The Computer System Concept.
A computer is more than a high-powered collection of electronic devices performing a variety of information processing chores. A computer is a system, an interrelated combination of components that performs the basic system functions of input, processing, output, storage, and control, thus providing end users with a powerful information processing tool. Understanding the computer as a computer system is vital to the effective use and management of computers.
A computer is system of hardware devices organized according to the following system functions.
- Input. The input devices of a computer system include keyboards, touch screens, pens, electronic mice, optical scanners, and so on.
- Processing. The central processing unit( CPU) is the main processing component of a computer system. (In microcomputers, it is the main microprocessor.) In particular, the electronic circuits of the arithmetic-logic unit one of the CPU’s major components, perform the arithmetic and logic functions required in computer processing.
- Output. The output devices of a computer system include video display units, printers, audio response units , and so on, They convert electronic information produced by the computer system into human intelligible form for presentation to end users.
- Storage. The storage function of a computer system takes place in the storage circuits of the computer’s primary storage unit, or memory, and in secondary storage devices such as magnetic disk and tape units. These devices store data and program instructions needed for processing.
- Control. The control unit of the CPU is the control component of a computer system. Its circuits interpret computer program instructions and transmit directions to the other components of the computer system.
The Central Processing Unit.
The central processing unit is the most important hardware component of a computer system. It is also known as the CPU, the central processor or instruction processor, and the main microprocessor in a microcomputer. Conceptually, the circuitry of a CPU can be subdivided into two major subunits the arithmetic-logic unit and the control unit. The CPU also includes circuitry for devices such as registers and cache memory for high —speed, temporary storage of instruction operations, input/output, and telecommunications support.
The control unit obtains instructions from software segments stored in the primary storage unit and interprets them. Then it transmits electronic signals to the other components of the computer system to perform required operations. The arithmetic-logic unit performs required arithmetic and comparison operations .A computer can make logical changes from one set of program instructions to another (e.g, overtime pay versus regular pay calculations) based on the results of comparisons made in the ALU during processing.
Main Memory and Primary Storage Unit.
A computer’s primary storage unit is commonly called main memory, and holds data and program instructions between processing steps and supplies them to the control unit and arithmetic-logic unit during processing. Most of a computer’s memory consists of microelectronic semiconductor memory chips known as RAM (random access memory ). The contents of these memory chips can be instantly changed to store new data. Other, more permanent memory chips called ROM (read only memory) may also be used.
Secondary storage devices like magnetic disks and optical disks are used to store data and programs and thus greatly enlarge the storage capacities of computer system. Also, since memory circuits typically lose their contents when electric power is turned off, most secondary storage media provide a more permanent type of storage. However the contents of hard disk drives floppy disks, CD-ROM disks, and other secondary storage media cannot be processed without first being brought into memory. Thus secondary storage devices play a supporting role to the primary storage of a computer system.
Multiple Processors.
Many current computers, from microcomputers to large mainframes, use multiple processors for their processing functions. Instead of having one CPU with a single control unit and arithmetic-logic unit, the CPUs of these computers contain several type of processing units. Let’s briefly look at the major types of such multiprocessor designs.
A support processor design relies on specialized microprocessors to help the main CPU perform a variety of functions. These microprocessors may used for input/output, memory management, arithmetic computations, multimedia processing, and telecommunications, thus freeing the main processor to do the primary job of executing program instructions For example, many microcomputers rely on support microprocessors such as arithmetic co-processing load on their main microprocessors. A large computer may use support microprocessors called channels to control the movement of data between the CPU and input/output devices. Advanced microprocessor designs integrate the functions of several support processors on a single main microprocessor.
A coupled processor design uses multiple CPUs or main microprocessors to do multiprocessing, that is, executing more than one instruction at the same time. Some configurations provide a fault-tolerant capability in which multiple CPUs provide a built-in backup to each other should one of them fail.
A parallel processor design uses a group of instruction processors to execute several program instructions at the same time. Some times, hundreds or thousands of processors are organized in clusters or networks in massively parallel processing (MPP) computers. Other parallel processor designs are based on simple models of the human brain called neural networks. All of these systems can execute many instructions at a time in parallel. This is a major departure from the traditional design of current computers, called the Von Neuman design, which executes instructions serially (one at a time). Though difficult to program, many experts consider parallel processor systems the key to providing advanced capabilities to future generations of computers.
RISC Processors. Many advanced technical workstations and other computers rely on a processor design called RISC (reduced instruction set computer). This contrasts with most current computers that use CISC (complex instruction set computer) processors. RISC processor designs optimize a CPU’s processing speed by using a smaller instruction set. That is, they use a smaller number of the basic machine instruction that a processor is capable of executing. By keeping the instruction set simpler than CISC processors and using more complex software, a RISC processor can reduce the time needed to execute program instructions.
Computer Processing Speeds.
Computer operating speeds that were formerly measured in milliseconds (thousands of a second) and microseconds (millionths of a second) are now in the nanosecond (billionth of a second) range, with picosecond (trillionth of a second) speed being attained by some computers. Such speeds seem almost incomprehensible. For example, an average person taking one step each nanosecond would circle the earth above 20 times in one second. Many microcomputers and midrange computers, and most mainframe computers, operate in the nanosecond range, and can thus process program instructions at million instructions per second (MIPS) speeds. Another measure of processing speed is megahertz (MHs), or millions of cycles per second. It is commonly called the clock speed of a microprocessor, sine it is used to rate microprocessors by the speed of their timing circuits or internal clock.
However, megahertz, ratings can be misleading indicators of the effective processing speed of microprocessors as measured in MIPS and other measures. That’s because processing speed depends on a variety of factors besides a microprocessor’s clock speed. Important examples include the size of circuitry paths, or busses, that interconnect microprocessor components, the capacity of instruction processing registers, the use of high-speed memory caches, and the use of specialized microprocessors such as a math co-processor to do arithmetic calculations faster. For example, Intel’s Pentium microprocessor runs at 66 to 200 MHz and is rated at over 100 MIPS, which the Pentium Pro microprocessor has a top processing rating of over 200 MIPS at similar megahertz speeds.