WHO AM I

The fundamental goal of computer systems is to execute user programs and to make solving user problems easier.

The common functions of controlling and allocating resources are then brought together into one piece of software: the operating system.

Operating system is the one program running at all times on the computer—usually called the kernel.

For a computer to start running—for instance, when it is powered up or rebooted—it needs to have an initial program to run. This initial program, or bootstrap program, tends to be simple. The bootstrap program must know how to load the operating system and how to start executing that system.To accomplish this goal, the bootstrap program must locate and load into memory the operating system kernel. The operating system then starts executing the first process, such as “init,” and waits for some event to occur.

Hardware may trigger an interrupt at any time by sending a signal to the CPU, usually by way of the system bus. Software may trigger an interrupt by executing a special operation called a system call (also called a monitor call).

When the CPU is interrupted, it stops what it is doing and immediately transfers execution to a fixed location. The fixed location usually contains the starting address where the service routine for the interrupt is located. The interrupt service routine executes; on completion, the CPU resumes the interrupted computation.

After the interrupt is serviced, the saved return address is loaded into the program counter, and the interrupted computation resumes as though the interrupt had not occurred.

The CPU can load instructions only from memory, so any programs to run must be stored there.

Main memory is usually too small to store all needed programs and data permanently. Main memory is a volatile storage device that loses its contents when power is turned off or otherwise lost.

Useful links :

http://dmoz.org/Computers/Software/Operating Systems/Open Source/

Source :

Operating System Concepts Essentials ABRAHAM SILBERSCHATZ Yale University PETER BAER GALVIN Corporate Technologies, Inc. GREG GAGNE Westminster College