PC Hardware Overview

Dept of IT | III YEAR | V SEMESTER IT E51 | COMPUTER HARDWARE AND TROUBLESHOOTING | UNIT 1
1 |Prepared By : Mr. PRABU.U/AP |Dept. of Computer Science and Engineering | SKCET |
UNIT I
Pc Hardware Overview: Introduction – Basic Parts of PC – Functional block diagram –
system board – Microprocessor – Interrupts – DMA – SMPS – BIOS – POST sequence -
System configuration – Memory – Mass storage – I/O interface standards.
1. 1 INTRODUCTION
A Computer is an electronic device which stores and processes information or
data to give meaningful results.
Processing is done, with the help of instructions given by the user, which are
also stored within the computer.
The data, as well as the instructions provided to the computer, are known as
INPUT, and the Processed result obtained at the end is known as OUTPUT.
A Computer comprises of
1. The Hardware
2. The Operating System (OS)
3. The Software
1.1.1 The Hardware
All physical components of a computer system are called hardware. The
hardware components of a computer system are the electronic and mechanical parts.
The hardware are the parts of computer itself including the Central Processing
Unit (CPU) and related microchips and micro-circuitry, keyboards, monitors, case and
drives (hard disk, CD, DVD, floppy disk, optical, tape drives, etc.).
Other extra parts called peripheral components or devices include mouse,
printers, modems, scanners, digital cameras and cards (sound, color, video) etc.
Together they are often referred to as a Personal Computer (PC).
1.1.2 The Operating System (OS)
The OS is integrated set of specialized programs that coordinates the various
functions of the computer. The OS communicates our requests to the computer in its
own language and gives user the results. In other words it acts as an interface between
the user and the computer hardware.
An Operating System (OS) is a system software that resources, both hardware
and software, of a computer. An OS usually pre-define basic functionalities that are
common to most applications and provide such functionalities, such as
 File management
 Device management
 Disk management
 Memory management
 Input/output management
 Graphical user interface rendering and
 Computer networking, to the applications that run on that OS.

Examples:
Various versions of Microsoft Windows, MacOS, and RedHat Linux.
An operating system links you to your programs (also called applications or
software) and then links your programs to the computer's hardware. It controls how
the computer does its most basic tasks, like storing files or talking to printers.
Microsoft Windows XP is an example of an operating system. It uses graphics
(pictures) to connect you to the computer's hardware and software in an easy-to-
understand way.
Microsoft Windows XP also comes with many free, built-in programs that can
help you create documents, movies and images, calculate figures, enjoy some music,
browse the Internet, or play a few games.
1.1.3 The Software
Software refers to the set of instructions required for running a computer
system. It is responsible for controlling, integrating and managing the hardware
components of a computer and to accomplish specific tasks.
While the OS is technically a software program, it is distinct from other software
specifically referred to as "application software". Application software, we'll just call it
software, are programs that you install onto a PC that make the PC useful.
Software is a complex series of instructions telling the computer what to do. The
instructions are very detailed because they have to tell the computer every single step
to be performed.
Example:
A word processing program has instructions for what to do when you press the
letter "A". The software tells the computer to take the letters as they are pressed on the
keyboard, then it tells it to add the letter to the letters you already typed and it tells the
computer to display the letter on the screen so you can see what you have typed. The
computer doesn't do anything without explicit instructions.
1.2 BASIC PARTS OF PERSONAL COMPUTER(PC)
Personal Computer (PC)
A small, single-user computer based on a microprocessor. In addition to the
microprocessor, a personal computer has a keyboard for entering data, a monitor for
displaying information, and a storage device for saving data.
A typical PC contains the following hardware:
 The Case
 The Power Supply
 The Motherboard
 The CPU
 The Memory
 Random Access Memory (RAM)
 Read Only Memory (ROM)
 The Hard Disk
 The Floppy Disk Drive
 The CD-ROM Drive
 The Graphics Card or Video Card

 The Sound Card
 The Modem
 The Mouse
 The Keyboard
1. The Case
The computer case is the metal and plastic box that contains the main
components of the computer. It houses the motherboard, central processing unit (CPU),
power supply, and more. The front of the case usually has an on/off switch and one or
more optical drives. Computer cases come in different shapes and sizes.
 A desktop case lies flat on a desk, and the monitor usually sits on top of it.
 A tower case is tall and sits next to the monitor or on the floor.
2. The Power Supply
In personal computer, the power supply unit is a metal box usually found in a
corner of the system unit. This box is known as SMPS (Switch Mode Power Supply) box.
Where the power cord connects to the back of the PC is the power supply.
The SMPS converts AC current from the wall outlet to the appropriate DC
voltages for the various components of the computer. Switch Mode Power Supply
(SMPS) provides different voltage levels.
The use of SMPS is to operate different DC voltage levels in the range of +5V, +12
V, -5 V, -12V to all the internal units like motherboard, hard disk, CD-ROM, floppy disk
drive etc.
3. The Motherboard
The motherboard is the largest and most fundamental component of a PC. Every
other hardware component is somehow attached to the motherboard. The motherboard
is the common link for every component to communicate and work together.
The motherboard has a series of slots, sockets and connectors for connecting
the various components of a PC. The memory, accessory cards, and CPU are installed
directly onto the motherboard in most cases. The drives and peripherals communicate
with the motherboard through wired connections.
There are a wide range of motherboards to choose from. They differ in features,
speed, capacity and the CPU supported. They also differ in size, shape and layout; this is
commonly referred to as the "form factor".
4. The CPU
The CPU, which stands for Central Processing Unit, is the brain of the PC. It is
often referred to as the "processor" or "chip". The CPU directs, coordinates and
communicates with the hardware components and performs all of the "thinking". The
speed of the CPU is measured in MegaHertz(MHz).
5. The Memory
Random Access Memory (RAM)
Every computer has a physical temporary internal storage place. Such temporary
internal storage place is known as RAM. RAM chips come in memory sizes of 64MB,
128MB, 256 MB, 1 GB, 2GB, 4GB and 8GB.
The RAM comes in two different technologies, Static RAM (SRAM) and Dynamic
RAM (DRAM). The two types differ in the technology they use to hold data, Dynamic
RAM being the more common type

Read Only Memory (ROM)
Unlike RAM, the Read Only Memory (ROM) retains its contents even when the
computer is turned off. ROM is referred to as being nonvolatile, whereas RAM is volatile.
Most personal computers contain a small amount of ROM that stores critical programs
such as the program that boots the computer.
6. The Hard Disk
The hard disk is a device which stores all programs and data in the computer.
Hence, the hard disk is referred to as the memory bank of a computer. Nowadays, the
capacity of the hard disk is measured in Tera bytes (TB).
Earlier, the memory capacity of computers was limited to Giga bytes (GB). But,
today PCs having hard disks of capacities like 40GB or 80GB, 1TB are popular. Larger
the hard disk capacity, more the amount of software programs and information that can
be stored in it.
The Hard Disk Assembly (HDA) is a sealed metal unit, which contains
components, consists of platters, spindle and spindle motor, read/write heads, head
actuator.
7. The Floppy Disk Drive
The floppy disk drive is a device that records data onto a removable storage disk
called a floppy disk. Floppy disks, also called "floppies" or “diskette”, are the most basic
storage medium for data. However their limited capacity, typically 1.44 megabytes,
makes them of limited use.
8. The CD-ROM Drive
The CD-ROM drive is a device that reads information or music off of a compact
disk (CD). A CD-ROM is an abbreviated term for Compact Disc-Read Only Memory. The
information stored in a CD-ROM can neither be changed nor can new information be
added to it. That is why; it is called Read Only Memory (ROM). Most software is
distributed on CDs because of their low cost and large capacity (650MB or more).
Many PCs are now built with a CD-RW drive, which stands for Compact Disc,
Read-Write. With the use of RE-writable CDs (CD-RWs) you can reuse the disk and
rewrite over it again many times. The speeds of a CD-RW are expressed like this, 4X,
16X and 32X.
9. The Graphics Card or Video Card
The video adapter card or graphics adapter translates information into graphics
and text that appear on the monitor screen.
Two types of display cards are available. These are the PCI graphics card
(Peripheral Component Interconnect) and the AGP card (Advanced Graphics Port).
10. The Sound Card
The sound card plugs into a slot on the motherboard or is incorporated directly
into the motherboard. With a basic sound card a microphone, speakers, joystick and an
auxiliary sound source can be connected to it.

11. The Modem
Modem is technically called the Modulator-Demodulator. The modem is a device
that enables the PC to use a telephone line to communicate with other PCs and devices.
The name comes from "MOdulation DEModulation".
12. The Mouse
Palo Alto Research Centre (PARC) developed the concept of pointing device
known as the mouse. Mouse is pointing device used in all the modern computers. The
mouse is a user input device that enables you to communicate with your PC. By moving
the mouse and pressing the two or three buttons, you can highlight and select images on
the screen to give directions to your PC.
A mouse is usually connected by a wire but wireless mice are also available
known as cordless mouse. Wired mice may use a serial, PS/2 or a USB port. Other
variations of mice available include the trackball and touchpad.
The most common mouse alternatives include:
Trackball: A trackball has a ball on top that can rotate freely. Instead of moving the
device like a mouse, you can roll the ball with your fingers to move the pointer. Some
mobile devices have miniature trackballs that can be controlled with your thumb.
Touchpad: A touchpad—also called a trackpad—is a touch-sensitive pad that lets you
control the pointer by making a drawing motion with your finger. Touchpads are
common on laptop computers.
13. The Keyboard
The keyboard is the primary user input device. It is the most popular and
commonly used input device with every computer. All the keyboards follow the
typewriter pattern to place the letters QWERTY on the top row of alphabetic characters.
Generally most of the keyboards have 101 keys and windows
keyboards/multimedia keyboards have 104 or 105 keys.
Keys Identification and use
------ -------------------------------
Alphabetic keys - keys with letters A to Z
Numeric keys - keys with numbers 0 to 9
Function keys - F1 to F12, specific functions on various software
Navigation keys - 
Enter key - used to process a command or data keyed in
Shift key - used to type upper case characters in combination with
alphabetic keys
1.3 FUNCTIONAL BLOCK DIAGRAM OF PC
 The blocks inside the system box are called as internal units.
 The blocks outside the system are called as external units.
 The expansion box provides additional memory.
 The external units are connected to the system box by separate cable.
 Switch mode power supply (SMPS) provides different voltage levels as +5v, -5v
to all the internal units

Figure 1.1: Functional Block Diagram
Functional Block Diagram of an IBM Personal Computer
Figure 1.2: Functional Block Diagram of an IBM Personal Computer
The following things are present in the block diagram
 CPU
 CPU logic
 Motherboard logic
 RAM logic
 ROM logic
 Bus arbitration logic
CRT monitor
System box
Keyboard
Expansion box
Printer

 DMA logic
 Interrupt logic
 Memory refresh logic
 WAIT STATE logic
1. CPU
ALU is a 10 bit register which provides the high speed processing power.
2. CPU logic:
It contains the microprocessor and the subroutine chips. It provides address,
control and data bus.
3. Motherboard logic
Motherboard logic is a microcomputer with add-on peripheral interface
controller. Keyboard interface is a part of motherboard logic. Other interface controllers
are mounted as separate board on the motherboard.
ROM Chips:
There are one or more ROM chips on the motherboard. When the power is ON,
the microprocessor reads the data from the ROM. The ROM chips in the PC are byte
organized. The location of each ROM contains 8 bits.
RAM Chips:
The RAM chip in the PC are bit organized. The location of each Ram contains one
bit. 9th bit is used as the parity bit.
RAM parity logic:
It is a part of motherboard logic; this logic contains odd parity generator and odd
parity checker.
 Odd parity generator:
 It is used to generate the parity bit or extra bit into the original data/user
data written into the RAM.
 Odd parity checker:
 It checks the number of one’s whether it is even or not. If it is odd means
‘no error’ and if it is even means ‘error’.
Keyboard interface:
Keyboard interface is also a part of motherboard logic. Keyboard interface
receive the keyboard serial data, it should be converted into parallel data.
4. RAM logic
RAM logic is used to interface the RAM chips into the address and control signal.
RAM logic is used to select RAM bank with the location.
5. ROM logic
It is used to select ROM bank with the location.
Figure 1.3: ROM logic

6. Bus arbitration logic
It provides service for DMA controller. It generates the wait state to the CPU.
7. DMA Logic
It is used for data transfer between the memory and IO controller. It receives the
request from floppy disk controller and hard disk controller.
8. Interrupt Logic
It receives several interrupt from interrupt sources. Interrupt logic send an input
to the CPU. CPU acknowledges the interrupt to the interrupt logic. Interrupt logic send
vector code to the CPU. CPU used with a code and branch this into interrupt subroutine.
9. Memory refresh logic
It generates the refresh indication to the RAM logic every 50 microsecond.
10. WAIT STATE logic
It generates the wait state to the CPU or DMA to synchronize with the CPU or
DMA controller with the memory or IO ports.
1.4 MOTHERBOARD / SYSTEM BOARD
The motherboard is the main circuit board inside the PC. It holds the CPU and
memory, provides expansion slots for peripherals, and, whether directly or indirectly,
connects to every part of the PC.
A motherboard, also known as a mainboard, system board, or logic boards
on Apple computers is the central or primary circuit board making up a complex
electronic system, such as a modern computer.
Connects to: Devices via Cables
Chips via Sockets
Riser Cards via one of
 PCI
 AGP
 PCI Express
Form Factors:
 AT (Advanced Technology)
 ATX (Advanced Technology eXtension)
 microATX
Common Manufacturers:
 Intel
 MSI
 Gigabyte
 Asus

Form factors
Motherboards are available in a variety of form factors, which usually
correspond to a variety of case sizes. The following is a summary of some of the more
popular PC motherboard sizes available:
 PC/XT - the original open motherboard standard created by IBM for the first
home computer, the IBM-PC. It created a large number of clone motherboards
due to its open standard and therefore became the de facto standard.
 AT form factor (Advanced Technology) - the first form factor to gain wide
acceptance, successor to PC/XT. AT was released in 1984 by IBM. Also known as
Full AT, it was popular during the 386 era. Now obsolete, it is superseded by
ATX. Baby AT – In 1985, IBM introduced Baby AT. IBM's successor to the AT
motherboard, it was functionally equivalent to the AT but gained popularity due
to its significantly smaller physical size. It usually comes without AGP port.
 ATX – In 1995, Intel introduced ATX- a modern form factor which quickly
replaced older Baby AT computers. The evolution of the Baby AT form factor, it is
now the most popular form factor available today.
 ETX, used in embedded systems and single board computers.
 Mini-ATX - essentially the same as the ATX layout, but again, with a smaller
footprint.
 microATX – Introduced in the late 1990s, the MicroATX is basically a smaller
version of Intel’s ATX specification, intended for compact, low-cost consumer
systems with limited expansion potential. It is commonly used in the larger cube-
style cases such as the Antec ARIA.
 FlexATX – The FlexATX is a natural evolution of the Intel’s microATX form factor
which was first unveiled in late 1999. A subset of microATX allowing more
flexible motherboard design, component positioning and shape.
 LPX – (Low Profile eXtension) form factor was a loosely defined motherboard
format (form factor) widely used in the 1990s. Based on a design by Western
Digital, it allows for smaller cases based on the ATX motherboard by arranging
the expansion cards in a riser. This design allows the cards to rest parallel to the
motherboard as opposed to perpendicular to it. The LPX motherboard is
generally only used by large OEM manufacturers.
 Mini LPX - a smaller subset of the LPX specification.
 NLX – Intel’s NLX design, introduced in 1997, is an improvement on the LPX
design for low-profile systems, with an emphasis on ease of maintenance. A low-
profile motherboard, again incorporating a riser, designed in order to keep up
with market trends. NLX never gained much popularity.
 BTX (Balanced Technology Extended) - a newer standard proposed by Intel as
an eventual successor to ATX.
 microBTX and picoBTX - smaller subsets of the BTX standard.

 Mini-ITX - VIA's highly integrated small form factor motherboard, designed for
uses including thin clients and set-top boxes.
 WTX (Workstation Technology Extended) - a large motherboard (more so
than ATX) designed for use with high-power workstations (usually featuring
multiple processors or hard drives.
1.5 MICROPROCESSOR
The processor is an electronic device about a one inch square, covered in plastic.
Inside the square is an even smaller square of silicon containing millions of tiny
electrical parts. A processor may contain 100 million transistors. The processor is the
"brain" of the computer system.
The processor is sometimes called the Central Processing Unit or CPU. A
particular computer will have a particular type of processor, such as a Pentium or a
SPARC chip. (Processors are often called "chips.")
The following table shows the differences between the different processors that
Intel has introduced over the years.
Name Date Transistors Microns Clock speed Data width MIPS
8080 1974 6,000 6 2 MHz 8 bits 0.64
8088 1979 29,000 3 5 MHz
16bits
8-bit bus
0.33
80286 1982 134,000 1.5 6 MHz 16 bits 1
80386 1985 275,000 1.5 16 MHz 32 bits 5
80486 1989 1,200,000 1 25 MHz 32 bits 20
Pentium 1993 3,100,000 0.8 60 MHz
32bits
64-bit bus
100
Pentium II 1997 7,500,000 0.35 233 MHz
32bits
64-bit bus
~300
Pentium III 1999 9,500,000 0.25 450 MHz
32bits
64-bit bus
~510
Pentium 4 2000 42,000,000 0.18 1.5 GHz
32 bits
64-bit bus
~1,700
Pentium 4
"Prescott"
2004 125,000,000 0.09 3.6 GHz
32 bits
64-bit bus
~7,000
1.5.1 Processor Performance Factors
There are a number of factors that decide the performance of a processor. Some
important factors are:
1. Clock speed: The number of cycles (clock ‘ticks) per second the processor
operates. AN instruction normally requires several cycles, - faster the clock
speed the more instructions executed per second (the MIPS rate); 8086
processors run at 0.25MIPS – Pentium processors at over 100 MIPS.

2. Memory (system) bus speed: as improvement as processor speed – the
processor spends a large amount of time ‘waiting’ – the faster the system bus the
shorter the waits.
3. Integer and floating point performance: a measure of number processing
efficiency; mainly integer processing is measured because intensive floating
point calculations are unusual; floating point performance is measured in
megaflops (MFLOPS). A megaflop is one million floating-point operations per
second.
4. Benchmark: a set of test that reflect real-world usage; give a P rating – based
mainly on integer performance.
1.5.2 Processor Modes
1. Real, or Native mode: The original 8086 mode restricted to 1 MB of memory
and only one program at a time (except for special memory-resident programs
which run in the background, called TSR [terminate and stay resident]
programs).
2. Protected mode: Introduced with 80286 processor; supports multitasking
(running multiple programs) – each program’s assigned memory is protected
from other programs (hence protected mode); supports extended memory;
803386 and later processors can switch on the fly between real and protected
modes.
3. Virtual real mode: Introduced with 80386 processors, it runs real mode from
within protected mode to allow DOS programs to run under MS windows – i.e.
open a DOS window.
1.5.3 Processor Instruction Set
An instruction set is a collection of machine code instructions which enable a
processor to carry out all its tasks.
A high level program is complied into a number of machine instructions – one
high level instruction (for example, a print command issued by a programming software
like, visual basic) must be changed into a series of low-level machine instructions ( or
machine language) which perform that single operation.
The total number of instructions in an instruction set can be up to 200.
Processors use one or two instruction set types.
1. CISC: Most Personal computers use a CISC (Complex Instruction Set
Computer) architecture, in which the CPU supports as many as two hundred
instructions. AN alternative architecture, used by many workstation and also
by some personal computers, is RISC (reduced Instruction Set Computer),
which supports fewer instructions.
2. RISC: Reduced Instruction Set Computer (RISC) is a type of instruction set for
a microprocessor that recognizes a relatively limited number of instructions.
One advantage of RISC is that it can execute their instructions very fast
because the instructions are so simple. Another important advantage is that
RISC chips require fewer transistors, which makes them cheaper to design
and produce.

3. Superscalar: Superscalar describes a microprocessor design that makes it
possible for more than one instruction at a time to be executed during a
single clock cycle. Superscalar design is sometimes called “second generation
RISC”.
1.5.4 Processor associates
There are several key components and sub-systems that work closely with the
processor. Some of the supporting components and sub-systems are:
1. Clock: A processor’s clock governs the speed at which the processor runs. The
clock determines how many cycles (pulses of power through the system) will
occur in a second.
2. Motherboard: The motherboard is the main circuit board into which the
processor is inserted. The motherboard runs at its own clock speed – the internal
bus speed. The motherboard contains all the other chipsets, controllers and
expansion slots used by the computer system.
3. System bus: The system bus is the main memory-access highway. A fast system
bus (100 MHz) is necessary for fast memory access. The Pentium II architecture
introduced a new bus connecting the processor and the integrated level 2 cache.
This is known as the backside bus, which typically runs at the same speed at the
processor. To make the distinction, the regular system bus is known as the front
side bus.
4. Cache memory: Cache memory is random assess memory (RAM) that a
computer microprocessor can access more quickly than it can access regular
RAM. Cache memory is sometimes described in levels of closeness and
accessibility to the microprocessor. L2 cache is usually a separate static RAM
(SRAM) chip.
5. Instruction Pipelining: Instruction pipelining is a method to increase the
capability of a CPU. The pipeline (a technique used in advanced microprocessors
where the microprocessor begins executing a second instruction before the first
has already been completed. That is several instructions are in the pipeline
simultaneously, each at a different processing stage) is divided into segments
and each segment can execute its operation concurrently with the other
segments.
6. Microprogram: A microprogram is a program consisting of microcode that
controls the different parts of a computer’s central processing unit (CPU). The
memory in which it resides is called a control store. It is a modern form of logic
of a computer’s control unit.
1.6 INTERRUPTS
In computing, an interrupt is an asynchronous signal from hardware indicating
the need for attention or a synchronous event in software indicating the need for a
change in execution.
Interrupts are a commonly used technique for computer multitasking, especially
in real-time computing. Such a system is said to be interrupt-driven. An act of
interrupting is referred to as an interrupt request.
Interrupts may be implemented in hardware as a distinct system with control
lines, or they may be integrated into the memory subsystem.

If implemented in hardware, a Programmable Interrupt Controller (PIC) or
Advanced Programmable Interrupt Controller (APIC) is connected to both the
interrupting device and to the processor's interrupt pin.
If implemented as part of the memory controller, interrupts are mapped into
the system's memory address space.
Hardware Interrupt
A hardware interrupt causes the processor to save its state of execution via a
context switch, and begin execution of an interrupt handler.
Hardware interrupts were introduced as a way to avoid wasting the processor's
valuable time in polling loops, waiting for external events.
Instead, an interrupt signals the processor when an event occurs, allowing the
processor to process other work while the event is pending.
Software Interrupt
Software interrupts are usually implemented as instructions in the instruction
set, which cause a context switch to an interrupt handler similarly to a hardware
interrupt. Software interrupts were introduced as a mechanism for access to shared
system routines that may also execute at higher privilege levels.
1.6.1 Categories of interrupt
Interrupts may be classified into one of two categories:
 Synchronous interrupts are predictable interrupts that occur at known times,
such as the execution of software interrupt instructions.
 Asynchronous interrupts are unpredictable interrupts that may occur at any
time, such the generation of an interrupt by a hardware device when it needs
servicing.
1.6.2 Types of Interrupt
1. Software interrupt
A software interrupt is an interrupt generated within a processor by
executing an instruction. Examples of software interrupts are system calls.
2. Maskable interrupt
A maskable interrupt is essentially a hardware interrupt which may be
ignored by setting a bit in an interrupt mask register's (IMR) bit-mask.
3. Non-Maskable Interrupt
Likewise, a non-maskable interrupt is a hardware interrupt which typically
does not have a bit-mask associated with it allowing it to be ignored.
4. Interprocessor interrupt
An interprocessor interrupt is a special type of interrupt which is generated
by one processor to interrupt another processor in a multiprocessor system.
5. Spurious interrupt
A spurious interrupt is a hardware interrupt which is generated by system
errors, such as electrical noise on one of the PICs interrupt lines.

1.6.3 Various class of interrupt
The various class of interrupts are
1. Level-triggered interrupt
2. Edge-triggered interrupt
3. Hybrid interrupt
4. Message-signalled interrupt
1. Level-triggered interrupt
 A level-triggered interrupt is a class of interrupts where the presence of an
unserviced interrupt is indicated by a high level (1), or low level (0), of the
interrupt request line.
 A device wishing to signal an interrupt drives the line to its active level, and then
holds it at that level until serviced. It ceases asserting the line when the CPU
commands it to or otherwise handles the condition that caused it to signal the
interrupt.
 Multiple devices may share a level-triggered interrupt line if they are designed
to. The interrupt line must have a pull-down or pull-up resistor so that when not
actively driven it settles to its inactive state.
Problems with sharing level-triggered interrupts
 There are also serious problems with sharing level-triggered interrupts. As long
as any device on the line has an outstanding request for service the line remains
asserted, so it is not possible to detect a change in the status of any other device.
 Deferring servicing a low-priority device is not an option, because this would
prevent detection of service requests from higher-priority devices. If there is a
device on the line that the CPU does not know how to service, then any interrupt
from that device permanently blocks all interrupts from the other devices.
2. Edge-triggered interrupt
 An edge-triggered interrupt is a class of interrupts that are signalled by a level
transition on the interrupt line, either a falling edge (1 to 0) or (usually) a rising
edge (0 to 1).
 A device wishing to signal an interrupt drives a pulse onto the line and then
returns the line to its quiescent state. If the pulse is too short to detect by polled
I/O then special hardware may be required to detect the edge.
 Multiple devices may share an edge-triggered interrupt line if they are designed
to. The interrupt line must have a pull-down or pull-up resistor so that when not
actively driven it settles to one particular state.
 Devices signal an interrupt by briefly driving the line to its non-default state, and
let the line float (do not actively drive it) when not signalling an interrupt. The
line then carries all the pulses generated by all the devices.
 The elderly Industry Standard Architecture (ISA) bus uses edge-triggered interrupts, but
does not mandate that devices be able to share them. However, ISA motherboards
include pull-up resistors on the IRQ lines, so well-behaved devices share ISA interrupts
just fine.
Problems of Edge-triggered interrupt
 Edge-triggered interrupts do not suffer the problems that level-triggered
interrupts have with sharing. Service of a low-priority device can be postponed

arbitrarily, and interrupts will continue to be received from the high-priority
devices that are being serviced.
 If there is a device that the CPU does not know how to service, it may cause a
spurious interrupt, or even periodic spurious interrupts, but it does not interfere
with the interrupt signalling of the other devices.
3. Hybrid Interrupt
 Some systems use a hybrid of level-triggered and edge-triggered signaling. The
hardware not only looks for an edge, but it also verifies that the interrupt signal
stays active for a certain period of time.
 A common hybrid interrupt is the NMI (non-maskable interrupt) input. Because
NMIs generally signal major-or even catastrophic-system events, a good
implementation of this signal tries to ensure that the interrupt is valid by
verifying that it remains active for a period of time. This 2-step approach helps to
eliminate false interrupts from affecting the system.
4. Message-signalled interrupt
 A message-signalled interrupt does not use a physical interrupt line. Instead, a
device signals its request for service by sending a short message over some
communications medium, typically a computer bus. The message might be of a
type reserved for interrupts, or it might be of some pre-existing type such as a
memory write.
 Message-signalled interrupts behave very much like edge-triggered interrupts, in
that the interrupt is a momentary signal rather than a continuous condition.
Interrupt-handling software treats the two in much the same manner.
 Typically, multiple pending message-signalled interrupts with the same message
(the same virtual interrupt line) are allowed to merge, just as closely-spaced
edge-triggered interrupts can merge.
 Message-signalled interrupt vectors can be shared, to the extent that the
underlying communication medium can be shared. No additional effort is
required. Because the identity of the interrupt is indicated by a pattern of data
bits, not requiring a separate physical conductor, many more distinct interrupts
can be efficiently handled. This reduces the need for sharing.
 Interrupt messages can also be passed over a serial bus, not requiring any
additional lines. PCI Express, a serial computer bus, uses message-signalled
interrupts exclusively.
1.6.4 Typical uses
Typical interrupt uses include the following: system timers, disks I/O, and
power-off interrupts. Other interrupts exist to transfer data bytes using UARTs
(Universal Asynchronous Receiver/Transmitter) or Ethernet; sense key-presses;
control motors; or anything else the equipment must do.
System Timers
A classic system timer interrupt interrupts periodically from a counter or the
power-line. The interrupt handler counts the interrupts to keep time. The timer
interrupt may also be used by the OS's task scheduler to reschedule the priorities of
running processes.

Counters are popular, but some older computers used the power line frequency
instead, because power companies in most Western countries control the power-line
frequency with an atomic clock.
Disk Interrupt
A disk interrupt signals the completion of a data transfer from or to the disk
peripheral. A process waiting to read or write a file starts up again.
Power-off interrupt
A power-off interrupt predicts or requests a loss of power. It allows the
computer equipment to perform an orderly shutdown.
1.7 DIRECT MEMORY ACCESS
 Direct memory access (DMA) is a feature of modern computers, that allows
certain hardware subsystems within the computer to access system memory for
reading and/or writing independently of the central processing unit.
 Many hardware systems use DMA including disk drive controllers, graphics
cards, network cards, and sound cards.
 Computers that have DMA channels can transfer data to and from devices with
much less CPU overhead than computers without a DMA channel.
 Without DMA, using programmed input/output (PIO) mode, the CPU typically
has to be occupied for the entire time it's performing a transfer. With DMA, the
CPU would initiate the transfer, do other operations while the transfer is in
progress, and receive an interrupt from the DMA controller once the operation
has been done. This is especially useful in real-time computing applications
where not stalling behind concurrent operations is critical.
 A typical usage of DMA is copying a block of memory from system RAM to or
from a buffer on the device. Such an operation does not stall the processor, which
as a result can be scheduled to perform other tasks.
 Two buses plays a major role BR(bus request),BG(bus grant)
 BR- DMA sends a request bus to CPU to get data
 BG- in turn CPU sends response signal to DMA, and DMA starts fetching
data from memory
1.7.1 Modes of Operation
1. Burst mode
2. Cycle stealing mode
3. Transparent mode
1. Burst mode: a entire block of data is transferred in one contiguous sequence, this
mode is useful loading the entire program or the data files into the memory but takes
more time for transferring.
2. Cycle stealing mode: in cycle stealing mode BR(bus request) and BG(bus grant)
signal plays a major role, in this mode the data is transferred byte by byte.
3. Transparent mode: it is same as a burst mode but data transferring is done very
fast.

1.7.2 Cache Coherency Problem
 DMA can lead to cache coherency problems. Imagine a CPU equipped with a
cache and an external memory, which can be accessed directly by devices using
DMA.
 When the CPU accesses location X in the memory, the current value will be
stored in the cache.
 Subsequent operations on X will update the cached copy of X, but not the
external memory version of X.
 If the cache is not flushed to the memory before the next time a device tries to
access X, the device will receive a stale value of X.
 Similarly, if the cached copy of X is not invalidated when a device writes a new
value to the memory, then the CPU will operate on a stale value of X.
Figure 1.4: Cache Coherency problem
Example
ISA
 A PC's ISA DMA controller has 16 DMA channels of which 7 are available for use
by the PC's CPU.
 Each DMA channel has been associated with a 16-bit address register and a 16-
bit count register.
 To initiate a data transfer the device driver sets up the DMA channel's address
and count registers together with the direction of the data transfer, read or write.
 It then instructs the DMA hardware to begin the transfer. When the transfer is
complete, the device interrupts the CPU.
 DRQ stands for DMA request; DACK for DMA acknowledge. They represent
electronic signaling lines between the CPU and DMA controller.
1.8 SMPS (SWITCH MODE POWER SUPPLY)
 A switched-mode power supply, switch-mode power supply, or SMPS, is an
electronic power supply unit (PSU) that incorporates a switching regulator.
 Switching regulators are used as replacements for the linear regulators when
higher efficiency, smaller size or lighter weight are required.
 The power output to cost crossover point between SMPS and linear regulating
alternatives has been falling since the early 1980s as SMPS technology was
developed and integrated into dedicated silicon chips
1.8.1 Various Types of SMPS
1. Buck converter (single inductor; output voltage < input voltage)
2. Boost converter (single inductor; output voltage > input voltage)
3. Buck-boost converter (single inductor; output voltage can be more or less than
the input voltage)

1.8.2 Working of SMPS
Figure 1.5 Working of SMPS
1. Input rectifier and filter
Figure 1.6: AC, half-wave and full wave rectified signals

 If the SMPS has an AC input, then its first job is to convert the input to DC. This is
called rectification.
 The rectifier produces an unregulated DC voltage which is then sent to a large
filter capacitor. The current drawn from the mains supply by this rectifier circuit
occurs in short pulses around the AC voltage peaks. These pulses have significant
high frequency energy which reduces the power factor.
2. Inverter or “Chopper”
 The inverter stage converts DC, whether directly from the input or from the
rectifier stage described above, to AC by running it through a power oscillator.
 The switching is implemented as a multistage (to achieve high gain) MOSFETs
amplifier. MOSFETs are a type of transistor with a low on-resistance and a high
current-handling capacity. This section refers to the block marked "Chopper" in
the block diagram.
3. Output Transformer
 Basically transformer contains primary and secondary windings, these winding
again converts the high voltage to low voltage current by stepping down the
current.
 The inverted AC is used to drive the primary winding of a high-frequency
transformer. This converts the voltage up or down to the required output level
on its secondary winding. The output transformer in the block diagram serves
this purpose.

4. Output Rectifier
 Ordinary silicon diodes are commonly used. For lower voltages, Schottky diodes
are commonly used as the rectifier elements; they have the advantages of faster
recovery times than silicon diodes (allowing low-loss operation at higher
frequencies) and a lower voltage drop when conducting. For even lower output
voltages, MOSFET transistors may be used as synchronous rectifiers; compared
to Schottky diodes, these have even lower "on"-state voltage drops
 The rectified output is then smoothed by a filter consisting of inductors and
capacitors.
5. Chopper Controller
 A feedback circuit monitors the output voltage and compares it with a reference
voltage, which is set manually or electronically to the desired output
 If there is an error in the output voltage, the feedback circuit compensates by
adjusting the timing with which the MOSFETs are switched on and off.
1.8.3 SMPS Compared with Linear Power Supply (LPS)
COMPARISON
FACTOR
LINEAR POWER
SUPPLY
SMPS
Size Large Small
Weight Heavy Small
Operating Cost High Low
Noise level at output Low High
1.9 BIOS (BASIC INPUT AND OUTPUT SYSTEM)
 BIOS stands for basic input/output system also incorrectly known as basic
integrated operating system
 BIOS was first invented by GARY KILDALL in 1976
 BIOS refers to the software code run by a computer when first powered
 The primary function of the BIOS is to prepare the machine so other software
programs stored on various media(such as hard drives, floppies) can load,
execute, and assume control of the computer this process is known as booting up
 BIOS stored on PROM, EPROM, flash memory
 Common manufacturers are american megatrends, phoenix technologies
1.9.1 Booting Process
1. When you turn on the pc‘s power switch the internal power supply initializes itself
the power supply does not provide power to the rest of the pc immediately the power
first transmitted to BIOS battery.
2. The system reset command sent by the mother board‘s chipset causes the CPU to read
its first instruction from what is called the JUMP instruction, JUMP instruction contains
the physical address of the BIOS boot program on the ROM or PROM.
3. The CPU executes first instruction which copies the BIOS program into system
memory and starts the BIOS running.

4. The BIOS next performs the POST(power on self test) process, the POST verifies and
tests the hardware configuration stored in the BIOS configuration information if detect
any problem it sounds "beep codes".
5. If the POST finds no problem the boot process continuous, at this point the system
BIOS looks for the video adapter‘s BIOS and starts it and virtually all peripheral devices
on the PC have their own BIOS.
6. In this stage the monitor turns on and display the video adapter‘s information
followed by information about the system BIOS itself this include information on the
manufacturer and version of the BIOS.
7. Next the BIOS begins a series of tests on the system including the amount of memory
detected on the system, this test is usually displayed on the screen as run-up counter.
8. If the BIOS supports Plug and Play(PnP) technology, information on each PnP device
is displayed on the screen.
9. At the end of the test and configuration sequence, the BIOS should display a summary
data screen.
10. At last to start the operating system running, the BIOS must first find it, in most
cases, the BOOT sequence parameters will be set to look for the OS on first the floppy
disk, second hard disk drive, third CD-ROM drive
1.9.2 Types of Booting
Booting is classified into two types they are
 Cold booting
 Warm booting
Cold booting- the booting process done when the computer turns on after completely
shutting down the computer.
Warm booting- the booting process done when we restart the computer.
1.10 POWER-ON SELF-TEST (POST) SEQUENCE
 Power-on self-test (POST) is the common term for a computer's, router’s or
printer’s pre-boot sequence. The same basic sequence is present on all
computer architectures.
 It is the first step of the more general process called initial program load (IPL),
booting, or bootstrapping.
 The term POST has become popular in association with and as a result of the
proliferation of the PC.
 It can be used as a noun when referring to the code that controls the pre-boot
phase or when referring to the phase itself.
1. General internal workings
2. Fundamental structure
3. Error reporting
4. Original IBM POST error codes
5. POST AMI BIOS beep codes
6. Beeps and meanings
1. General internal workings
The principal duties of the main BIOS during POST are as follows:
 verify the integrity of the BIOS code itself
 determine the reason POST is being executed

 find, size, and verify system main memory
 discover, initialize, and catalog all system buses and devices
 pass control to other specialized BIOSes (if and when required)
 provide a user interface for systems configuration
 identify, organize, and select which devices are available for booting
 construct whatever system environment that is required by the target OS
2. Fundamental structure
The POST section, or POST code, is responsible for the tasks mentioned above,
and the environment POST constructs for the OS is known as the runtime code, the
runtime BIOS, or the runtime footprint.
3. Error reporting
The original IBM BIOS reported errors detected during POST by outputting a
number to a fixed I/O port address, 80. Using a logic analyzer or a dedicated POST card,
an interface card that shows port 80 output on a small display, a technician could
determine the origin of the problem.
4. Original IBM POST error codes
 1 short beep - Normal POST - system is OK
 2 short beeps - POST error - error code shown on screen
 No beep - Power supply or system board problem
 Continuous beep - Power supply, system board, or keyboard problem
 Repeating short beeps - Power supply or system board problem or keyboard
 1 long, 1 short beep - System board problem
 1 long, 2 short beeps - Display adapter problem (MDA, CGA)
 1 long, 3 short beeps - Enhanced Graphics Adapter (EGA)
5. POST AMI BIOS beep codes
 1 - Memory refresh timer error
 2 - Parity error in base memory (first 64 KiB block)
 3 - Base memory read/write test error
 4 - Mother board timer not operational
 5 - Processor error
 6 - 8042 Gate A20 test error (cannot switch to protected mode)
 7 - General exception error (processor exception interrupt error)
 8 - Display memory error (system video adapter)
 9 - AMI BIOS ROM checksum error
 10 - CMOS shutdown register read/write error
 11 - Cache memory test failed
6. Beeps and Meanings
Beeps Meaning
Steady, short beep Power supply may be bad
Long continuous beep
tone
Power supply bad or not plugged into motherboard correctly
Steady, long beeps Power supply bad
No beep Power supply bad, system not plugged in, or power not turned on

No beep
If everything seems to be functioning correctly there may be a problem with
the 'beeper' itself.
One long, two short
beeps
Video card failure
1.11 SYSTEM CONFIGURATION
 Open System Configuration by clicking the
 Start button -> clicking Control Panel -> clicking System and Security -> clicking
Administrative Tools -> System Configuration.
 If you're prompted for an administrator password or confirmation, type the
password or provide confirmation.
 In Windows 7, search for "system" or "system configuration" in the Start Menu
and click its shortcut.
 Another method that works in all operating systems is to type msconfig.exe in
the Run window, the Start Menu search box.
Figure 1.7: The Windows Startup Selection
 In the General tab, there are options available for how you want Windows to
start up:
 Normal startup - starts Windows as is, with ALL the installed startup items,
drivers and services.
 Diagnostic startup - this mode is similar to booting in Safe Mode. Safe Mode
runs only Windows services and drivers. This mode might run, on top of them,
networking services or important services from third-party applications such as
your antivirus, firewall or security suite.
 Selective startup - starts Windows with its basic services and drivers. Also, it
allows you to select other services and startup items you want to run, from the
Services and Startup tabs.

1.12 MEMORY
 Internal storage areas in the computer. The term memory identifies data storage
that comes in the form of chips, and the word storage is used for memory that
exists on tapes or disks.
 Moreover, the term memory is usually used as shorthand for physical memory,
which refers to the actual chips capable of holding data. Some computers also
use virtual memory, which expands physical memory onto a hard disk.
 There are several different types of memory:
1. RAM (Random-Access Memory)
2. ROM (Read-Only Memory)
 PROM (Programmable Read-Only Memory)
 EROM (Erasable Read-Only Memory)
1. RAM(Random-Access Memory)
 This is the same as main memory. When used by itself, the term RAM refers to
read and write memory; that is, you can both write data into RAM and read data
from RAM.
 This is in contrast to ROM, which permits you only to read data.
 Most RAM is volatile, which means that it requires a steady flow of electricity to
maintain its contents.
 As soon as the power is turned off, whatever data was in RAM is lost.
 RAM is the most common type of memory found in computers and other devices,
such as printers.
 There are two basic types of RAM:
 Dynamic RAM (DRAM)
 Static RAM (SRAM)
The two types differ in the technology they use to hold data, dynamic RAM being
the more common type. Dynamic RAM needs to be refreshed thousands of times per
second.
Static RAM does not need to be refreshed, which makes it faster; but it is also
more expensive than dynamic RAM. Both types of RAM are volatile, meaning that they
lose their contents when the power is turned off.
2. ROM (Read-Only Memory)
Computers almost always contain a small amount of read-only memory that
holds instructions for starting up the computer. Unlike RAM, ROM cannot be written to.
PROM (Programmable Read-Only Memory)
 A PROM is a memory chip on which you can store a program. But once the PROM
has been used, you cannot wipe it clean and use it to store something else. Like
ROMs, PROMs are non-volatile.
 Once a program has been written onto a PROM, it remains there forever. Unlike
RAM, PROMs retain their contents when the computer is turned off.
 The difference between a PROM and a ROM (read-only memory) is that a PROM
is manufactured as blank memory, whereas a ROM is programmed during the
manufacturing process. To write data onto a PROM chip, you need a special
device called a PROM programmer or PROM burner. The process of
programming a PROM is sometimes called burning the PROM.

EROM (Erasable Read-Only Memory)
 An EPROM (erasable programmable read-only memory) is a special type of
PROM that can be erased by exposing it to ultraviolet light. Once it is erased, it
can be reprogrammed. An EEPROM is similar to a PROM, but requires only
electricity to be erased.
 The ultraviolet light clears its contents, making it possible to reprogram the
memory. To write to and erase an EPROM, you need a special device called a
PROM programmer or PROM burner.
 An EPROM differs from a PROM in that a PROM can be written to only once and
cannot be erased. EPROMs are used widely in personal computers because they
enable the manufacturer to change the contents of the PROM before the
computer is actually shipped. This means that bugs can be removed and new
versions installed shortly before delivery.
 EEPROM is similar to flash memory (sometimes called flash EEPROM). The
principal difference is that EEPROM requires data to be written or erased one
byte at a time whereas flash memory allows data to be written or erased in
blocks. This makes flash memory faster.
1.13. MASS STORAGE
The earliest storage devices were punched paper cards, which were used as early
as 1804.
Modern mass storage devices include all types of disk drives and tape drives.
Mass storage is distinct from memory, which refers to temporary storage areas within
the computer. Unlike main memory, mass storage devices retain data even when the
computer is turned off.
The main types of mass storage are:
1. Floppy disks
2. Hard disks
3. Optical disks
 CD-ROM
 WORM
 Erasable
4. Tapes
5. Auxiliary storage
1. Floppy Disks
 Relatively slow and have a small capacity, but they are portable, inexpensive, and
universal.
 A soft magnetic disk. It is called floppy because it flops if you wave it.
 Unlike most hard disks, floppy disks (often called floppies or diskettes) are
portable, because you can remove them from a disk drive.
 Disk drives for floppy disks are called floppy drives.
 Floppy disks are slower to access than hard disks and have less storage capacity,
but they are much less expensive and most importantly, they are portable.
 Floppies come in three basic sizes:
 8-inch
 5¼-inch
 3½-inch

2. Hard Disks
 Very fast and with more capacity than floppy disks, but also more expensive.
Some hard disk systems are portable (removable cartridges), but most are not.
 It is a magnetic disk on which you can store computer data. The term hard is
used to distinguish it from a soft, or floppy, disk. Hard disks hold more data and
are faster than floppy disks. A hard disk, for example, can store anywhere from
10 to more than 100 gigabytes, whereas most floppies have a maximum storage
capacity of 1.4 megabytes.
3. Optical Disks
 Optical disks have very large storage capacity, but they are not as fast as hard
disks.
 In addition, the inexpensive optical disk drives are read-only. Read/write
varieties are expensive.
 A storage medium from which data is read and to which it is written by lasers.
 Optical disks can store much more data -- up to 6 gigabytes (6 billion bytes) --
than most portable magnetic media, such as floppies. There are three basic types
of optical disks:
CD-ROM: Like audio CDs, CD-ROMs come with data already encoded onto them. The
data is permanent and can be read any number of times, but CD-ROMs cannot be
modified.
WORM: Stands for write-once, read -many. With a WORM disk drive, you can write data
onto a WORM disk, but only once. After that, the WORM disk behaves just like a CD-
ROM.
Erasable: Optical disks that can be erased and loaded with new data, just like magnetic
disks. These are often referred to as EO (erasable optical) disks.
These three technologies are not compatible with one another; each requires a
different type of disk drive and disk. Even within one category, there are many
competing formats, although CD-ROMs are relatively standardized.
4. Tapes
Relatively inexpensive and can have very large storage capacities, but they do
not permit random access of data.
5. Auxiliary storage
It is measured in kilobytes (1,024 bytes), megabytes (1,024 kilobytes), gigabytes
(1,024 megabytes) and terabytes (1,024 gigabytes). Auxiliary is sometimes called Mass
storage.
1.14. I/O INTERFACE STANDARDS
1. Keyboard Interface
2. Printer Controller
3. CRT Controller
4. Floppy Disk Drive (FDD) Controller
5. Hard Disk Controller

1. Keyboard Interface
Key board is serial data.
 If we press a key, the keyboard send a scan code for the key by data.(bit by bit)
 SIPO(Serial-in Parallel-out) is used to convert serial bit into parallel bit to form 1
byte.
 After forming byte, it sends IRQ1 to CPU by interrupt controller.
 KB logic receives then scan by PPI(Programmable Peripheral Interface )
 Again the PPI converts the scan code into.
2. Printer Controller
 It is a parallel interface.
 Printer controller supports 2 modes
 Programmed mode Interrupt mode
 Recent form of PC is LPT(Line Print Terminal)1, LPT2, and LPT3.
3. CRT Controller
 CGA supports both text and graphics mode and display or color CRT monitor
 Video buffer acts as memory between CRT and CPU.
 CPU is used to store text and graphics in video buffer.
 CRT monitor retrieves the text and graphics
4. Floppy Disk Drive Controller
 FDC is connected to the SMPS bus and DMA controller.
 Data transfer is done in DMA mode.
 NEC 765 FDC is a programmable IC, is used to convert parallel to serial and serial
to parallel.
 The BIOS is used to supply the command to the FDC.
 FDC IC has a set of registers to store command.
5. Hard Disk Controller
 HDC takes data from sector buffer and converts into serial data.
 HDC adds clock pulses into the data and send MFM (Modified Frequency
Modulation) write data.
 During RD operation MFM contains data and pulses.
 HDC is used to separate data pulses and clock pulses. It should be converted into
parallel data and stored in sector buffer. finally it is stored in memory(DFA
mode)

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