Once you hit your computer's on switch, a number of very important, and a couple of not-so-important, things happen almost simultaneously. In fact, when you break it down, it's amazing how quickly PCs boot up given the gigantic checklist of things they have to do internally before you can log on to your e-mail, check your bank balance, or look up cat pictures.
Powering Up
Computer
On boot up, your PC's power supply will send power in differing levels of voltage to all of the components (motherboard, central processing unit, disk drives, etc.). The power supply has to be exactly right or some of the components won't be allowed to start up, so the quality of the power signal is monitored from the get-go.
The BIOS
BIOS stands for basic input/output system and it is the first code that a PC runs upon boot up. Its main job is to start up the operating system of your computer. It locates and activates hardware and then executes your start up software. You've undoubtedly seen the BIOS screen on your monitor during boot up. BIOS is also where you can make important system changes if needed.
The BIOS is not contained within Windows, it's encoded onto the Motherboard so that it can be a launching pad for other things.
Hardware
At an early stage, drivers for different hardware components are launched. A driver is a piece of software that acts as a mediator between a device (like a printer or monitor or camera) and the operating system. Once installed, drivers initiate when the computer does so that thee hardware devices are quickly accessed and made ready while you're computing.
Software
Windows
More generally, the operating system. Windows is, after all, just another program. It's just a really big program and one in which other programs can operate. After the initial boot stage via the BIOS and the driver loading sequence, the operating system revs up.
StartUp Commands
Windows supports a seemingly endless supply of start-up commands. These are little agent programs, set up by bigger applications that tell the computer to run all or part of that application on bootup. This can be good or bad as it can launch anything, but generally the idea behind startups is helpful. If a program takes a long time to load, it's useful for the computer to load a part of it right away. That way, when you need it, you only have to wait a few seconds for all of the needed components to get started.
Startups can launch whole applications, but usually the idea is to launch piece of an application, something that can stand by and be ready when needed.
Services
Services are the background applications on a computer, and they're often the things that the startup commands are launching. Why would a program need to run something constantly in the background? Well there are few reasons...
- To speed things up. As mentioned, big programs, including applications like Microsoft Word and Excel take a while to load. By having a single MS Office service, shared components can be pre-loaded and run continuously. That way they're ready as soon as they're needed.
- To check for updates. Lots of applications have ongoing updaters. These programs run in the background and every now and then check the Web looking for updates. If you run Windows long enough, you've probably seen prompts from the Adobe Updater or the Java Update Scheduler.
- To watch for changes. Most security and optimization programs use an on-going service to monitor for changes. It's because of a service that anti-virus applications can halt a worm or Trojan as soon as it tries to activate itself.
With services in place, the computer is fully on, with the operating system standing by.
What Happens When You Boot a Computer?
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