Flipping the Switch

Daily hundreds of people, probably including you, switch on their computer. You’ve heard me talk a lot about Operating Systems and hardware and how they work together. But I never explained what happens when you push the start button of your computer.

First of all you press the start button on your laptop or your computer. This issues the power supply to send a certain amount of voltage to the computer components (CPU, RAM, keyboard , etc). At this moment, your computer is still useless. The RAM memory is empty and no OS has been loaded. To begin booting, the CPU looks at the ROM, the read only memory, it will find the basic input output system, the BIOS. The BIOS will explain where to find an operating system, or at least, where the CPU could possibly find an OS. Like a hard drive, DVD drive, flash drive.

But this is the second step of the booting process, first something else happens. The POST test is issued, the Power On Self Test. This test will check the system clock & hardware information, compares the system settings to hardware, tests RAM, Disk Drivers, Optical Drives, Hardware Drives. When this has run successfully the BIOS will look for the OS. It does this by following a certain boot sequence, typically this is Floppy Drive, CD/DVD Drive, Hard Drive.

The best case scenario is that an OS is found (which is true in most cases). The next step is to load the OS. And frankly, an OS is like a giant script that is executed, a big program that has to run. Once it is initialized it copies certain files to the memory (RAM) to be able to access them. Following to this, the OS will take note of the available system memory.

Now the next step is to make sure that the OS can communicate with the hardware properly. Drivers will be loaded for the printer, scanner, optical drives, graphics card, etc. Once this is done, the final step will be executed, and this hasn’t got much to do with actually booting the OS. It’s booting the programs that are started along with the OS. Most of these things we have set up ourselves, like an antivirus.

This is an overview of what happens when you flip the switch, the detailed description is somewhat more complex. But knowing this will get you a long way.

It’s alive!


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