Powering Up a PC

A typical computer session begins with turning on the power. Here's what happens in that process:

1. You press the "On" button on the computer and the monitor.

2. You see the BIOS software doing its thing, called the power-on self-test (POST). On many machines, the BIOS displays text describing such data as the amount of memory installed in your computer and the type of hard disk you have. During this boot sequence, the BIOS does a remarkable amount of work to get your computer ready to run.

• The BIOS determines whether the video card is operational. Most video cards have a miniature BIOS of their own that initializes the memory and graphics processor on the card. If they don't, there is usually video-driver information on another ROM on the motherboard that the BIOS can load.


• The BIOS checks to see if this is a cold boot or a reboot. It does this by checking the value at memory address 0000:0472. A value of 1234h indicates a reboot, in which case the BIOS skips the rest of POST. Any other value is considered a cold boot.


• If your computer is undergoing a cold boot, the BIOS verifies RAM by performing a read/write test of each memory address. It checks for a keyboard and a mouse. It looks for an expansion bus and, if it finds one, checks all the connected cards. If the BIOS finds any errors during the POST, it notifies you with a series of beeps or a text message displayed on the screen. An error at this point is almost always a hardware problem.


• The BIOS displays some details about your system. This typically includes information about the following:
  
  
  
  • Processor
  
  
  • Floppy and hard drive
  
  
  • Memory
  
  
  • BIOS revision and date
  
  
  • Display
  
  
  
  


• Any special drivers, such as those for expansion cards, are loaded from the adapter and the BIOS displays the information.


• The BIOS looks at the sequence of storage devices identified as boot devices in the complementary metal-oxide semiconductor (CMOS) setup. Boot is short for "bootstrap," as in the old phrase "Pull yourself up by your bootstraps." Boot refers to the process of launching the operating system. The BIOS tries to initiate the boot sequence from the first device using the bootstrap loader.

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This animation walks you through a typical PC session.
3. The bootstrap loader loads the operating system into memory and allows it to begin operation. It does this by setting up the divisions of memory that hold the operating system, user information and applications. The bootstrap loader then establishes the data structures that are used to communicate within and between the sub-systems and applications of the computer. Finally, it turns control of the computer over to the operating system.