2nd puc computer science chapter 1 backdrop of computers

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CHAPTER 1: COMPUTER SYSTEM ORGANISATION
CPU( Central Processing Unit) is the brain of computer. It processes all the data and
instructions given by the user. To process data, it takes the data and instructions from
memory. Data is moved from one unit to another by means of electrical signal pathways
called Bus. Bus is a set of wires that carries electrical signals from one component to
another component. CPU consists of Registers, Control Unit and Arithmetic and Logic Unit.
It is also known as microprocessor.
1. Program Counter-PC- Program Counter or Instruction Pointer stores the address
of next instruction to be executed. It helps the CPU to know where the next
instruction to be executed is located in the memory.
2. Memory Address Register- MAR-It is connected to Address bus. It stores the
address of memory location where data can be stored or data can be found. Program
counter transfers the address to MAR which transfer this address to address bus at
the time of read signal.
3. Memory Data Register- MDR-Also known as Memory Buffer Register. It acts as an
interface between CPU and main memory. At the time of read signal data from
memory is copied to MDR through the data bus.. At the time of write signal data
from CPU is copied to MDR.
4. Instruction Register- It stores the instruction to be currently executed. The
instruction is then sent to Instruction Decoder.
5. Instruction Decoder-ID-It analyses the type of instruction to perform the correct
operation. It decodes the instruction to a set of micro operations so that it can be
executed directly.
6. Timing and Control Unit-TCU-It generates all the internal and external control
signals for executing the instructions in a proper sequence. These signals are sent to
ALU and other registers.

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7. Arithmetic and Logic unit-ALU- It performs all arithmetic operations(+,-,*,/)logical
operations(AND,OR,NOT) and relational operations(<,<=,>,>=,==,!=).For all
operations, one operand is taken from accumulator and another from memory
buffer register. The results of arithmetic and logic operation are usually transferred
back to the accumulator.
8. Accumulator- It stores all intermediate results, final output for further use.
9. General Purpose Registers- R0 to R5 are the general purpose registers used for
storing intermediate data.
Motherboard:
The main circuit board of the computer which contains CPU, memory, expansion slots, bus etc is
known as a Motherboard. Also known as System board. It provides a platform for all the
components and peripherals to communicate with each other.
A Motherboard can be characterized by three features
1. Form factor- it refers to motherboard’s geometry,dimensions,arrangement and electrical
requirements.ATX standard is followed by most of the motherboards today
2. Chipset-it controls all the resources of the computer. It controls and coordinates the
transfer of data between various components of the computer.
3. Processor socket-it is a rectangular connector into which the processor is mounted
vertically or a square shaped connector with many small connectors into which the
processor is directly inserted.
Types of motherboard: Depending on the processors used. Motherboard is classified into
different types
1. XT motherboard- XT stands for eXtended Technology. It is an old model motherboard
having DIMM sockets, ISA slots, 12 pin power connector and no ports.
Ex: Pentium I, Pentium MMX, Pentium II processors
2. AT motherboard- AT stands for Advanced Technology. It have PGA socket(Pin Grid Array),
20 pin power connector, SDRAM slots, PCI slot and ISA slots.
Ex: Pentium III
3. Baby AT motherboard- It has the features of both XT and AT motherboards. It have PGA
processor sockets, SDRAM slots and DDRRAM slots, PCI slots and ISA slots,12 pin power
connector and 20 pin power connector.
Ex: Pentium III and Pentium IV
4. ATX motherboard- It stands for Advanced Technology Extended. Latest motherboards are
ATX motherboards. It has MPGA processor sockets, DDRRAM slots, PCI slots, AGP slots,
SATA connectors, 20 pin and 24 pin ATX power connector and ports.
Components of motherboard:
1 .Bus

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2. Communication Ports
3. Input device ports
4. RAM Slot
5. Processor Slot
6.Cache memory
7. Hard disk controller
8. Floppy controller
9. BIOS chip
10. CMOS Battery
11 .Expansion Slots
❖ Bus:
Different components of computer like CPU, I/O unit and memory unit are connected to each other
using bus.
A Bus is a set of wires that carry electrical signals(binary information)from one component to
another. It carries data, address and control signals. Each wire carries one bit of data.
Bus width refers to number of wires in the bus. There are two types of buses-Internal bus and
External bus.
Internal bus also known as System bus connects the major components of the computer system
like CPU,memory unit etc.
External bus is used to connect external or peripheral devices, expansion slots, ports etc to the
computer. It is slower than system bus.
Types of Buses:
1. Data bus –carries data between CPU and memory. Its width ranges from 32,64,128 bits.
It is bidirectional. When width of the data bus is higher, speed of the CPU also increases.
2. Control bus- carries necessary control signals generated by TCU to access the use of data
and addresses
3. Address bus- carries address of memory location between CPU and memory.It is
unidirectional.The width of the address bus determines the maximum number of
memory locations the computer can address. 36 bit address bus can address 236 bytes or
64 GB memory.
CPU Memory input output
Control bus
Data bus
Address bus

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❖ Communication Ports(I-O Ports): Port is a connector available on the back side of the
CPU to connect external devices to the computer like printers, keyboard, scanner etc.. The
different ports are
a) Serial port
➢ also called RS232C Port/COM port
➢ used to connect mouse and modem
➢ can send only one bit of data at a time. So it is slower in data transmission
➢ can be used for long distance transmission
b) Parallel port->
➢ also called Printer port. It is used to connect Printers
➢ can send 8 bits of data at a time
➢ Faster in data transmission.
➢ used for short distance transmission
c) IDE port(Integrated Digital Electronics Port)
➢ IDE devices like hard disk drives or CD ROM drives are connected to the motherboard
using IDE port
d) USB Port( Universal Serial Bus port)
➢ It is a easy to use way to connect a variety of new peripheral devices to the computer
➢ USB port is known as ‘Plug and Play interface’ because external devices can be
connected to the computer without switching off the system or without adding an
adapter card.
➢ Devices that can be connected to USB port are Plug and Play devices. It include
Pendrives, modems, scanners, audioplayers, card readers, joysticks etc.
e) PS/2 port
➢ Developed by IBM
➢ Used to connect keyboards, mouse, trackballs etc.
➢ Also known as Mouse port
f) MIDI port(Musical Instruments Digital Interface Port)
➢ through MIDI port, musical instruments can be connected to the computer
g) AGP interface- (Accelerated Graphics Port)-
➢ It is used to insert graphics card (video card) to a PC to make graphics application( like
games and other multimedia) run faster.
h) SCSI Port
➢ Small Computer System Interface Port
➢ used to connect high speed hard disk, CD ROMs, printers, scanners etc
➢ Fast and expensive
➢ Need separate adapters to function

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i) VGA Port(Visual Graphics Adapter Port)
➢ Connects monitor to computer video card
j) Modem(Modulator-Demodulator)
➢ Connect PC modem to the telephone network
k) Game Port ->used to connect gaming devices like joystick.
l) Ethernet port-used to connect to a network and internet
m) Digital Video Interface port-connects flat panel LCD monitor to computers video graphic
cards
n) Sockets are used to connect microphone, speakers to sound card of the computer.
o) Firewire port- transfer large amount of data at a high speed between camcorder or any
video devices to the computer.
p) Power connector- it has a three pronged plug that connects to the computer power cable.
❖ Expansion Slot: Also called as Motherboard slot. It is used to add peripheral cards to a PC
to give additional functionality.
Ex: ISA (Industrial Standard Architecture)-
• non plug n play in nature
• Introduced by IBM
PCI (Peripheral Component Interconnect)-
• plug n play in nature
• Introduced by Intel
• PCI Express Slot is introduced. It is faster than Intel.
AGP (Accelerated Graphics Port)-
• to insert graphics card (video card) to a PC to make graphics application run faster.
• Designed by Intel
❖ BIOS(Basic Input Output System)
ROM BIOS is a chip that stores a set of instructions that controls booting process and
all the input and output operations. BIOS runs when the computer is switched on. It
helps to perform Power On Self Test(POST).
Four functions of BIOS are

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➢ POST-It is a testing step that includes checking all the computer hardware
devices and make sure that they are properly connected and functioning
properly before loading the operating system.
➢ Bootstrap Loader-BIOS is used to locate the operating system to the memory
and it passes the control to the operating system.
❖ CMOS battery
➢ Complementary Metal Oxide Semiconductor battery gives the motherboard a small
amount of power in order to store vital data like date and time when the power is
off.
❖ RAM memory slot
➢ Random Access Memory stores programs and data currently being used by the CPU.
➢ Capacity of RAM is measured in bytes.
➢ RAM is packed in a slot called DIMM(Dual Inline Memory Module)
➢ Four standard of DIMM are Single Data Rate,Double Data Rate,DDR2 and DDR3.
❖ Disk Controller
➢ Disk Controller is the circuit which enables the CPU to communicate with a hard disk,
floppy disc or any other disk drive.
➢ FDD Controller –Floppy Disc Drive Controller is used to connect floppy drive to the
motherboard.It is a 33 wire ribbon cable.
➢ HDD Controller-Hard disk Controller is used to connect hard disk drives or CD/DVD
drives to the motherboard.The first hard disk controller is the IDE standard
drive(integrated digital electronics) which is attached to the motherboard using a 40
wire ribbon cable. Another standard is EIDE standard that allows four drives to be
connected.
❖ Processor slot
➢ It is used to insert the processor chip which is the largest chip on the motherboard.It
can be identified, as a heat sink or fan is located on top of it.
**************************************************************************************
❖ Switched Mode Power Supply
➢ It is an electronic power supply that incorporates a switching regulator to
convert electrical power from 230 volts AC to 5-12 DC.
➢ It supply power to all components of the CPU
❖ Clock speed
➢ Clock speed is the measurement of processors operating speed.It is measured
in megahertz or gigahertz.
➢ Megahertz(MHz) represent million pulses in one second and Gigahertz(GHz)
represents one bllion pulses in one second
➢ When clock speed increases, speed of the CPU also increases.
❖ Uninterruptible Power Supply(UPS)

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➢ UPS is a device that includes a battery to maintain power and protects
electronic equipment from power uncertainties.
➢ It supply power to the system in case of power line disturbances, power
outages, overvoltage, under voltage, voltage spikes, Lightening etc.
➢ Voltage spike refers to short term overvoltage
➢ It helps the user to save data to hard disk from RAM and then shut down
the system successfully. It also helps to protect the electrical components
from damage.
There are two types of UPS……..
a) Offline UPS(Standby UPS)---it monitors the powerline and supply power as soon
as it detects any problems like overvoltage,undervoltage,powerfailure etc.It
takes some milliseconds to carry out this switching. During that time computer
is not receiving any power.
b) Online UPS----------It is more efficient as it constantly supply power from its
own inverter but it is expensive than offline UPS. Apersonal computer needs
ups of 500 VA to 600VA.
❖ Memory:
A computer memory is the storing space for data and instructions which are needed by the
CPU for processing
There are two types of memory……..internal memory and secondary memory
Internal memory includes registers, cache memory and primary memory.It is used for
temporary storage of data and instructions which are currently needed by the CPU.It is faster
and more expensive.It has limited storage capacity.
❖ Registers
Registers are high speed temporary storage areas inside the CPU. It stores the data from Cache
memory or RAM.These registers are controlled by the control unit. The size of register affects
the speed of CPU. If register size is more,speed of CPU increases.
❖ Cache memory
➢ Cache memory: is a small fast intermediate memory between CPU and RAM. It stores
the frequently used data or programs by the CPU.
➢ There are three types of cache……Level1 cache(L1 cache) which is integrated within
the processor and Level2 cache(L2 cache)which is located outside the processor on the
motherboard and L3 cache is currently available in Intel core i7 processor.
➢ It is used to increase the speed of CPU.
➢ It is smaller in size but expensive.
➢ L1 cache have storage capacity of 256 KB,L2 cache have 6MB and L3 cache have 12MB.
PRIMARY MEMORY
➢ Faster memory
➢ Low storage capacity
➢ consumes less power
➢ Expensive

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➢ Store data currently needed by CPU
Types of Primary Memory
1. Random Access Memory:
▪ It is called as RAM or Read/Write memory.The users can read information from
RAM and also write /store data to it. So RAM is called Read/Write memory.
▪ It is called a volatile memory because the contents of RAM will be lost when the
power supply is switched off. The programmer has to reload his program into RAM
when the power supply is resumed.
▪ There are two types of RAM-------
❖ Static RAM-It retains stored information only as long as the power
supply is on. It is costlier and consumes more power. It doesn’t need any
refreshing circuitry. They have more speed than DRAM.
❖ Dynamic RAM-It loses its stored information in a very short time even
though the power supply is on. So it should be refreshed periodically to
store data. Refreshing occurs automatically 1000times per second.It is
cheaper and consumes less power. It has refreshing circuitry.
❖ SDRAM-Synchronous Dynamic Random Access Memory-it is a special
type of DRAM and is synchronized to the system clck.So it has the data
ready whenever CPU requests it.It increases the efficiency of CPU.
❖
❖ DDR SDRAM-It works like SDRAM.but it has double data transfer rate
than SDRAM.
2. Read Only Memory (ROM) - ROM stores all the information that is to be stored
permanently. It is Non volatile either information stored in it is not lost even when
the power supply goes off. The information stored in ROM can only be read. We
can’t write any information to it. Only manufacturers can decide it. But there is
user programmable ROM’s like PROM, EPROM etc.
▪ PROM (Programmable ROM)-User decides the contents of PROM.
But PROM is programmable once. i.e., user writes information to it
only once and can read the information again and again. Cannot
perform writing again.
▪ EPROM (Erasable PROM)-The data stored in erasable PROM can be
erased by exposing it to high intensity ultraviolet light for about 20
minutes. It is cheap and reliable. It is used to store programs, which
are permanent but need updating.
▪ EEPROM (Electrically Erasable Programmable Read Only Memory)-
Data stored can be erased with the help of electrical signals.
Current configuration of PC in market
To purchase a new computer, user has to follow these guidelines

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✓ Select a good processor by seeing the brand,model and speed
Bit size of a processor indicates how many bytes of data can be accessed from RAM at a
time.64 bit processor can access 8 bytes of data from RAM.Megahertz is the clock
speed of processor.A 64 bit 800MHz processor can process 8 bytes simultaneously at
800 million times per second.
Ex: AMD FX-6100,Intel Core i3-4130,Intel core i5-4430 etc
✓ Select a good motherboard according to the type of processor,no of ports,expansion
slots,ram capacity needed etc.
Ex:ASUS P9X79
✓ Select RAM chip according to the RAM slot on the motherboard.
Ex:SDRAM,DDR,DDR2,DDR3
✓ Select harddisk of high storage capacity 250 GB or 500 GB or 1 TB
✓ Monitor can be of CRT type,LCD(Liquid Crystal Display) or LED or OLED(Organic Light
Emitting Display)
✓ Select regular keyboard with 84 keys or Enhanced keyboard with 101 keys or
multimedia keyboards ar wireless keyboards
✓ Mouse can be optical mouse,optomechanical or wireless mouse
✓ ATX cabinet is needed to house good quality SMPS(400 watts)
✓ Speaker should be good quality Ex:Creative 5.1 speakers
Summary
Processor
Intel or AM (AMD FX 6100 six core processor 3.3GHZ or Intel Core i7 -4930K 3.9GHZ)
Motherboard
Gigabyte GA-78LMT-S2
8 GB DDR3 RAM
1 TB Seagate harddisk with 7200 rpm
Monitor 21 inch LCD monitor
Keyboard and Optical mouse
Zebronix ATX cabinet
DVD writer Drive
Creative 5.1 soeakers
SMPS of 400 watts
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2nd puc computer science chapter 1 backdrop of computers