What is the BIOS doing during the bootup sequence?
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When you turn on your computer, several events occur automatically:
- The CPU "wakes up" (has power) and reads the x86 code in the BIOS chip.
- The code in the BIOS chip runs a series of tests, called the
POST for Power On Self Test, to make sure the system devices are working correctly.
In general, the BIOS:
- Initializes system hardware and chipset registers
- Initializes power management
- Tests RAM (Random Access Memory)
- Enables the keyboard
- Tests serial and parallel ports
- Initializes floppy disk drives and hard disk drive controllers
- Displays system summary information
- During POST, the BIOS compares the system configuration data obtained from POST with the system information stored on a CMOS - Complementary Metal-Oxide Semiconductor - memory chip located on the motherboard. (This CMOS chip, which is updated whenever new system components are added, contains the latest information about system components.)
- After the POST tasks are completed, the BIOS looks for the boot program responsible for loading the operating system. Usually, the BIOS looks on the floppy disk drive A: followed by drive C:.
- After being loaded into memory, the boot program then loads the system configuration information (contained in the registry in a Windows environment) and device drivers.
- Finally, the operating system is loaded, and, if this is a Windows environment, the programs in the Start Up folder are executed.
Go back to our BIOS FAQ.
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