LIS 505 - Inside a computer

control unit

The part of a CPU that extracts instructions from memory and decodes and executes them, calling on the ALU when necessary.


Short for arithmetic/logic unit. The part of a CPU that performs computations (arithmetic operations) and comparisons (logical operations).


Symbols that represent specific actions, including arithmetic operations and comparisons. ,

relational operators

Operators that compare values , such as <, =, >.


Internal storage areas in a computer, currently in the form of chips. , Also known as primary storage, primary memory, main storage, internal storage, main memory, or RAM.

clock ticks

The smallest units of time recognized by a device. Also called cycles. A CPU in a modern personal computer can execute several instructions in one tick of the computer system's clock. ,

system clocks

The main clocks that control timing within computers. ComputerUser

instruction sets

The basic sets of instructions that CPUs understand. ,

binary system

A number system that has just two unique digits.


Short for binary digits. The digits in the binary system.


Units of storage each of which can hold a single character, typically 8 bits.


Units of either 1,000 or 1,024 (=210) bytes. The abbreviations KB and K (with the capital K) more usually mean the larger number, but usage varies.


Units of either 1,000,000 or 1,048,576 (=220) bytes. Abbreviated to MB or M. The smaller value typically applies to data transfer rates and the larger to storage space.


Units of 1,073,741,824 (=230) bytes. Abbreviated to GB or G.


Units of either 1,000,000,000,000 (one trillion) or 1,099,511,627,776 (=240) bytes. . Abbreviated to TB or T.


The smallest data units with which a CPU works, from 1 to 64 bytes, depending on the CPU. .


Sets of symbols for representing characters, etc. . Also known as coding schemes.


Short for American Standard Code for Information Interchange. The most common computer coding scheme for text, It consists of 128 (=27) different values, most for letters, numerals, and symbols. Two different common 128-code extensions bring the total to 256: Extended ASCII and ISO Latin 1. .


A standard code consisting of up to 216 different values, useful for representing texts in non-Western-European languages. .

printed circuit boards

Thin plates on which chips and other components are placed in a computer. Also called PCBs, cards, or adapters. .


Main printed circuit boards of small computers. .

volatile memory

Memory that loses its content when the power is turned off. .


Short for static random access memory. A relatively fast but expensive kind of memory, typically used for a memory cache. .


Short for dynamic random access memory. The main type of memory used in personal computers, relatively slower than SRAM. .


Short for single in-line memory modules. Small printed circuit boards containing up to 9 memory chips. Easier to install than individual chips. They transmit and receive 32 bits of data at a time. .


Short for dual in-line memory modules. Small printed circuit boards containing memory chips. They transmit and receive 64 bits of data at a time. .

non-volatile memory

Memory that does not lose its content when the power is turned off. .


Short for complementary metal oxide semiconductor. Because of its low power requirements, this type of memory can be powered for long periods by batteries. It is therefore used to store the date and time and basic system settings while a small computer is turned off. .


Short for read-only memory. Non-volatile memory on which data have been prerecorded. .


Short for programmable read-only memory. Non-volatile memory on which data can be written only once. .

flash memory

Non-volatile memory on which data can be erased and rewritten in blocks. Often used in modern computers instead of regular ROM as well as in digital cameras.


Collections of wires through which data are transmitted from one part of a computer to another.

system buses

Buses that connect CPUs to main memory.

ISA bus

Short for Industry Standard Architecture bus. A standard local (internal) bus used for slower devices.

PCI bus

Short for Peripheral Component Interconnect bus. A more recent standard local bus used for faster devices.


Short for Accelerated Graphics Port. A standard designed to improve output of graphic information to the monitor. It is faster than PCI and allows 3-d textures to be stored in main memory.


Short for Universal Serial Bus. A standard external bus that supports data transfer rates up to 12 megabits per second, used for devices such as mice, keyboards, and modems.

IEEE 1394 bus

A very fast standard external bus that supports data transfer rates up to 400 million bits per second. Apple uses the tradename FireWire.

PCMCIA standard

A standard of the Personal Computer Memory Card International Association, originally designed for credit-card-sized devices called PC Cards.

expansion slots

Openings in a computer where circuit boards can be inserted to add new capabilities.


Interfaces on computers to which users can connect devices, such as disk drives, modems, printers, and mice.

serial ports

Ports used for serial devices, such as mice and modems. The traditional standard for personal computers is RS-232C. In DOS and Windows, these are the COM (for communication) ports.

parallel ports

Ports used for parallel devices, such as most computer printers. The traditional standard for personal computers is the Centronics standard. In DOS and Windows, these are the LPT (for line printer) ports.


Megahertz. Millions of cycles per second. A measure of the speed of a device, often quoted for CPUs.


Abbreviated as GHz. Billions of cycles per second. Personal computer clock speeds reached 1 GHz in March of 2000.


Short for million instructions per second. An older measure of a computer's speed.


Short for floating point operations per second. Another measure of CPU speed. Hence megaFLOPS (millions of floating-point operations per second) and teraFLOPS (trillions of floating-point operations per second).


Special high-speed temporary storage. Memory caches (also called RAM caches) are special high-speed memory (typically SRAM) in which copies of frequently used data and instructions are stored. They include level 1 caches (L1 caches or internal caches), which are built into CPUs, and level 2 caches (L2 caches or external caches), which are situated between CPUs and conventional main memory and are much larger than level 1 caches. Disk caches use conventional main memory to store recently accessed data from disks.


Short for reduced instruction set computer (or computing). CPUs that recognize relatively limited sets of instructions. More efficient for certain types of tasks. Many workstations are RISC.


Short for complex instruction set computer (or computing). CPUs that recognize relatively larger sets of instructions (up to 200). Most personal computers are CISC.

serial processing

Performing instructions one after another.


A technique by which a CPU begins executing a second instruction before the first has been completed. Several instructions may be "in the pipeline" simultaneously, each at a different processing stage. Also called pipeline processing.

parallel processing

Simultaneous use of more than one CPU to execute a program.

Last updated October 29, 2002.
This page maintained by Prof. Tim Craven
E-mail (text/plain only):
Faculty of Information and Media Studies
University of Western Ontario,
London, Ontario
Canada, N6A 5B7