What Is A System Unit Computer Science Essay

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A system unit, also known as a base unit . The system unit refers to the computer itself but does not include the monitor, the keyboard, the mouse, or any other peripherals. I suppose most people will probably know what you mean when you refer to "the box," but saying "system unit" will definitely make you sound more sophisticated. It is the main body of a desktop computer, typically consisting of a plastic enclosure containing the motherboard, power supply, cooling fans, internal disk drives, and memory modules and expansion cards that are plugged into the motherboard, such as video and network cards.


The motherboard is sometimes alternatively known as the main board, system board, or, on Apple computers, the logic board. A motherboard is the central printed circuit board (PCB) in many modern computers and holds many of the crucial components of the system, while providing connectors for other peripherals.

Most computer motherboards produced today are designed for IBM-compatible computers, which currently account for around 90% of global PC sales. A motherboard, like a backplane, provides the electrical connections by which the other components of the system communicate, but unlike a backplane, it also connects the central processing unit and hosts other subsystems and devices.




A backplane (or "backplane system") is a circuit board, usually a printed circuit board (PCB) that connects several connectors in parallel to each other, so that each pin of each connector is linked to the same relative pin of all the other connectors, forming a computer bus. It is used as a backbone to connect several printed circuit boards together to make up a completecomputer system. A backplane is generally differentiated from a motherboard by the lack of on-board processing power where the CPU is on a plug-in card.

*Computer Bus

In computer architecture, a computer bus is a subsystem that transfers data between computer components inside a computer or between computers.


A microprocessor incorporates most or all of the functions of a computer's central processing unit (CPU) on a single integrated circuit (IC, or microchip)

Power Supply


Cooling Fan

When use any machines for a long time, heat will be generated, just like that computer is also a machine in which heat is generated from various parts. To cool down the machine and to protect against the heat, CPU cooling fans are used.The functions of the CPU cooling fans are that it flushes out the heat and draws cool air in the system. Cooling fans are also fixed in mother boards and hard drives. There are more than two to three fans are attached in the central processing unit.

Internal Disk Drives

Disk storage or disc storage is a general category of storage mechanisms, in which data are digitally recorded by various electronic, magnetic, optical, or mechanical methods on a surface layer deposited of one or more planar, round and rotating platters. A disk drive is a device implementing such a storage mechanism with fixed or removable media; with removable media the device is usually distinguished from the media as in compact disc drive and the compact disc. Notable types are the hard disk drive (which is today almost always use fixed media), the floppy disk drive and its floppy disk, and various optical disc drives and associated media

Memory Module.

Memory modules perform an important function in PCs, Macs and various laptops. They help make computers run. There is not one type of memory module since the term expresses a broad array of memory chips that are used in the computer's motherboard to help it function.

Extension Cards


e.g Video Cards, Sound Cards, Memory Expansion Cards,etc.



Arithmetic operations

The basic arithmetic operations are addition, subtraction, multiplication and division, although this subject also includes more advanced operations, such as manipulations of percentages, square roots, exponentiation, and logarithmic functions. Arithmetic is performed according to an order of operations. Any set of objects that all four arithmetic operations (except division by zero) can be performed on, and where these four operations obey the usual laws, is called a field.

Addition (+)

Addition is the basic operation of arithemetic. In its simplest form, addition combines two numbers, the addends or terms, into a single number, the sum of the numbers.


Addition can be given geometrically as follows:

If a and b are the lengths of two sticks, then if we place the sticks one after the other, the length of the stick thus formed is aÂ +Â b.

Subtraction (âˆ’)


Subtraction is neither commutative nor associative. For that reason, it is often helpful to look at subtraction as addition of the minuend and the opposite of the subtrahend, that is aÂ âˆ’Â bÂ =Â aÂ +Â (âˆ’b). When written as a sum, all the properties of addition hold.


Suppose an amount P is given to pay the required amount Q, with P greater than Q. P - Q will equal to the difference between the two amounts. But if amount Q is greater than of amount P, the difference will be in a negative state.

Multiplication (Ã-, Â·, or *)



Division (÷ or /)


It is helpful to look at subtraction as addition, it is helpful to look at division as multiplication of the dividend times the reciprocal of the divisor, that is aÂ ÷Â bÂ =Â aÂ Ã-Â 1â„b. When written as a product, it obeys all the properties of multiplication.

Logic Operation

An operation that analyzes one or more inputs and generates a particular output based on a set of rules. See AND-OR-NOT and Boolean logic.


The fundamental operations of Boolean logic. To learn how they function and how they are wired together to build circuits, see Boonlean Logic.

Boolean logic

The "mathematics of logic," developed by English mathematician George Boole in the mid-19th century. Its rules govern logical functions (true/false) and are the foundation of all electronic circuits in the computer. As add, subtract, multiply and divide are the primary operations of arithmetic, AND, OR and NOT are the primary operations of Boolean logic. Boolean logic is turned into logic gates on the chip, and the logic gates make up logic circuits that perform functions such as how to add two numbers together.

The rules, or truth tables, for AND, OR and NOT follow.

Comparison Operation

Comparison operators, as their name implies, allow you to compare two values. You may also be interested in viewing the type of comparison tables, as they show examples of various type related comparisons.

Machine Cycle Definition

A machine cycle, also called a processor cycle or a instruction cycle, is the basic operation performed by the computer processor for each machine language instruction received. A machine cycle consists of a sequence of 3 steps that is performed continuously and at a rate of millions per second while a computer is in operation. They are fetch, decode and execute. There also is a fourth step, store, in which input and output from the other three phases is stored in memory for later use; however, no actual processing is performed during this step.

In the fetch step, the control unit requests that main memory provide it with the instruction that is stored at the address (i.e., location in memory) indicated by the control unit's program counter.

The control unit is a part of the CPU that also decodes the instruction in the instruction register. A register is a very small amount of very fast memory that is built into the CPU in order to speed up its operations by providing quick access to commonly used values; instruction registers are registers that hold the instruction being executed by the CPU. Decoding the instructions in the instruction register involves breaking the operand field into its components based on the instructions opcode.

Opcode (an abbreviation of operation code) is the portion of a machine language instruction that specifies what operation is to be performed by the CPU. Machine language, also called machine code, refers to instructions coded in patterns of bits (i.e., zeros and ones) that are directly readable and executable by a CPU.

A program counter, also called the instruction pointer in some computers, is a register that indicates where the computer is in its instruction sequence. It holds either the address of the instruction currently being executed or the address of the next instruction to be executed, depending on the details of the particular computer. The program counter is automatically incremented for each machine cycle so that instructions are normally retrieved sequentially from memory.

The control unit places these instructions into its instruction register and then increments the program counter so that it contains the address of the next instruction stored in memory. It then executes the instruction by activating the appropriate circuitry to perform the requested task. As soon as the instruction has been executed, it restarts the machine cycle, beginning with the fetch step.



CMOS is also sometimes referred to as complementary-symmetry metal-oxide-semiconductor (or COS-MOS). The words "complementary-symmetry" refer to the fact that the typical digital design style with CMOS uses complementary and symmetrical pairs of p-type and n-type metal oxide semiconductor field effect transistors (MOSFETs) for logic functions.


CMOS controls a variety of functions, including the Power On Self Test (POST). When the computer's power supply fires up, CMOS runs a series of checks to make sure the system is functioning properly. One of these checks includes counting up random access memory (RAM). This delays boot time, so some people disable this feature in the CMOS settings, opting for a quick boot. If installing new RAM it is better to enable the feature until the RAM has been checked.

Once POST has completed, CMOS runs through its other settings. Hard disks and formats are detected, along with Redundant Array of Independent Disk (RAID) configurations, boot preferences, the presence of peripherals, and overclocking tweaks. Many settings can be manually changed within the CMOS configuration screen to improve performance. However, changes should be made by experienced users. Changing settings improperly can make the system unstable, cause crashes, or even prevent the computer from booting.

Most motherboard manuals provide a complete list of available CMOS options. These will vary according to motherboard design and BIOS manufacturer. Two of the most well known BIOS manufacturers for clone PCs are Phoenix and Award, while companies like Dell and Compaq produce their own BIOS chips.




The word RAM is often associated with volatile types of memory (such as DRAM memory modules), where the information is lost after the power is switched off. Many other types of memory are RAM, too, including most types of ROM and a type of flash memory called NOR-Flash.

Random access memory is volatile memory, meaning it loses its contents once power is cut. This is different from non-volatile memory such as hard disks and flash memory, which do not require a power source to retain data. When a computer shuts down properly, all data located in random access memory is committed to permanent storage on the hard drive or flash drive. At the next boot-up, RAM begins to fill with programs automatically loaded at startup, and with files opened by the user.

There are several different types of random access memory chips which come several to a "stick." A stick of RAM is a small circuit board shaped like a large stick of gum. Sticks of RAM fit into "banks" on the motherboard. Adding one or more sticks increases RAM storage and performance.

Random access memory is categorized by architecture and speed. As technology progresses, RAM chips become faster and employ new standards so that RAM must be matched to a compatible motherboard. The motherboard will only support certain types of random access memory, and it will also have a limit as to the amount of RAM it can support.

Since random access memory can improve performance, the type and amount of RAM a motherboard will support becomes a major factor when considering a new computer. If there is a faster, better random access memory chip on the market, the buyer will want to consider purchasing a motherboard capable of using it. A year down the road, that 'new' RAM might be standard, while the buyer may be stuck with an old style motherboard. A new variety of non-volatile random access memory made with nanotubes or other technologies will likely be forthcoming in the near future. These RAM chips would retain data when powered down.

RAM varies in cost depending on type, capacity and other factors. Brand name random access memory often comes with a lifetime guarantee at a competitive price. That's one guarantee that can't be beat.





Use of ROM for program storage

Every stored-program computer requires some form of non-volatile, or erasable, storage to store the initial program that runs when the computer is powered on or otherwise begins execution (a process known as bootstrapping, often abbreviated to "booting" or "booting up"). Likewise, every non-trivial computer requires some form of mutable memory to record changes in its state as it executes.

Forms of read-only memory were employed as non-volatile storage for programs in most early stored-program computers. Read-only memory was simpler to implement since it required only a mechanism to read stored values, and not to change them in-place, and thus could be implemented with very crude electromechanical devices. The ROM and its mutable counterpart static RAM were implemented as arrays of transistors in silicon chips.


In modern PCs, "ROM" (or Flash) is used to store the basic bootstrapping firmware for the main processor, as well as the various firmware needed to internally control self contained devices such as graphic cards, hard disks, DVD drives, TFT screens, etc, in the system. Today, many of these "read-only" memories - especially the BIOS - are often replaced with Flash memory (see below), to permit in-place reprogramming should the need for a firmware upgrade arise. However, simple and mature sub-systems (such as the keyboard or some communication controllers in the ICs on the main board, for example) may employ mask ROM or OTP (one time programmable).



Types of ROMs

Programmable read-only memory (PROM), or one-time programmable ROM (OTP), can be written to or programmed via a special device called a PROM programmer. Consequently, a PROM can only be programmed once.

Erasable programmable read-only memory (EPROM) can be erased by exposure to strong ultraviolet light (typically for 10 minutes or longer), then rewritten with a process that again requires application of higher than usual voltage. Repeated exposure to UV light will eventually wear out an EPROM, but the endurance of most EPROM chips exceeds 1000 cycles of erasing and reprogramming.


