ICT - Lecture Notes 3.pdf

LECTURE NOTES 3
Information & Communications Technology

The Computer
Processing System

Overview
Instructions and data needed for immediate processing are
usually placed in memory. A computer has two types of
memory: volatile and non-volatile.
They are used for the short- and long-term retention of data,
respectively. RAM is an example of volatile memory—the
primary memory. ROM is an example of non-volatile memory.
Random-access memory (RAM) temporarily holds data, the
operating system (instructions that control the computer’s
operation), and application software (instructions that
manipulate data).

A component of an operating system resides in RAM only while
the computer is turned on. The application software remains in
RAM only while it is being used.
Unless there is enough RAM to hold the application software for
more than one program at a time, when new application
software is retrieved from secondary storage, it is loaded into
RAM, replacing the application software that was previously
residing there.
Most current computers have enough RAM to run several
applications simultaneously, a process called multitasking.

Data and instructions about to be processed are in RAM, as are
the output of the processes. The data and instructions in
working memory (i.e., RAM) are electronically stored, as
opposed to being magnetically or optically stored.
Thus, when the computer is shut down, all instructions and
data in RAM are lost because the flow of electricity ceases.
In RAM, any address can be randomly accessed at any time;
hence the name random-access memory. Cache memory is
very fast RAM. It is used to store frequently used instructions.
Computers that have cache memory process data faster than
computers without it because data travels faster from cache to
the CPU than from RAM to the CPU.

CENTRAL PROCESSING UNIT (CPU). The CPU consists of —
an arithmetic/logic unit (ALU) and a control unit; as the major
components.
The ALU performs arithmetic calculations (addition, subtraction,
multiplication, and division), comparisons (greater than, less
than, and equal to), and logical operations (and, or, and not).
All transformations of "meaningless" data into "useful"
information ultimately are the result of these three functions.
The control unit retrieves instructions and data from primary
memory and determines which instructions the ALU and other
CPU Components will carry out.

The CPU also contains registers, which are temporary holding
areas where data resides as it is being processed.
Note the multiple layers of information retention required by the
computer:
Storage (where information is kept long-term),
Memory (where information is kept short-term), and
Registers (where information is kept during processing).
Memory – Although memory is technically any form of
electronic storage, it is used most often to identify fast,
temporary forms of storage.
Accessing the hard drive for information takes time. When the
information is kept in memory, the CPU can access it much
more quickly.

The Central Processing Unit Components
and Processes

The Computer System Architecture

CPU Components and Processes
The Central Processing Unit (CPU), sometimes referred to as
the Processor, performs the system’s calculation and
processing activities.
The CPU is a microscopic circuitry that serves as the main
information processor in a computer. It is made of
semiconducting material, usually silicon, with millions of
electrical components on its surface.
On a higher level, the CPU is actually a number of
interconnected processing units that are each responsible for
one aspect of the CPU’s function.

The CPU is composed of five basic components:
- RAM (Random Access Memory),
- Registers,
- Buses,
- the ALU (Arithmetic and Logic Unit),
- the Control Unit.
Each of these components are pictured in the diagram
below. The diagram shows a top view of a simple CPU.

To better understand the basic components of the CPU, we will
consider each one in detail.
RAM: Random Access Memory. RAM and Memory are
normally used interchangeably. RAM holds the program
instructions and the data that is required for processing. It is
located external to CPU and is much slower than Registers.
Generally, data has to be loaded into a CPU Register from
RAM before the CPU can process it. RAM is temporary; that
is, its contents can be changed at any time and it is erased
when power to the computer is turned off.
` Cache is also a local memory that is used to temporarily
store the information available in the RAM for faster access
during the operations.

Registers: these components are special memory locations
that can be accessed very fast. They are small local memory
locations inside the chip that temporarily stores the
instructions (and / or data) which is currently being worked on
by the processing units.
There are many components, with different functions, that
are generally referred to as Registers. The main types
include Accumulator register (ACC), status register,
instruction register, ordinal / program counter, and buffer
register.
Buses: these components are the information highway for the
CPU. Buses are bundles of tiny wires that carry data
between components. The three most important buses are
the address, the data, and the control buses.

Each bus contains several wires that allow for the parallel
transmission of information between various CPU hardware
components.
ALU: this component is the number cruncher of the CPU.
Arithmetic and Logic Unit (ALU) perform:
basic arithmetic (such as add, subtract, multiply and divide),
comparisons (greater than, less than, and equal to), and
logical operations (and, or, and not) and
a host of other calculations on binary numbers.
The Arithmetic and Logic Unit is composed of complex
circuitry.

Control Unit: this component is responsible for directing the
flow of instructions and data within the CPU by issuing control
signals to different components. It interprets the instructions.
The Control Unit is actually built of many other selection
circuits such as decoders and multiplexors.
In the diagram above, the Decoder and the Multiplexor are
part of the Control Unit.

Fetch-Decode-Execute Cycle
In order for a CPU to accomplish meaningful work, it must have
two inputs: instructions and data.
Instructions tell the CPU what actions need to be performed on
the data. Data is that which must be handled according to the
instructions. Both inputs to the CPU are stored in RAM.
CPU functions by following a cycle of fetching an instruction,
decoding it, and executing it. This process is known as the
fetch-decode-execute cycle.
The cycle begins when an instruction is transferred from RAM
to the Instruction Register (IR) along the data bus. In the IR, the
unique bit patterns that make up the machine-language are
extracted and sent to the Decoder.

The Decoder is responsible for the second step of the cycle,
which is, recognizing which operation the bit pattern represents
and activating the correct circuitry to perform the operation.
Sometimes this involves reading data from memory, storing
data in memory, or activating the ALU to perform a
mathematical operation.
Once the operation is performed, the cycle begins again with
the next instruction.
The CPU always knows where to find the next instruction
because the Program Counter holds the address of the current
instruction.
Each time an instruction is completed, the program counter is
advanced by one memory location.

ROM – Read-Only Memory
ROM is permanent (non-volatile) and is used to store
the initial boot up instructions of the machine.
Read Only Memory (ROM) is computer memory that
can permanently store data and applications within it.
Unlike RAM, when a computer is powered down, the
contents of the ROM are not lost.

BIOS – Basic Input / Output System
It is also normally referred to as System BIOS, ROM BIOS, PC
BIOS or Computer BIOS. It is software stored on a small
memory chip on the motherboard.
BIOS is a set of routines stored in memory that enable a
computer to start the operating system. BIOS instructs the
computer on how to perform a number of basic functions during
the booting process.
It is also used to identify, configure and communicate with the
various devices in the system, such as disk drives, floppy drive,
memory, CPU, keyboard, monitor, printer, communication ports,
etc, on the computer system and ensure that they are all
functioning well.

Here are some common things you can do in most BIOS
systems:
- Change the Boot Order
- Load BIOS Setup Defaults
- Remove a BIOS Password
- Create a BIOS Password
- Change the Date and Time
- Change Floppy Drive Settings
- Change Hard Drive Settings
- Change CD/DVD/BD Drive Settings
- Change CPU Settings
- Change Memory Settings
- Change System Voltages
- Change Power-on Settings
- View Fan Speeds
- View System Voltages

Difference between Virtual Memory and Physical RAM?
Physical RAM – (sometimes referred to as Main Memory or
just RAM) is directly addressable and accessible on demand by
any program.
Virtual Memory – an area of a hard disk set aside for
temporary storage and used as an extended memory area, but
not directly addressable / accessible.
The Computer’s total system memory is made up of physical
memory, in the form of random access memory (RAM), and
virtual memory.
Virtual Memory is a portion on the secondary storage which
however is volatile; ie does not retains its content when the
computer is turned off.

Difference between Virtual Memory and Physical RAM?
Virtual memory is only used when the computer runs out of
physical memory. Some instructions and / or data will be moved
out of physical memory to virtual memory to create space in the
physical memory for other new processes.
Because this moving of data happens automatically, you don't
even know it is happening, and it makes your computer feel like
is has unlimited physical memory.
Since hard disk space is so much cheaper than RAM chips, it
also has a nice economic benefit. However, if the system has to
rely too heavily on virtual memory, there will be a significant
drop in performance. This is because the read/write speed of a
hard drive is much slower than RAM, and the technology of a
hard drive is not geared toward accessing small pieces of data
at a time.

RAM vs Cache Memory
Memory of a computer is organized into a hierarchy and they
are organized in consideration to the time taken to access
them, the cost and the capacity.
RAM and cache memory are two members in this memory
hierarchy.
RAM (Random Access Memory) is the primary memory used
in a computer. Its individual memory cells can be accessed in
any sequence, and therefore it is called the random access
memory.
Cache memory is a special memory used by the CPU (Central
Processing Unit) of a computer for the purpose of decreasing
the average time required to access memory.

Cache Memory
Cache memory is a special memory used by the CPU for the
purpose of decreasing the average time taken for memory
accesses.
Cache memory is relatively a smaller and also a faster memory,
which stores most frequently accessed data of the main
memory.
When there is request for a memory read, cache memory is
checked to see whether that data exists in cache memory.
If that data is in the cache memory, then there is no need to
access the main memory (which takes longer time to be
accessed), therefore making the average memory access time
smaller.

Typically, there are separate caches for data and instructions.
Data cache is typically set up in a hierarchy of cache levels
(sometimes called multilevel caches).
L1 (Level 1) and L2 (Level 2) are the top most caches in this
hierarchy of caches.
L1 is the closest cache to the main memory and is the cache
that is checked first.
L2 cache is the next in line and is the second closest to main
memory.
L1 and L2 vary in access speeds, location, size and cost.

Difference between RAM and Cache Memory?
In the memory hierarchy, cache memory is the closer
memory to the CPU when compared with the RAM.
Cache memory is much faster and also expensive
when compared with the RAM.
However, the capacity of the RAM memory is larger
than the capacity of the cache memory.
Further, the cache memory is also organized as a
hierarchy as L1, L2 and L3 caches that differ in speed,
cost and capacity.

L1 vs L2 Cache
L1 Cache (also known as primary cache or Level 1 cache) is
the top most cache in the hierarchy of cache levels of a CPU.
L1 is the fastest cache in the hierarchy. It has a smaller size and
a smaller delay (zero wait-state).
L2 Cache (also known as secondary cache or Level 2 cache) is
the cache that is next to L1 in the cache hierarchy.
L2 is usually accessed only if the data looking for is not found in
L1.
L2 is a bigger memory in comparison to L1. Access speed is
also less than that of L1.

Representing Data
We have all seen computers do seemingly miraculous things
with all kinds of sounds, pictures, graphics, numbers, and text. It
seems that we can build a replica of parts of our world inside
the computer.
You might think that this amazing machine is also amazingly
complicated - it really is not. In fact, all of the wonderful multi-
media that we see on modern computers is all constructed from
simple ON/OFF switches - millions of them - but really nothing
much more complicated than a switch.
The trick is to take all of the real-world sound, picture, number,
etc data that we want in the computer and convert it into the
kind of data that can be represented in switches, as shown in
the figure below:

Representing Data
Representing Real-World Data In The Computer

Representing Data
01100001,01100010,01100011
00000001,00000010,00000011
01001100010101000110100…
10001001010100000100111...
00110000001001101011001…
Discrete/digital and binary!
Text a,b,c
Number 1,2,3
Sound
Image
Video
What we see/hear Inside computers

Computers Are Electronic Machines
The computer uses electricity, not mechanical parts, for its data
processing and storage. Electricity is plentiful, moves very fast
through wires, and electrical parts fail much less frequently than
mechanical parts.
The computer does have some mechanical parts, like its disk
drive (which are often the sources for computer failures), but the
internal data processing and storage is electronic, which is fast
and reliable (as long as the computer is plugged in).
Electricity can flow through switches: if the switch is closed, the
electricity flows; if the switch is open, the electricity does not
flow. To process real-world data in the computer, we need a way
to represent the data in switches. Computers do this
representation using a binary coding system.

Binary Coding Vs Electricity Flow
States of a Bit
0
FALSE OFF LOW VOLTAGE
1
TRUE ON HIGH VOLTAGE

Binary and Switches
Binary is a mathematical number system: a way of
counting.
We have all learned to count using ten digits: 0-9. One
probable reason is that we have ten fingers to
represent numbers.
The computer has switches to represent data and
switches have only two states: ON and OFF.
Binary has two digits to do the counting: 0 and 1 - a
natural fit to the two states of a switch (0 = OFF, 1 =
ON).

Data Storage in the Computer
Bits and Bytes. One binary digit (0 or 1) is referred to
as a bit, which is short for binary digit. Thus, one bit
can be implemented by one switch, as shown in the
Figure below:

In the following table, we see that bits can be grouped
together into larger chunks to represent data.
0 1 bit
1 1 bit
0110 4 bits
01101011 8 bits

The fundamental / smallest unit of data storage in a computer is
called a bit or binary digit.
A bit is similar to a two-way switch. Just like a switch has two
states (off or on), a bit also has two states (0 or 1).
Often these two states represent the values false or true and
are implemented inside a computer by using a low voltage
value or a high voltage value.
Since bits provide the foundation for all data storage, it is not
surprising that the binary number system is very important to
computers.

The reason computers use the base-2 system is because it
makes it a lot easier to implement them with current electronic
technology.
Computer designers use eight bit chunks called bytes as the
basic unit of data. A byte is implemented with eight switches as
shown in Figure below.
We could still wire up and build computers that operate in base-
10, but they would be extremely expensive right now. On the
other hand, base-2 computers are relatively cheap.

Computer manufacturers express the capacity of memory and
storage in terms of the number of bytes it can hold. The number
of bytes can be expressed as kilobytes.
Kilo represents 2 to the tenth power, or 1024. Kilobyte is
abbreviated KB, or simply K. A kilobyte is 1024 bytes. Thus, the
memory of a 640K computer can store 640x1024, or 655,360
bytes.
Memory capacity may also be expressed in terms of megabytes
(1024x1024 bytes). One megabyte, abbreviated MB, means
roughly one million bytes. With storage devices,
manufacturers sometimes express memory amounts in terms of
gigabytes (abbreviated GB); a gigabyte is roughly one billion
bytes.

Computer memory, or RAM, in some computers might hold 6
GB, or roughly 6 billion bytes. Some computer hard disks hold
terabytes (e.g. 1 TB).
Bits are the building blocks for all information processing that
goes on in digital electronics and computers.
Bits actually represent the state of a transistor in the logic
circuits of a computer.
The number 1 (meaning on, yes, or true) is used to represent a
transistor with current flowing through it—essentially a closed
switch.
The number 0 (meaning off, no, or false) is used to represent a
transistor with no current flowing through it—an open switch.

All computer information processing can be understood in
terms of vast arrays of transistors (3.1 million transistors on
the Pentium chip) switching on and off, depending on the
bit value they have been assigned.
Bits are usually combined into larger units called bytes. A
byte is composed of eight bits.
The values that a byte can take, range between 00000000
(0 in decimal notation) and 11111111 (255 in decimal
notation).
This means that a byte can represent 2^8 (2 raised to the
eighth power) or 256 possible states (0-255).

Because a byte represents only a small amount of information,
amounts of computer memory and storage are usually given in
1,024 bytes = 1 Kilobyte,
1,048,576 bytes = 1,024 Kilobytes = 1 Megabyte
1,024 megabytes = 1 Gigabyte
1,024 gigabyte = 1 Terabyte
A terabyte is equal to 1,024 gigabytes and there are lots of
things bigger than one terabyte.
1,024 Terabytes = 1 Petabyte.
1,024 Petabytes = 1 Exabyte.
1,024 Exabytes = 1 Zettabyte.
1,024 Zettabyte = 1 Zottabyte.

A bit is a BInary digiT. So a bit is a zero or a one. Bits can be
implemented in computer hardware using switches.
If the switch is on then the bit is one and if the switch is off then
the bit is zero. A bit is limited to representing two values.
Since the alphabet contains more than two letters, a letter
cannot be represented by a bit.
A byte is a sequence of bits. Since the mid 1960's a byte has
been 8 bits in length. 01000001 is an example of a byte.

ASCII, the American Standard Code for Information
Interchange, is the code that is most commonly used today.
EBCDIC, Extended Binary Coded Decimal Interchange Code,
was used by IBM on its large mainframe computers in the past.
Since these codes are limited to 256 possible combinations,
certain character sets, such as Chinese, Arabic, Japanese,
Klingon and others, cannot be represented using these codes.
This problem is solved by using another code, Unicode, which
uses 2 bytes for each character. This extension allows 2^16
different symbols to be represented, a total of 65,536.
The use of Unicode gives more flexibility in the representation
of data. The drawback of using Unicode is that it takes twice as
much space to store the same number of characters.

A word is the number of bits that are manipulated as a unit by
the particular CPU of the computer.
The size of the words used by a computer’s central processing
unit (CPU) depends on the bit-processing ability of the CPU.
A 32-bit processor, for example, can use words that are up to
four bytes long (32 bits). Computers are often classified by the
number of bits they can process at one time.
Today most CPUs have a word size of 32 or 64 bits.
Data is fetched from memory to the processor in word size
chunks and manipulated by the ALU in word size chunks.
All other things being equal, (and they never are), larger word
size implies faster and more flexible processing.

Bits, Bytes and Words
In Summary:
- A Bit: (Short for binary digit) is the smallest unit of information
on a machine "computer". A single bit can hold only one of
two values: 0 or 1.
- A Byte: A sequence of adjacent bits, usually eight, that is used
to represent a character: 1 Byte = 8 Bits.
- A Word: It is a number of bits that can be manipulated as a
unit. The size of a word varies from one computer to another,
depending on the CPU. For computers with a 32-bit
processors, a word size is 32 bits (4 bytes). On some
computers with 64-bit processors, a word size is 64 bits (8
bytes).

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