The term a computer usually means a combination of hardware devices and software that manages information and processes data. This term most specifically describes the system unit because this is where computing actually takes place. In the system unit program instructions are performed (executed) and data is processed.
COMPONENTS OF THE SYSTEM UNIT
The system unit is made up of very many parts called components that are covered in the plastic/metallic casing (the box) some of the components found in the system unit include:-
- Power Supply Unit (PSU)
- Mother Board
- Memory chips and slots
- Micro-processor/central processing unit
- Bus Lines
- Ports and connectors
- Expansion slots etc.
a) The Power Supply Unit (PSU)
Computers require direct current (DC) to power the electronic components and to represent data and instructions. The Power Supply Unit converts alternating current (AC) coming from the wall outlets or directly from batteries to a lower voltage that the computer uses. The power supply unit also has the fan that provides the cooling system for the components inside the system unit. Desktop computers have a PSU located directly within the system unit while notebook computers use AC adapters that are located outside the system unit.
b) Mother Board
The motherboard is sometimes called the main board, system board or circuit board. The system board connects all system components and allows input and output devices to communicate with the system unit. External devices such as the keyboard and mouse communicate to the system unit through the motherboard. Some of the components found on the motherboard include, processor, memory slots, expansion slots and cards etc.
c) Ports and Connectors
A port is a socket used to connect external devices such as peripheral devices e.g. keyboard, monitor to a system unit. Some ports connect directly to the system board while others connect to the cards inserted into slots on the system board. Today’s computers have ports at the front and back of the system unit. There are three types of ports.
i) Parallel ports
Parallel ports are used to connect devices that send/receive large amounts of data over a short distance such as the printers and tape devices. They transfer eight bits of data simultaneously using a cable that has eight parallel data lines.
ii) Serial ports
These transfer small amounts of data over a long distance and are used to connect devices such as keyboards, mice, etc. A serial port transmits data one bit at a time.
iii) Universal serial bus (USB) ports
A USB port, short for Universal Serial Bus port, can connect several devices together with a single connector type. These ports send and receive very large amounts of data. A USB port connects any device that has a USB interface on it. USB ports can transfer up to 12 MB (mega bytes) of data at a time. Personal computers typically have four to eight USB ports either on the front or back system unit.
The USB 2.0 port is designed for devices that transfer a lot of data, such as MP3 music players, CDs, DVDs, and removable hard disks.
A USB hub is a device that can be used to plug into a USB port on the system unit and contains multiple USB ports into which you plug cables from USB devices. Some newer peripherals may attach only to a USB port. USB also supports hot plugging (Plug and Play), which means you, can attach peripherals while the computer is running.
Cable Connectors
Devices have different types of connectors attached to the cable that is used to connect the device to the system unit. There are basically two types of connectors. The connectors are generally categorized as follows:
Male connectors
They have one or more exposed pins at the back of the system unit or on the cable connector
Female connectors
They have one or more matching receptive holes to accept the pins from the male connectors.
d) Buses:
Data is transported and processed as a series of electronic bits. These bits transfer internally within the circuitry of the computer along electrical channels. Each channel, called a bus, allows the various devices both inside and attached to the system unit to communicate with each other. Buses transfer bits from input devices to memory, from memory to the processor, from the processor to memory, and from memory to output or storage devices. Buses also link the CPU to various other components on the system board.
Buses consist of two parts: a data bus and an address bus. The data bus transfers actual data and the address bus transfers information about where the data should reside in memory. The size of a bus, called the bus width, determines the number of bits that the computer can transmit at one time. For example, a 32-bit bus can transmit 32 bits (4 bytes) at a time.
e) Bays
A bay is an open area inside the system unit used to install additional equipment such as disk drives.
f) MICRO PROCESSOR/PROCESSOR
In personal computers, the processor or central processing unit (CPU) interprets and carries out the basic operations that operate a computer. The processor is contained on a single chip called the microprocessor. Microprocessors are mounted onto a carrier package that plugs into a special slot on the system board. The microprocessor is the “brains” of the computer system.
In the world of personal computers, the terms microprocessor and CPU are used interchangeably. At the heart of all personal computers and most workstations sits a microprocessor. Microprocessors also control the logic of almost all digital devices, from clock radios to fuel-injection systems for automobiles.
Three basic characteristics differentiate microprocessors:
- Instruction Set: The set of instructions that the microprocessor can execute.
- Bandwidth: The number of bits processed in a single instruction.
- Clock Speed: Given in megahertz (MHz), gigahertz (GHZ), the clock speed determines how many instructions per second the processor can execute.
For example, a 32-bit microprocessor that runs at 50MHz is more powerful than a 16-bit microprocessor that runs at 25MHz. In both cases, the higher the value, the more powerful the CPU.
The central processing unit contains two basic components: the control unit and arithmetic logic unit (ALU). These two units work together to perform processing operations.
Control unit:
The control is the component of the processor that directs and coordinates most of the operations in the computer. It directs the movement of electric signals between memory and the arithmetic and logic unit. It also directs the signals between the CPU and the input and output devices. The control unit interprets each instruction issued by a program and then initiates the appropriate action to carry out the instruction.
Machine Cycle:
For every instruction, a processor repeats a set of four basic operations called the machine cycle. The operations include:
- i) Fetching
Fetching is the process of obtaining the program instructions or data items from memory.
ii) Decoding
Decoding is the process of translating and interpreting the program instructions into signals a computer can execute.
iii) Executing:
Executing is the process of carrying out the computer commands.
iv) Storing:
After executing, the results of the instructions are written back to memory, thus storing.
The Arithmetic Logic Unit (ALU):
The ALU contains electronic circuits necessary to perform arithmetic, comparison and logical operations on data. The arithmetic operations include basic calculations such as addition, subtraction, multiplication and division. Comparison operations involve comparing one data item with another to determine if the first data item is greater than, less than or equal to the other item. The logical operations include conditions along with logical operators such as AND, OR and NOT.
Registers:
Processors contain high speed storage locations called registers that temporarily hold data and instructions for specific types of data in the system unit during processing. Registers are part of the processor not part of memory or storage devices. Processors have many different types of registers each with a specific storage function. The specific functions of registers include:
- Storing the location from where the instruction was fetched.
- Storing an instruction while it’s being decoded by the control unit.
- Storing data while the ALU computes it.
- Storing the results of a calculation.
The System Clock
A system clock is a small quartz crystal circuit. The system clock controls the timing of all computer operations. It generates regular electronic pulses or ticks that set the operating pace of system unit components. The pace of the system clock measured by the number of ticks per second is called the clock speed. Today’s PCs have clock speeds in the gigahertz range. A Giga is a prefix that stands for billion, and a hertz is one cycle per second. A computer operating at 2.8 GHz has 2.8 billion (giga) clock cycles in one second (hertz).
Coprocessors:
A coprocessor is a special additional processor chip that assists the processor in performing specific tasks and increases the performance of the computer.
Personal Computer Processors
The leading processor chip manufacturers are Intel, AMD (Advanced Micro Devices), IBM (International Business Machines) and Motorola. Most PCs use some type of Pentium Processor (i.e. Pentium II, Pentium Pro). Notebooks use Pentium M processors. AMD produces Intel-compatible chips called Athlon and Hammer. Intel-compatible processors are able to process programs originally written specifically for Intel chips
Older PCs process data and instructions in millionths of a second or microseconds. Newer PCs, which are much faster process data and instructions in billionths of a second, or nanoseconds. Supercomputers operate at speeds measured in trillionth of a second or picoseconds.
g) EXPANSION SLOTS
These are sockets designed on circuit boards for other devices such as video cards; network interface cards (NIC) sound cards that add more capabilities to the computer system. The card that is added into the expansion slot is sometimes referred to as an adapter card, interface card or expansion cards.
h) MEMORY
Memory are small chips that temporarily store program instructions and data during processing. Memory chips are basically installed on the motherboard and today many devices such as printers, monitors have small circuit boards with memory installed on them.
The basic unit of measuring memory is a byte. Each byte in memory has an address called a memory address.
What is a memory address?
A memory address is a number that identifies a location of a byte in memory. Today, memory is measured in kilobytes, megabytes, and gigabytes.
Today some of the common memory chips have the following capacities: 64 megabytes, 128 megabytes, 256 megabytes, 512 megabytes, 1 gigabyte. There are three types of memory:
a) Random Access Memory (RAM)
RAM is an integrated circuit or chip that can be read and written by the microprocessor or other computer devices. RAM chips temporarily store data and program instructions during and after processing. RAM is the working memory in any computer system. RAM chips store the following during and after processing:
- Operating system instructions (Windows, Linux, DOS)
- Application program instructions (Word, Excel, games)
- Data currently being processed
RAM is volatile. This means memory can lose data and programs stored in it when power to the computer is turned off.
b) Read-Only Memory (ROM)
ROM chips store information or instructions that don’t change. ROM chips store the startup instructions when the computer is turned on. Therefore it can be read as many times as possible but not written to or erased from, hence the term Read-Only.
The instructions in ROM are recorded permanently in memory when it is manufactured. ROM is non-volatile. This means memory retains its contents when power to the computer is turned off.
c) Complementary Metal Oxide Semi-conductor (CMOS) Memory
These memory chips store information about the computer system such as:
- The type of keyboard used
- The type of mouse used
- The type of monitor used
- Date and time in the computer system
- The amount of RAM installed
- The type and capacity of disk drives, etc
A small battery installed on the motherboard maintains the contents in CMOS memory.
