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CS110: Introduction to Computers

PC Architecture Terminology Guide - Fall 2002

Note: These items are intentionally not in alphabetical order. I’ve attempted to group related things together, for example the types of memory.



A device that accepts input, processes & stores data, and produces output.

CPU (Central processing unit)

The main processing unit in a computer, consisting of circuitry that executes instructions to process data. The key component in a microprocessor, such as the Intel Pentium. In general conversation, the terms CPU and microprocessor are used synonymously.


The computer circuitry that holds program instructions and data.

RAM (Random Access Memory)

A type of computer memory circuit that holds data, program instructions and the operating system while the computer is on. RAM holds raw data that is waiting to be processed, as well as the instructions that will process the raw data. In addition, RAM holds processed data before it is stored more permanently on disk.


Another attribute of RAM is that it is volatile. That is, when power to the chips, or system, is lost or turned off the contents of RAM memory are lost. That’s one of the reasons it seems to take so long to shut down your computer, because the Operating System is writing any important information,  back out to disk.

ROM (Read-only Memory)

One or more integrated circuits (i.e. chips) that contain (semi-) permanent instructions that the computer uses during the boot process.


ROM is necessary because, when you turn on the computer, the CPU receives electrical power and is ready to begin executing instructions, but because the computer was just turned on, RAM is empty and does not have any instructions for the CPU to execute. This is where ROM comes into play; its ROM BIOS instruction set tells the computer how to access the disk drives, and other peripheral devices.


The instructions on some ROM chips are permanent, hence the only way to change them is to remove the ROM chips from the motherboard and replace them with another set. However, many computers utilize a type of programmable ROM, also called Flash Rom, that allow for the BIOS program instructions to be upgraded in place. (This is sometimes necessary on older computers when installing new technology, such as a new Operating System version or new peripherals, e.g. new rewriteable DVD or new disk drive technology.)

ROM BIOS (Basic Input/Output System)

A small set of basic input/output system instructions stored in ROM, which cause the computer system to load critical operating files when the user turns on the computer, and tell the computer how to access the disk drives and other peripheral devices (based on configuration information stored in the computers’ CMOS memory). Once the operating system is loaded, the computer can understand & respond to your input, display output, run software & access your data.

CMOS memory (pronounced “SEE moss”, stands for Complementary metal oxide semiconductor but no need to memorize that).

A type of integrated circuit that holds data, but requires very little power to retain its contents. Because of its low power requirements, a CMOS chip can be powered by a battery (i.e. CMOS is also volatile) that’s integrated into the motherboard. The battery trickles power to the CMOS chip so that it can retain vital data about your computer system configuration, even when your computer power is turned off. In many of today’s microcomputers, the CMOS chip is housed within the same chip carrier as the ROM BIOS.


When your system configuration changes, the data in the CMOS memory must be updated. Some operating systems have a special utility, or feature, called “plug and play” that automatically updates the configuration information in CMOS when hardware components are upgraded or new peripherals are added, e.g. you install a new hard drive.

Virtual Memory


A computer's ability to use storage space on a hard disk drive to simulate RAM.

Virtual memory allows computers without sufficient amounts of real memory to run large programs, manipulate large data files, and run multiple programs simultaneously. It does this by temporarily writing the contents of the least-used sections of RAM to the hard drive to free up RAM memory when needed. For example, if you are running a word processing program and open a spreadsheet program, the operating system frees up memory, as needed, by copying the contents of the least-used memory segments used by the word processing instructions and data to the disk; thereby creating space for the spreadsheet program and its data. (This is one example of the operating system managing the computer system’s resources).

Virtual memory is not as fast as RAM, because your computer takes longer to retrieve data from virtual memory because the disk is a mechanical device (vs. an electronic circuit).

Like data held in RAM, data in virtual memory becomes inaccessible if the power fails. Data in virtual memory is not erased from the disk if the power fails, but the instructions that direct the computer to the location of virtual memory are stored in RAM and are lost when the power fails.


Special high-speed memory that gives the CPU more rapid access to data than from memory located elsewhere on the motherboard (also called RAM cache or cache memory). A very fast CPU can execute an instruction so quickly that it often must wait for data to be delivered from RAM, which slows processing. As you begin a task, the computer anticipates what data the CPU is likely to need and loads this data in the cache area. When an instruction calls for data, the CPU first checks to see if the required data is in the cache. If so, it takes the data from cache instead of fetching it from RAM, or the hard drive (even slower).


Note: disk drives typically also have memory devices, i.e. caching mechanisms, built in to speed up access to data on the disk.

The Boot Process


The sequence of events that occur when you turn the power to your computer on. They are:

  1. Power up – power distributed to fan, motherboard and peripheral devices
  2. Start boot program – the microprocessor begins to execute instructions stored in ROM BIOS
  3. POST – the computer performs diagnostic tests of crucial system components (based on configuration data stored in CMOS)
  4. Load operating system – operating system instructions copied from disk to RAM
  5. Check (operating system related) configuration and customization – the microprocessor reads configuration data and executes any customized startup routines specified by the user
  6. Ready – the computer is ready for user input.

POST (Power-on Self-test)

A diagnostic process that runs during startup (i.e. boot process) to check components of the computer such as the graphics card, RAM, keyboard and disk drives. The computer tests RAM by placing data in each memory location (i.e. address) in RAM, then retrieving that data to see if it is correct. The computer then displays the amount of memory tested on the monitor. When it checks the disk drives you will see the drive activity lights flash on for a moment, and you will hear the drives spin.

Safe Mode

A menu option that appears when Windows is unable to complete the boot sequence. It is a limited version of Windows that allows you to use your mouse, keyboard and limited use of your monitor, but no other peripheral devices. This mode is designed for troubleshooting. By entering safe mode, a user can gracefully shut down the computer, and then try to reboot it.

Peripheral Device

Components and equipment that expand a computer’s input, output, and storage capabilities. By some definitions this includes devices that are included with most basic computer systems, such as keyboards, monitors, disk drives and mice, but also include devices that extend a computer’s capabilities, such as a printer, scanner, and modem.


The main circuit board in the computer that houses the chips that control the processing functions. Chips may be soldered on the motherboard or plugged into the board. Chips, or cards, that are plugged into the motherboard may be upgraded. These include the microprocessor chip (CPU), as well as chips for memory (RAM) or that handle basic input and output (ROM). Circuits are etched into the motherboard and act like wires, providing a path so the computer can transport data from one chip to another as needed for processing.

Data Bus

An electronic pathway or circuit that connects the electronic components (mainly the processor and RAM) on a computer’s motherboard.

Expansion Slots

The motherboard also contains expansion sockets (or slots) that allow you to plug in circuit boards, called expansion cards. A computer usually contains sets of different types of slots.

Expansion Card

A circuit board that is plugged into a slot on a PC motherboard in order to add extra functions, devices, or ports. For example, a video, sound or network interface card. Any given card is manufactured to fit in a specific type of expansion slot. Also known as an “expansion board”.

Expansion Port

A socket, located on an expansion card, into which the user plugs a cable from a peripheral device, allowing data to pass between the computer and the peripheral device.

Expansion Bus

The segment of the data bus that transports data between RAM and the peripheral devices. That is, it connects RAM to the expansion slots/cards, which in turn are connected to peripheral devices through their expansion ports(s) and the cables. All input & output travels through such a path.

Bit, byte, ASCII & the Binary number system

As microcomputers are digital devices, all data, information, programs, graphics, music, etc. are stored as a digital series of 1s and 0s. A bit is the smallest unit of information handled by a computer. By the above definition then a bit can hold one of two values, either a 0 or a 1. Eight bits comprise a byte which, by use of coding mechanisms, can represent a letter or number. ASCII is the coding system used to represent characters, while the Binary number system is a method for representing numbers using only two digits.

Network Interface Card or NIC (pronounced “nick”)

A NIC is the key hardware component for connecting a computer to a local area network (LAN). It is a small circuit board/card that sends data from a workstation out over a network, and collects incoming data for the workstation. Every device must have one in order to connect to a network. It is sometimes built into a device; otherwise an NIC expansion card is required.



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