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The Booting Process of the PC

Version 1.00 January 23, 2002

The process begins when the power supply is switched on:

  1. The power supply performs a self-test:
  2. The microprocessor timer chip receives the "Power Good" signal:
  3. The CPU starts executing the ROM BIOS code:
  4. The BIOS searches for adapters (usually video adapters) that may need to load their own ROM BIOS routines:
  5. The ROM BIOS checks to see if this is a 'cold boot' or a 'warm boot':

    To determine whether this is a "cold boot" or a "warm boot" the ROM BIOS startup routines check the value of the two bytes located at memory location "0000:0472".

  6. POST (Power-On Self-Test):

    The POST is a series of diagnostic tests that run automatically when you turn your computer on. The actual tests can differ depending on how the BIOS is configured, but usually the POST tests the following:

    -Note:"The POST operations are not the same for the all BIOS software"

    Any errors found during the POST are reported by a combination of beeps and displayed error messages. The errors which occur during the POST can be classified as either 'fatal' or 'non-fatal'. A non-fatal error (e.g. problem in the extended memory) will typically display an error message on the screen and allow the system to continue the boot process. A fatal error (e.g. problem in the processor), on the other hand, stops the process of booting the computer and is generally signaled by a series of beep-codes. However, successful completion of the POST is indicated by a single beep.

  7. The BIOS locates and reads the configuration information stored in CMOS:

    CMOS (Complementary Metal-Oxide Semiconductor) is a small area of memory (64 bytes) which is maintained by the current of a small battery attached to the motherboard. Most importantly, for the ROM BIOS startup routines (boot sequence), CMOS determines the order in which drives should be examined for an operating system (floppy disk first, CD-Rom first, or fixed disk first). Furthermore, it holds some essential information such as hard drive size, memory address location, and Date & Time.

  8. Shadow RAM: (Optional, you can turn it off/on using the CMOS settings)

    Shadow RAM is where a copy of BIOS routines from ROM is stored a special area of RAM, so that the BIOS routines can be accessed more quickly.

  9. Loading the OS (Operating System):

    The BIOS will attempt booting using the boot sequence determined by the CMOS settings, and examine the MBR (Master Boot Record) of the bootable disk.
    The MBR is the information in the first sector (512 bytes) of any hard disk or diskette that identifies how and where an operating system is located so that it can be loaded into the RAM (booted).

    The MBR is also sometimes called the "partition sector" or the "master partition table" because it includes a table that locates each partition that the hard disk has been formatted into. In addition to this table, the MBR also includes a program that reads the boot sector record of the partition containing the operating system to be booted into RAM. In turn, that record contains a program that loads the rest of the operating system into RAM.

By: S. Ebrahim Shubbar

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