System Loading

How does an Operating System load itself?
In this section we look at how the operating system loads when the computer is first turned on. 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.

What is the BOOTSTRAP PROCESS?
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), reads a special load routine from the diskette.

In floppy based system, this routine is normally located on Track 00, sector 00 (or 01), and is called the boot sector. The code contained in the sector is transferred into RAM, then executed. It has the sole responsibility for loading the rest of the operating system into memory.