Operating System : Computer System Structure

Computer-System Structure:

1. Single-Processor System : On a single processor system, there is one main CPU capable of executing a general-purpose instruction set, including instructions from user processes. Almost all single processor systems have other special-purpose processors as well. They may come in the form of device-specific processors, such as disk, keyboard, and graphics controllers; or, on mainframes, they may come in the form of more general-purpose processors, such as I/ O processors that move data rapidly among the components of the system. All of these special-purpose processors run a limited instruction set and do not run user processes. If there is only one general purpose CPU, then the system is a single-processor system.

2. Multiprocessor Systems: Multiprocessor systems (also known as parallel systems or multicore systems) have begun to dominate the landscape of computing. Such systems have two or more processors in close communication, sharing the computer bus and sometimes the clock, memory, and peripheral devices.

Multiprocessor systems have three main advantages:

1. Increased throughput: By increasing the number of processors, we expect to get more work done in less time. The speed-up ratio with N processors is not N, however; rather, it is less than N.

2. Economy of scale. Multiprocessor systems can cost less than equivalent multiple single-processor systems, because they can share peripherals, mass storage, and power supplies.

3. Increased reliability. If functions can be distributed properly among several processors, then the failure of one processor will not halt the system, only slow it

down. If we have ten processors and one fails, then each of the remaining nine processors can pick up a share of the work of the failed processor.

The multiple-processor systems in use today are of two types.:

·        Some systems use asymmetric multiprocessing , in which each processor is assigned a specific task. A boss processor controls the system; the other processors either look to the boss for instruction or have predefined tasks. This scheme defines a boss–worker relationship. The boss processor schedules and allocates work to the worker processors.

·        The most common systems use symmetric multiprocessing (SMP), in which each processor performs all tasks within the operating system. SMP means that all processors are peers; no boss–worker relationship exists between processors.