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Operating System is a software package that makes the computer hardwares operated as a predesigned system.

BIOS

Basic Input/Output System (BIOS) is a standard softwre which is stored on a ROM of a PC compatible computer as the first run program of the computer. A computer is a collection of hardware elements. There is usually a main board with a processor, rams, device controllers, extension interface, on-board devices and connectors. However every hardware element itself is usually provided with limited functions which is unable to perform very useful tasks without a well programmed software. Besides, hardware elements in a computer always work on its own way when the computer is first powered up and therefore an initial setup is needed to test and initial the hardware elements, and to search and load software from mass storage device for transfering the control of the processor to. 

Old operating systems, e.g. DOS, use BIOS services while modern operating systems, e.g MS Windows do not use BIOS after loading.

Unified Extensible Firmware Interface (UEFI) becomes more popular and is used to replace the Basic Input/Output System (BIOS).

Bootloader

For a mainboard with a BIOS, bootloader is the second software to be executed by the processor. BIOS first locates the boot device, then loads and executes the boot loader program stored on the boot sector. A boot sector is also called master boot record (MBR). Since the boot loader program is loaded from the boot sector of a boot device instead of embeded in the ROM chip of the main board, the design of the boot loader program can be more flexible. For instance, the boot loader program can be used to select an operating system from a list or pass parameters to an operating system. And eventually, the kernel of an operating system is loaded into memory and the control is then handed over to the kernel.

The Kernel

The kernel of an operating system is the third software to be executed by the processor. A kernel is the top level software running on the computer to provide the necessary functions for ensuring that both hardwares and softwares can operate properly on the computer. The kernal also provides a computing platform for all other compatible programs to be processed by kernel on the computer. The key functions are

• The first important function of the kernel is to detect and drive the hardware attached to the computer. The detected device is initiated and a corresponding device driver is assigned such that the detected device is available to other software with a simple standard programming interface provided by the kernel.

• Another important function is to create and manage the data filing systems of all physical data storage devices connected to the computer. Similarly, the indentified device is initiated and a corresponding device driver is assigned such that the indentified device is available to other software with a simple standard programming interface provided by the kernel. Information of drives, directories and files is translated according the file naming system so that data can be obtained without knowing the actural physical location of the data storage device.

•  The other important function is to provide core sharing functions for hardwares and softwares. For example, file handling functions are shared for all software to access the file systems, filesystem handling functions are shared by all physically storage devices.

• Processes Management functions is one of the core functions provided by the kernel. Processes Managment is the top level program.

• Rights management functions is an other core functions provided by the kernel. A rights management system is used to control the activity on the computer.

The User space

All other softwares other than the kernal are in the user space. Besides the user application softwares that is used to perform useful tasks for the user, there are also general purpose system software to share the common platform or framework in the form of software layers.