Analog computers are analog devices. That is, they have continuous states rather than discrete numbered states. An analog computer can represent fractional or irrational values exactly, with no round-off. Analog computers are almost never used outside of experimental settings.
A digital computer is a programmable clocked sequential state machine. A digital computer uses discrete states. A binary digital computer uses two discrete states, such as positive/negative, high/low, on/off, used to represent the binary digits zero and one.
The French word ordinateur, meaning that which puts things in order, is a good description of the most common functionality of computers.
what are computers used for?
Computers are used for a wide variety of purposes.
Data processing is commercial and financial work. This includes such things as billing, shipping and receiving, inventory control, and similar business related functions, as well as the “electronic office”.
Scientific processing is using a computer to support science. This can be as simple as gathering and analyzing raw data and as complex as modelling natural phenomenon (weather and climate models, thermodynamics, nuclear engineering, etc.).
Multimedia includes content creation (composing music, performing music, recording music, editing film and video, special effects, animation, illustration, laying out print materials, etc.) and multimedia playback (games, DVDs, instructional materials, etc.).
parts of a computer
The classic crude oversimplication of a computer is that it contains three elements: processor unit, memory, and I/O (input/output). The borders between those three terms are highly ambigious, non-contiguous, and erratically shifting.
A slightly less crude oversimplification divides a computer into five elements: arithmetic and logic subsystem, control subsystem, main storage, input subsystem, and output subsystem.
processor
The processor is the part of the computer that actually does the computations. This is sometimes called an MPU (for main processor unit) or CPU (for central processing unit or central processor unit).
A processor typically contains an arithmetic/logic unit (ALU), control unit (including processor flags, flag register, or status register), internal buses, and sometimes special function units (the most common special function unit being a floating point unit for floating point arithmetic).
Some computers have more than one processor. This is called multi-processing.
The major kinds of digital processors are: CISC, RISC, DSP, and hybrid.
CISC stands for Complex Instruction Set Computer. Mainframe computers and minicomputers were CISC processors, with manufacturers competing to offer the most useful instruction sets. Many of the first two generations of microprocessors were also CISC.
RISC stands for Reduced Instruction Set Computer. RISC came about as a result of academic research that showed that a small well designed instruction set running compiled programs at high speed could perform more computing work than a CISC running the same programs (although very expensive hand optimized assembly language favored CISC).
DSP stands for Digital Signal Processing. DSP is used primarily in dedicated devices, such as MODEMs, digital cameras, graphics cards, and other specialty devices.
Hybrid processors combine elements of two or three of the major classes of processors.
For more detailed information on these classes of processors, see processors.
arithmetic and logic
An arithmetic/logic unit (ALU) performs integer arithmetic and logic operations. It also performs shift and rotate operations and other specialized operations. Usually floating point arithmetic is performed by a dedicated floating point unit (FPU), which may be implemented as a co-processor.
An arithmetic/logic unit (ALU) performs integer arithmetic and logic operations. It also performs shift and rotate operations and other specialized operations. Usually floating point arithmetic is performed by a dedicated floating point unit (FPU), which may be implemented as a co-processor.
control
Control units are in charge of the computer. Control units fetch and decode machine instructions. Control units may also control some external devices.
A bus is a set (group) of parallel lines that information (data, addresses, instructions, and other information) travels on inside a computer. Information travels on buses as a series of electrical pulses, each pulse representing a one bit or a zero bit (there are trinary, or three-state, buses, but they are rare). An internal bus is a bus inside the processor, moving data, addresses, instructions, and other information between registers and other internal components or units. An external bus is a bus outside of the processor (but inside the computer), moving data, addresses, and other information between major components (including cards) inside the computer. Some common kinds of buses are the system bus, a data bus, an address bus, a cache bus, a memory bus, and an I/O bus.
For more information, see buses.
main storage
Main storage is also called memory or internal memory (to distinguish from external memory, such as hard drives).
RAM is Random Access Memory, and is the basic kind of internal memory. RAM is called “random access” because the processor or computer can access any location in memory (as contrasted with sequential access devices, which must be accessed in order). RAM has been made from reed relays, transistors, integrated circuits, magnetic core, or anything that can hold and store binary values (one/zero, plus/minus, open/close, positive/negative, high/low, etc.). Most modern RAM is made from integrated circuits. At one time the most common kind of memory in mainframes was magnetic core, so many older programmers will refer to main memory as core memory even when the RAM is made from more modern technology. Static RAM is called static because it will continue to hold and store information even when power is removed. Magnetic core and reed relays are examples of static memory. Dynamic RAM is called dynamic because it loses all data when power is removed. Transistors and integrated circuits are examples of dynamic memory. It is possible to have battery back up for devices that are normally dynamic to turn them into static memory.
ROM is Read Only Memory (it is also random access, but only for reads). ROM is typically used to store thigns that will never change for the life of the computer, such as low level portions of an operating system. Some processors (or variations within processor families) might have RAM and/or ROM built into the same chip as the processor (normally used for processors used in standalone devices, such as arcade video games, ATMs, microwave ovens, car ignition systems, etc.). EPROM is Erasable Programmable Read Only Memory, a special kind of ROM that can be erased and reprogrammed with specialized equipment (but not by the processor it is connected to). EPROMs allow makers of industrial devices (and other similar equipment) to have the benefits of ROM, yet also allow for updating or upgrading the software without having to buy new ROM and throw out the old (the EPROMs are collected, erased and rewritten centrally, then placed back into the machines).
Registers and flags are a special kind of memory that exists inside a processor. Typically a processor will have several internal registers that are much faster than main memory. These registers usually have specialized capabilities for arithmetic, logic, and other operations. Registers are usually fairly small (8, 16, 32, or 64 bits for integer data, address, and control registers; 32, 64, 96, or 128 bits for floating point registers). Some processors separate integer data and address registers, while other processors have general purpose registers that can be used for both data and address purposes. A processor will typically have one to 32 data or general purpose registers (processors with separate data and address registers typically split the register set in half). Many processors have special floating point registers (and some processors have general purpose registers that can be used for either integer or floating point arithmetic). Flags are single bit memory used for testing, comparison, and conditional operations (especially conditional branching). For a much more advanced look at registers, see registers.
For more information on memory, see memory
external storage
External storage (also called auxillary storage) is any storage other than main memory. In modern times this is mostly hard drives and removeable media (such as floppy disks, Zip disks, optical media, etc.). With the advent of USB and FireWire hard drives, the line between permanent hard drives and removeable media is blurred. Other kinds of external storage include tape drives, drum drives, paper tape, and punched cards. Random access or indexed access devices (such as hard drives, removeable media, and drum drives) provide an extension of memory (although usually accessed through logical file systems). Sequential access devices (such as tape drives, paper tape punch/readers, or dumb terminals) provide for off-line storage of large amounts of information (or back ups of data) and are often called I/O devices (for input/output).
input/output overview
Most external devices are capable of both input and output (I/O). Some devices are inherently input-only (also called read-only) or inherently output-only (also called write-only). Regardless of whether a device is I/O, read-only, or write-only, external devices can be classified as block or character devices.
A character device is one that inputs or outputs data in a stream of characters, bytes, or bits. Character devices can further be classified as serial or parallel. Examples of character devices include printers, keyboards, and mice.
A serial device streams data as a series of bits, moving data one bit at a time. Examples of serial devices include printers and MODEMs.
A parallel device streams data in a small group of bits simultaneously. Usually the group is a single eight-bit byte (or possibly seven or nine bits, with the possibility of various control or parity bits included in the data stream). Each group usually corresponds to a single character of data. Rarely there will be a larger group of bits (word, longword, doubleword, etc.). The most common parallel device is a printer (although most modern printers have both a serial and a parallel connection, allowing greater connection flexibility).
A block device moves large blocks of data at once. This may be physically implemented as a serial or parallel stream of data, but the entire block gets transferred as single packet of data. Most block devices are random access (that is, information can be read or written from blocks anywhere on the device). Examples of random access block devices include hard disks, floppy disks, and drum drives. Examples of sequential access block devcies include magnetic tape drives and high speed paper tape readers.
input
Input devices are devices that bring information into a computer.
Pure input devices include such things as punched card readers, paper tape readers, keyboards, mice, drawing tablets, touchpads, trackballs, and game controllers.
Devices that have an input component include magnetic tape drives, touchscreens, and dumb terminals.
output
Output devices are devices that bring information out of a computer.
Pure output devices include such things as card punches, paper tape punches, LED displays (for light emitting diodes), monitors, printers, and pen plotters.
Devices that have an output component include magnetic tape drives, combination paper tape reader/punches, teletypes, and dumb terminals.
