram below, the boot sequence for all standard computers and operating systems is shown:Īs you can see, the boot process is broken down into several major components, each of which is a completely-separate subsystem with many different options and variations. ![]()
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