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System Boot Sequence

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System Boot Sequence

The system BIOS is what starts the computer running when you turn it on. The following are the steps that a typical boot sequence involves. Of course this will vary by the manufacturer of your hardware, BIOS, etc., and especially by what peripherals you have in the PC. Here is what generally happens when you turn on your system power:

1. The internal power supply turns on and initializes. The power supply takes some time until it can generate reliable power for the rest of the computer, and having it turn on prematurely could potentially lead to damage. Therefore, the chipset will generate a reset signal to the processor (the same as if you held the reset button down for a while on your case) until it receives the Power Good signal from the power supply.

2. When the reset button is released, the processor will be ready to start executing. When the processor first starts up, it is suffering from amnesia; there is nothing at all in the memory to execute. Of course processor makers know this will happen, so they pre-program the processor to always look at the same place in the system BIOS ROM for the start of the BIOS boot program. This is normally location FFFF0h, right at the end of the system memory. They put it there so that the size of the ROM can be changed without creating compatibility problems. Since there are only 16 bytes left from there to the end of conventional memory, this location just contains a "jump" instruction telling the processor where to go to find the real BIOS startup program.

3. The BIOS performs the power-on self test (POST). If there are any fatal errors, the boot process stops. POST beep codes can be found in this area of the Troubleshooting Expert.

4. The BIOS looks for the video card. In particular, it looks for the video card's built in BIOS program and runs it. This BIOS is normally found at location C000h in memory. The system BIOS executes the video card BIOS, which initializes the video card. Most modern cards will display information on the screen about the video card. (This is why on a modern PC you usually see something on the screen about the video card before you see the messages from the system BIOS itself).

5. The BIOS then looks for other devices' ROMs to see if any of them have BIOSes.
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