How does a computer boot?

The main thing about how our computer works is that the CPU doesn’t really know anything about what it’s connected to. It communicates with the outside environment only through pins to which it is connected. There are three main ways by which the CPU and the outside environment communicate: memory address space, I/O address space, and interrupts.

A motherboard is a physical arrangement in a computer that contains the basic circuitry and components of a computer.Motherboard has a front side bus which connects the CPU to the north bridge. Whenever the CPU needs to read or write memory it does so via this bus. It uses some pins to transmit the physical memory address it wants to write or read, while other pins send the value to be written or receive the value being read. When the north bridge receives a physical memory request it decides where to route it.

The basic processing starts when the power button of the computer is turned on.Once the motherboard is powered up it initializes its own firmware and tries to get the CPU running. Thus the CPU starts running. In a multi-processor system one CPU is dynamically chosen to be the bootstrap processor (BSP) that runs the kernel initialization code. The remaining processors, called application processors (AP) at this point, remain halted until later on when they are explicitly activated by the kernel. The CPU then starts executing the BIOS code which initializes some of the hardware in the machine. After that the BIOS kicks off the Power On Self Test mode(POST) which tests the various components in computer.

After the POST the BIOS wants to boot up an operating system. The actual order in which the BIOS seeks a boot device is user configurable. If there is no suitable boot device the BIOS halts with a complaint like “Non-System Disk or Disk Error.” The BIOS now reads the first 512-byte sector (sector zero) of the hard disk. This is called the Master Boot Record and it normally contains two vital components: a tiny OS-specific bootstrapping program at the start of the MBR followed by a partition table for the disk.

Thus in this way the process of computer booting works.

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