POST tests the very core components, the ones it needs even just to complete POST, such as a small amount of memory, the processor, and the BIOS itself.
If there are any core failures, or if video cannot be started, BIOS may use the internal speaker to give you an audible clue, such as a beep code, or a recorded voice message.
Once the display is up, additional memory is either counted, or actually tested.
BIOS has a list of devices that should exist. This list was built the last time the computer was started, and includes anything that has been entered via the BIOS Setup program.
BIOS checks for each device in the list. Any changes will be displayed, possibly as an error. Some changes will automatically be applied to the list for the next time the computer is started, others may require you to go into BIOS Setup.
When BIOS finishes with it's list of devices, it scans the expansion bus of the computer for other installed devices that have their own BIOS.
Control is turned over to each other BIOS located in the computer, one at a time. Each BIOS may have it's own Setup program, depending on what the device is.
- Video cards often have their own BIOS, but they rarely have a Setup program.
- Add-in cards for hard drives will often have their own BIOS, with their own setup program.
After each device has finished initializing, the system BIOS regains control over the computer and begins looking for an Operating System to load.
- Where the BIOS looks for an OS is controlled via the BIOS setup program.
BIOS loads the very first OS it finds. Every OS has a small program that BIOS can recognize, called a 'bootstrap'.
The bootstrap is loaded into memory, and control is turned over to the bootstrap program. At this point, BIOS is done.

Loading the Operating System

The OS bootstrap locates the main files for the OS and begins loading them into memory. The OS core is called the 'kernel'.

Control is turned over to the kernel, which then repeats many of the same tasks the BIOS did previously.

Just like BIOS, the OS has a list of devices it knows are supposed to be installed. This list was built on previous starts, and includes items you installed while running the OS previously.
The OS loads special software for each device that allows the OS to talk to it. This software is called a 'driver'.
As each driver is loaded, the OS initializes each device and confirms that it passes basic tests.

Once all devices are initialized, the kernel loads the User Interface (UI). This is recognized by the controls beginning to show up on the screen.

This may be the mouse pointer, the 'desktop', buttons, and other icons.

Now the OS will begin startup processes. These are tasks that have been defined by the user, or more likely tasks defined by software the user has run.

Examples are startup tasks may be loading Anti-Virus software, starting a firewall, checking for software updates, or starting other programs that have been selected to run anytime the OS is running.

The computer is then ready for you to start using it.