The operating system may be loaded into the computers memory in two ways.  it is already present in ROM (so is permanent, immediately accessible and difficult to update)  it is loaded from disk when the computer is turned on.

 If the operating system is already present in ROM (for systems like industrial controllers, petrol pumps etc), it will gain control immediately the processor is powered on. This method is best suited for small appliances and hand held devices where the operating system is relatively simple and small.  For more complex systems, the operating system is usually stored on secondary media (such as disk), and is loaded into the computer memory (RAM) when the computer is powered on. Advantages of this type of system are that changes to the operating system are easier to make and implement.

The bootstrap process describes the task of initially loading the operating system from disk into RAM. A small routine stored in ROM, called the BOOTSTRAP LOADER or IPL (Initial Program Loader)..

BATCH: The earliest type, allowed only one program to run at a time. The program was entered into the computer, then run till completed. The data used by the program could not be modified whilst the program was running. Any errors in the program or data mean starting all over again.

INTER-ACTIVE: These allow the modification and entry of data whilst the program is running. Typical systems are airline reservations and languages such as BASIC. TIME-SHARING/MULTI-USER: These share the computer system amongst more than one user, and employ pre- emptive scheduling techniques.

MULTI-TASKING: More than one process may be executed at once. The processor is switched rapidly between the processes. A user may run more than one process at a time. REAL-TIME: Primarily used in process control, telecommunications, etc. The OS monitors various inputs which affect the execution of processes, changing the computers model of the environment, thus affecting the outputs.

MULTI-PROCESSOR: A computer that has more than one processor dedicated to running processes. EMBEDDED: An embedded operating system means the operating system is self-contained in the device and resident in ROM. Typical systems that use embedded operating systems are household appliances, car management systems, traffic control systems and energy management systems.

These are examples of embedded systems:

Embedded systems are also known as dedicated systems. This is because they only perform a specific task, and cannot run a wide variety of programs like a home computer (which we previously identified as a general purpose system). Windows NT workstation is an interactive, multitasking multiprocessor operating system. Windows 98 is an interactive, multitasking operating system. Linux is an interactive, multitasking, multiprocessor, multi-user operating system.

A shell is a program that handles user input and output. It provides routines for handling user input from a keyboard or mouse, as well as routines for displaying information on the terminal screen. A shell also provides a mechanism to interpret user commands and run additional programs that users request. In program called command.com was the shell in MS- DOS. In Windows 98 and NT workstation this shell is still present (it is called the command prompt and you run it by accessing the command prompt icon). The UNIX shell is called the bourne shell, and is a program called sh. There are other UNIX shell programs.