How computers work

While the technologies used in computers have changed dramatically since the first electronic, general-purpose, computers of the 1940s (see History of computing hardware for more details), most still use the von Neumann architecture.

The von Neumann architecture describes a computer with four main sections: the Arithmetic and Logic Unit (ALU), the control circuitry, the memory, and the input and output devices (collectively termed I/O). These parts are interconnected by a bundle of wires (a "bus") and are usually driven by a timer or clock (although other events could drive the control circuitry).

Memory

In this system, memory is a sequence of numbered cells, each containing a small piece of information. The information may be an instruction to tell the computer what to do. The cell may contain data that the computer needs to perform the instruction. Any cell may contain either, and indeed what is at one time data might be instructions later.

In general, the contents of a memory cell can be changed at any time - it is a scratchpad rather than a stone tablet.

The size of each cell, and the number of cells, varies greatly from computer to computer, and the technologies used to implement memory have varied greatly - from electromechanical relays, to mercury-filled tubes (and later springs) in which acoustic pulses were formed, to matrices of permanent magnets, to individual transistors, to integrated circuits with millions of capacitors on a single chip.

Processing (Processor)

A CPU

The arithmetic and logical unit, or ALU, is the device that performs elementary operations such as arithmetic operations (addition, subtraction, and so on), logical operations (AND, OR, NOT), and comparison operations (for example, comparing the contents of two bytes for equality). This unit is where the "real work" is done.

The control unit keeps track of which bytes in memory contain the current instruction that the computer is performing, telling the ALU what operation to perform and retrieving the information (from memory) that it needs to perform it, and transfers the result back to the appropriate memory location. Once that occurs, the control unit goes to the next instruction (typically located in the next slot (memory address), unless the instruction is a jump instruction informing the computer that the next instruction is located in another location). When referring to memory, the current instruction may use various addressing modes to determine the relevant address in memory. Also some computer motherboards will support dual or more processers. Computer servers generally make use of dual/multi processors.

Input and output

The I/O allows the computer to obtain information from the outside world, and send the results of its work back there. There is an broad range of I/O devices, from the familiar keyboards, mice, monitors and floppy disk drives, CD/DVD Drives, printers to the more unusual such as scanners & webcams.

What all input devices have in common is that they encode (convert) information of some type into data which can further be processed by the digital computer system. Output devices on the other hand, decode the data into information which can be understood by the computer user. In this sense, a digital computer system is an example of a data processing system.

Instructions

The machine set of instructions are not the rich instructions of a human language. A computer only has a limited number of well-defined, simple instructions. Typical sorts of instructions supported by most computers are "copy the contents of cell 5 and place the copy in cell 10", "add the contents of cell 7 to the contents of cell 13 and place the result in cell 20", "if the contents of cell 999 are 0, the next instruction is at cell 30".

Instructions are represented within the computer as binary code - a base two system of counting. For example, the code for one kind of "copy" operation in the Intel line of microprocessors is 10110000. The particular instruction set that a specific computer supports is known as that computer's machine language. In practice, people do not normally write the instructions for computers directly in machine language but rather use a "high level" programming language which is then translated into the machine language automatically by special computer programs (interpreters and compilers). Some programming languages map very closely to the machine language, such as assembler (low level languages); at the other end, languages like Prolog are based on abstract principles far removed from the details of the machine's actual operation (high level languages).

Architecture

Contemporary computers put the ALU and control unit into a single integrated circuit known as the Central Processing Unit or CPU. Typically, the computer's memory is located on a few small integrated circuits near the CPU. The overwhelming majority of the computer's mass is either ancillary systems (for instance, to supply electrical power) or I/O devices.

Some larger computers differ from the above model in one major respect - they have multiple CPUs and control units working simultaneously. Additionally, a few computers, used mainly for research purposes and scientific computing, have differed significantly from the above model, but they have found little commercial application, because their programming model has not yet standardized.

The functioning of a computer is therefore in principle quite straightforward. Typically, on each clock cycle, the computer fetches instructions and data from its memory. The instructions are executed, the results are stored, and the next instruction is fetched. This procedure repeats until a halt instruction is encountered.

Programs

Computer programs are simply large lists of instructions for the computer to execute, perhaps with tables of data. Many computer programs contain millions of instructions, and many of those instructions are executed repeatedly. A typical modern PC (in the year 2003) can execute around 2-3 billion instructions per second. Computers do not gain their extraordinary capabilities through the ability to execute complex instructions. Rather, they do millions of simple instructions arranged by people known as "programmers." Good programmers develop sets of instructions to do common tasks (for instance, draw a dot on screen) and then make those sets of instructions available to other programmers.

Nowadays, most computers appear to execute several programs at the same time. This is usually referred to as multitasking. In reality, the CPU executes instructions from one program, then after a short period of time, it switches to a second program and executes some of its instructions. This small interval of time is often referred to as a time slice. This creates the illusion of multiple programs being executed simultaneously by sharing the CPU's time between the programs. This is similar to how a movie is simply a rapid succession of still frames. The operating system is the program that usually controls this time sharing.

Operating system

A computer will always need at least one program running at all times to operate. Under normal operation (in the typical general-purpose computer), this program is the operating system (OS). The operating system decides which programs are run, when, and what resources (such as memory or input/output - I/O) the programs will get to use. The operating system also provides a layer of abstraction over the hardware, and gives access by providing services to other programs, such as code ("drivers") which allow programmers to write programs for a machine without needing to know the intimate details of all attached electronic devices.

Most operating systems that have hardware abstraction layers also provide a standardized user interface. The most popular OS remains the Windows family of operating systems.