A modern general-purpose C/S consists of a CPU and a number of device controllers that are connected through a common bus that provide access to shared memory. See figure (2.1).
Figure (2.1) A modern computer system
When the computer to start running, 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. It initializes all aspects of the system from CPU registers to device controllers to memory contents. The bootstrap program must know how to load the O.S. and how to start executing that system. The bootstrap program must locate and load into memory, the O.S kernel. The O.S then start executing the first process such as "init" and wait for some event to occur. The occurrence of an event is usually signaled by an interrupt from either H/W or S/W. The Hardware may trigger an interrupt at any time by sending a signal to the CPU by way of the system bus. Software may trigger an interrupt by executing a special operation called a system call.
