System unit is a box that contains different components of computer system. It is also called chassis. It is made of plastic or metal to protect the components inside it. Most storage devices reside inside the system unit.
Some system units sit on the top of a desk. A type called tower model stands vertically on the floor. A type called all-in-one computer contains the system unit and the monitor in the same box. In Laptop computers, keyboard and pointing devices are built on the top of system unit.
Network card Ports
Motherboard is the most important circuit board in system unit. It contains different chips.
A chip is a small semiconductor that contains integrated circuits on it.
Integrated circuit (IC) is a microscopic path that carries electrical current. An IC may contain millions of transistors. A transistor works as an electronic gate that opens or closes the circuit for electronic signals. Chips are attached to a circuit board like motherboard, memory module or card.
There are different types of chip packages that are as follows:
DIP – DIP stands for the dual-line package. It consists of two parallel rows and downward-pointing thin metal pins. The pins attach the chip package to the circuit board.
PGA – PGA stands for pin grid array. It holds a larger number of pins as the pins are mounted on the surface of the package.
FC-PGA – FC-PGA stands for flip chip-PGA. It is a higher performance PGA. It laces the chip on the opposite side of the pins.
SEC – SEC stands for single edge contact. It connects to the motherboard on one of its edge.
Central processing unit is also called processor. It interprets and executes the instructions to run the computer system. It is the brain of the computer. Il performs all operations on the data according to the instructions. It carries out instructions and tells other parts of computer system what to do.
CPU Consists of two main units:
Arithmetic and Logic Unit – ALU is a part of CPU. Actual execution of the instructions takes place in this part. All arithmetic and logical operations are performed in ALU.
Control Unit – Control Unit is an important component of CPU. It controls all activities of computer system. It is also called the supervisor of the computer.
CU fetches instructions from main memory. It interprets the instruction to find what operation is to be performed. It controls the execution of instruction.
It consists of two units:
Arithmetic Unit – Arithmetic unit of the ALU performs basic arithmetic functions such as addition, subtraction, multiplication and division.
Logic Unit – Logic unit of the ALU performs logical operations like comparing two data ilems to find which data item is greater than, equal to, or less than the other.
CPU performs following steps to execute an instruction:
Instruction Fetch – Instruction fetch is a process of getting an instruction from the memory to execute it. This process is performed by control unit.
Interpret instruction – Interpret instruction is a process to examine the nature of instruction to determine what further operations should be taken.
Execute Instruction – After interpreting the instruction and getting the required data, CPU finally executes that instruction. The instruction is executed by ALU.
Storing – The process of writing the result to the memory is called storing.
The time required to fetch and decode instruction is called Instruction time or i-time. The time required to execute and store is called Execution time or e-time.
A set of all instructions that a CPU can perform is called instruction set. The instruction set consists of the following types of instructions:
Arithmetic Instructions – These instructions are used to perform arithmetic operations.
Logical Instructions – These instructions are used to perform logical operations. Data
Transfer Instruction – These instructions are used to transfer data from one component to another component during program execution.
Control Transfer Instruction – These instructions are used to change the sequence of instructions of a program.
The speed of processor is also measured as millions of instructions per second (MIPS). Computers can process instruction with a speed of more than 300 MIPS.
Two types of CPU are as follows:
CISC – CISC stands for complex instruction set computing. It supports a large number of instructions. It executes complex instruction more quickly.
RISC – RISC stands for reduced instruction set computing. It reduces the instructions to only those used more frequently. It executęs simple instructions more quickly than CISC CPU.
CISC computers contain more machine instructions, and RISC ones have fewer instructions. RISC computers are faster.
Pipelining is a technique in which CPU fetches the next instruction before the completion of the first instruction. Pipelining is used because it results in faster processing.
Register is a small high-speed memory inside CPU. It is used to store temporary results. A register can be of one, two, four or eight bytes. Bigger size of register increases the performance of CPU.
Different registers are as follows:
Accumulative Registers – Accumulative registers are used in mathematical and logical operations.
Stack Control Register – Stack control registers are used to manage stack in computer.
Instruction Pointer Register – Instruction pointer register is used to store the memory location of the next instruction to be executed.
Address or Segment Registers – Address or segment register is used to store the memory location of the instruction that is being executed.
Flag Registers – Flag registers are used to indicate a particular condition.
System clock is an electronic component. It generates electric signals at a fast speed. It controls all functions of the computer using cluck ticks. Computer clock can tick from 100 million to 1000 million times in one second.
The speed at which the CPU executes instructions is called clock speed or clock rate. This speed depends on the model of the computer. Clock speed is measured in MHz.Clock speed tells us how much time a CPU lakes to execute each instruction.
Heat sink is a small ceramic or metal component used to absorb and ventilate the heat produced by electrical components of computer. Heat pipes are used to absorb heat in notebook computers.
A coprocessor is a special-purpose chip mounted on 3 processor board that can be installed into a computer. It is designed to handle common functions quickly and efficiently, and so expand on doing some of the processing of the chip. This frees up the CPU for other processing.
A method in which multiple processors are used to speeding up the processing is called parallel processing. These processors work simultaneously to complete a program. Parallel processing divides a task into multiple smaller tasks. Each processor solves the smaller task independently at the same time.
The two types of parallel processing are as follows:
Single instruction/multiple data (SMID): This method executes the same instruction on many data values simultaneously.
Multiple instruction/multiple data (MMD): This method connects a number of processors that run different programs or parts of a program on different sets of data.
Benchmarking is a technique used to evaluate how various computers will be able to handle the workload of a particular organization. Benchmarking can provide comparison information for selecting or configuring computer systems.
Computer works in binary system. This system consists of two digits 0 and I called bits. Bit stands for binary digit. It is the smallest unit of data that can be represented in computer. A collection of eight bits is called byte. One byte can represent 256 different characters.
Popular coding schemes are as follows:
ASCII – ASCII stands for American Standard Cude for Information Interchange. It is an 8-bit code. It is the most popular coding scheme for data representation. It can represent 256 characters.
EBCDIC – EBCDIC stands for Extended Binary Coded Decimal Interchange Code. It is an 8-bit code. It is normally used in mainframe computers. It can represent 256 characters.
Unicode – Unicode is a 16-bit code. So it can represent 65536 characters. It can represent the characters of all languages in the world.
Unlike ASCII and EBCDIC, which are 8-bit codes, Unicode is a 16-bit code. This means that instead of being limited to 256 different characters, Unicode has up to 65,536 characters. This allows the inclusion of most of the alphabets for the major foreign languages.
Data and instructions are stored in memory to be executed. This memory is temporary storage unit for data, instructions and information. It is also called primay storage. It consists of one or more chips on the motherboard.
A memory address points to an identifiable location in memory where data are stored. It allows the computer to determine exactly where an element of data or information is stored.
RAM is very important role in the processing speed of a computer. A bigger RAM size provides larger amount of space for processing. So the processing speed is increased.
Memory stores three items:
1. Operating system and other system software to control the use of computer system.
2. Application programs to perform specific tasks.
3. Data to be processed by application programs.
There are two types of memory in computer that are as follows:
Volatile Memory – Volatile memory loses its contents when computer is turned off.
Nonvolatile Memory – It does not lose its contents when computer is turned off.
RAM stands for Random Access Memory. It is also called main memory or Direct Access Memory. Random access means that each individual byte in the entire memory is directly accessible. A program must be loaded into RAM before execution.
The data and instructions are retrieved from RAM when CPU uses them for first time. A copy of that data or instructions is stored in cache. The next time the CPU needs that data or instructions, it first looks in cache. If the required data is found there, it is retrieved from cache memory instead of main memory. It speeds up the working of CPU.
Different types of cache memory are as follows:
Level 1 (L1) Cache – It is also called primary or internal cache. It is built directly into the processor chip. It has small capacity from 8 KB to 128 KB.
Level 2 (L2) Cache – it is slower than L1 cache. It is size is large i.e. from 64KB to 4MB. New processors contain advanced transfer cache on processor chip that is a type of L2 cache. The common size of this cache is 256 KB.
Level 3 (L3) Cache – This cache is separate from processor chip on the motherboard. It exists on computer that use L2 advanced transfer cache.
Cache memory is a special form of high-speed memory. It eliminates the need to repeatedly move data to and from the CPU to memory. Primary memory is slower than cache and located outside of the CPU. It is also less expensive than cache.
Flash memory is a type of non-volatile memory. It can be erased electronically and reprogrammed. Flash memory chip stores data on handheld computers like cellular phones, digital cameras etc.
Cell phones, pagers, portable computers, and handheld PDAs use flash memory.
CMOS stands for complementary metal-oxide-semiconductor. It stores configuration information of computer. The information include the type of disk drivers, keyboard and monitor, current data and Lime and other startup information required during the booting process.
Information such as the date, time, and calendar is stored in CMOS RAM.
The amount of time required by processor to read data, instructions and information from memory is called access time. Access time is defined as follows:
Millisecond – It is one thousandth of a second. It is denoted by ms.
Microsecond – It is one millionth of a second. It is denoted by us.
Nanosecond – It is one billionth of a second. It is denoted by ns.
Picosecond – It is one-trillionth of a second. It is denoted by ps.
Megahertz – It can be converted into nanoseconds by dividing it into 1 billion ns. It is denoted by MHz.
An expansion slot is a socket that is mounted on a motherboard to insert circuit board. Different circuit boards are inserted into the motherboard to add new devices and enhance the capabilities of a computer.
RAM is temporary memory. When the power is turned off, the information in this memory is lost. Thus it is called volatile memory.
RAM plays very important role in the processing speed of a computer. A bigger RAM size provides larger amount of space for processing. So the processing speed is increased.
There are two types of RAM:
Static RAM – It is more expensive. It requires more amount of space for a given amount of data than dynamic RAM. It does not need to be power-refreshed and is, therefore, faster to access.
Dynamic RAM – ll is the least expensive kind of RAM. It uses a kind of capacitor that should be recharged again and again. A power refresh is required after each read so it is slow.
DRAM and SRAM are two types of RAM. Both are random access and work only while the computer is turned on. DRAM holds its data dynamically. It does not hold it indefinitely. Each DRAM memory cell must be constantly refreshed to keep data in DRAM. SRAM does not require refreshing. It holds the data indefinitely as long as the computer remains on. It is faster than DRAM but it is more complex and less compact.
RAM is mounted on a small circuit board.called memory module. The memory module is mounted on the motherboard. Three types of memory modules are as follows:
SIMM – SIMM stands for single inline memory module. In this module, the pins on opposite sides of the circuit board connect together to form a single set of contacts.
DIMM – DIMM stands for double inline memory module. In this module, the pins on opposite sides of the circuit board do not connect and form two set of contacts.
RIMM – RIMM stands for Rambus inline memory module. It houses SDRAM chips.
ROM stands for read only memory. The instructions in ROM prepare the computer for use. These instructions can only be read but cannot be changed or deleted. It is not possible to write new information or instructions into the ROM.
ROM stores data and instructions permanently. When the power is switched off, the instructions stored in ROM are not lost. Therefore ROM is called non-volatile memory.
Different types of ROM are as follows:
PROM – PROM stands for programmable read-only memory. The user can write his own instructions once. If there is any error in writing the instructions, the error cannot be removed from the PROM.
EPROM – EPROM stands for Erasable Programmable Read-Only Memory. User can erase instructions or data stored in the EPROM chip by exposing the chip to ultraviolet light and write new program.
EEPROM – EEPROM stands for Electronically Erasable Programmable Read-Only Memory. The user can erase and write instructions with electrical pulses. The user can erase the contents electrically.
A cache is a small and very fast memory. It is designed to speed up the transfer of data and instructions. It is faster than RAM. The data and instructions that are most recently or most frequently used by CPU are stored in cache.
Different types of expansion cards are as follows:
Video Card – It is also called video adapter or graphics card. It converts computer output into a video signal and sends it to the monitor to display.
Sound Card – It enhances the sound-generating capabilities of a computer. It allows sound to be input through microphone and output through speakers.
Network Interface Card (NIC) – It is also called network card. It is a communication device that allows a computer to communicate via a network.
Modem Card – It is also called an internal modem. It is a communication device that enables computer to communicate via telephone lines or other means.
PC card is a thin credit card size device. It is used to add memory, disk drives, sound, fax/modem, communications and other capabilities to a mobile computer like laptop.
Three types of PC Cards are as follows:
Type I Card – it adds memory capabilities to the computer.
Type II Card – It contains communication devices such as modem.
Type III Card – It deals with devices like hard disk. It is the thickest card.
PC Card and flash memory cards can be attached to the computer without opening the system unit and restarting the computer. The operating system automatically detects the PC Card and flash memory card as it is attached to the system. This feature is called hot plugging or hot swapping.
A port is an interface or point of attachment. It is used to connect peripheral devices with computer such as printers, keyboards or mouse.
A connector joins a cable to a device. One end of the cable is attached to the connector on the system unit and the other end is attached to the connector on the peripheral device. Different genders of connectors are as follows:
Male Connectors – It has one or more exposed pins like the end of an electrical cord.
Female Connectors – It has matching holes to accept pins on male connector like electrical wall outlet
Gender changer is a device that is used to join two connectors of same gender i.e. two male connectors or two female connectors.
A serial port is used to connect devices to the system unit. A serial port transmits data one bit at a time. It is usually used to connect devices that do not require fast data transmission like mouse and keyboard etc.
The parallel port is used to connect devices that transfer many bits at a time. Printers connect to computer using a parallel port.
When using a serial port, the bits are sent and received sequentially.one at a time over that data wire. A parallel port has multiple data wires and the bits are sent simultaneously. Even though a serial port is slow, it can transmit data faster than a human can type.
Universal serial bus port (USB) can connect up to 127 different peripheral devices with a single connector type. Many computers have one or two USB ports.
There are three reasons of the popularity of USB:
1. Data can be transferred approximately 100 times faster than a serial port;
2. USB devices can be chained together;
3. USB devices can be hot-swapped.
Multiple devices can be attached to USB port through a daisy chain. It means that the first device is attached to the USB port of the computer, second USB device connects to the first USB device, the third USB device connects to the second USB device and so on.
USB hub is an alternate to daisy chain. USB hub plugs into the USB port on the system unit. It contains multiple USB ports used to connect many USB devices.
This port is also called FireWire port. It can connect multiple types of devices that require faster data transmission speeds like digital video cameras, digital VCR, color printers, scanners etc. It can connect up to 63 devices together.
MIDI stands for musical instrument digital interface. It connects the system unit to a musical instrument like an electronic keyboard. A system unit with MIDI port can record sounds created by a synthesizer and then process these sounds.
Synthesizer is a peripheral or chip that creates sound from digital instructions.
SCSI stands for small computer system interface. It is a special high-speed parallel port to attach SCSI peripheral devices like disk drives and printers. It can connect up to 15 devices using daisy chain.
IrDA stands for Infrared Data Association. It is also called Fast Infrared (FIR). A wireless device can transmit signals to a computer with IrDA port. Different devices use IrDA port like keyboard, mouse, printers, digital cameras etc. These devices Lise a high-speed IrDA port.
Bluetooth port is an alternative of IrDA port. It uses radio waves to transmit data between two devices. Bluetooth-enabled devices can communicate with one another without aligned to one another.
Buses are electrical paths or lines inside computer. These buses are used to carry electrical signals between different components of computer. One line of wire can carry one bit at a time.
The electrical path through which data is transferred between components of computer is called data bus. The data bus typically consists of 8, 16, 32 or 64 separate lines. The data bus lines are bidirectional. Different devices such as keyboard, memory and mouse work with data bus.
The address bus consists of parallel signal lines. The address bus is used to designate the source or destination of data on the data bus. The address bus typically consists of 8, 16, or 32 separate lines.
The control buses are used to control the use of the data and address buses. CPU provides enabling signal through control bus. For ekample many devices are connected to data bus like keyboard, mouse etc. but only one should be enabled at a given time. It is controlled by control bus.
Some devices connect to a port on a card which is inserted into an expansion slot. This expansion slot connects to the expansion bus. The expansion bus allows the processor to communicate with the peripheral devices attached to the card.
Different types of expansion buses are as follows:
ISA Bus – ISA stands for Industry Standard Architecture. It is common and slowest expansion bus.
Local Bus – It is a high-speed expansion bus. It connects higher speed devices like hard disk
AGP Bus – AGP stands for Accelerated Graphics Port. It is designed to improve the speed of 3-D graphics and video transmit. It provides a faster-dedicated interface between the video card and memory.
USB – USB stands for Universal Serial Bus. It eliminates the need to install cards into expansion slots. Many USB devices can be attached to computer with single USB port.
PC Card Bus – It is designed for PC Card. It transmits data between PC Card bus and PCI bus.
Bay is an open area inside the system unit to install additional equipment. Bays are also called drive bays. Two types of bays are as follows:
External or Exposed Drive Bay – It is used to access the drive from outside the system unit. Some examples of external drive bays are floppy disk drive, CD drive, DVD drive and Zip drive.
Internal or Hidden Drive Bay – It is hidden inside the system unit. An example of internal drive bay is hard disk drive.
The power supply of a computer is a simple electrical system. It is used to supply correct voltage to different components of computer. It takes 240 or 110 AC voltages as input. It converts AC voltage into 3V, 6V, 12V and 24V of Direct Current (DC).
Following are some important factors that affect that processing speed of a computer:
Registers – The size of the registers determines the amount of data with which the computer can work at one time. Bigger size of register increases the performance of computer.
RAM – Bigger amount of RAM means that more program instructions and data can be stored in memory System.
Clock -A faster clock can execute more instructions each second.
Buses – A higher bus width means that the use can carry more data. It increases the performance of computer.
Cache Memory – Cache holds the most recent data and instructions that can be reused without going to main memory. It speeds up the working of CPU.
A collection of related components that are designed to work together. A system includes hardware and software.