How Computers Boot Up

Posted on June 11, 2008  Comments (3)

How Computers Boot Up

Things start rolling when you press the power button on the computer (no! do tell!). Once the motherboard is powered up it initializes its own firmware – the chipset and other tidbits – and tries to get the CPU running. If things fail at this point (e.g., the CPU is busted or missing) then you will likely have a system that looks completely dead except for rotating fans. A few motherboards manage to emit beeps for an absent or faulty CPU, but the zombie-with-fans state is the most common scenario based on my experience. Sometimes USB or other devices can cause this to happen: unplugging all non-essential devices is a possible cure for a system that was working and suddenly appears dead like this. You can then single out the culprit device by elimination.

If all is well the CPU starts running. In a multi-processor or multi-core system one CPU is dynamically chosen to be the bootstrap processor (BSP) that runs all of the BIOS and 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. Intel CPUs have been evolving over the years but they’re fully backwards compatible, so modern CPUs can behave like the original 1978 Intel 8086, which is exactly what they do after power up. In this primitive power up state the processor is in real mode with memory paging disabled. This is like ancient MS-DOS where only 1 MB of memory can be addressed and any code can write to any place in memory – there’s no notion of protection or privilege.

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3 Responses to “How Computers Boot Up”

  1. Curious Cat Science and Engineering Blog » Demystifying the Memristor
    November 23rd, 2008 @ 2:43 pm

    “Engineers could, for example, develop a new kind of computer memory that would supplement and eventually replace today’s commonly used dynamic random access memory (D-RAM). Computers using conventional D-RAM lack the ability to retain information once they are turned off…”

  2. Yasir
    February 4th, 2010 @ 5:06 am

    I want to know from a naive point of view that what happens if there is no BIOS present in the system.

    As we all know that BIOS is the thing that is started when we power on the computer and it then gives instructions to all the devices to do some particular thing.

    But in the same way when the power is given to a particular hardware device say RAM then it may perform some self test or something to be ready for instructions from BIOS.


    After power on:

    – The RAM may initiate some of its registers.
    – May increment any counter.
    – May get itself ready for input.

    and other hardware components may do this as well …
    I want to get this information. The phenomenal change of hardware components when the get powered on!!

  3. Anonymous
    June 25th, 2012 @ 7:24 pm

    Faulty capacitors can also result in zombie-with-fans mode. To Yasir: if a computer didn’t have BIOS, it would at least have to have something similar or an operating system wouldn’t be able to boot.

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