Computing Theory guide

String (stings contain characters (text, numbers, symbols)#
Integer (a whole number without decimals you use integers for calculations)
Real e.g. floating point ( a number with decimals) , 6.786, 5.7.6
Boolean-yes/no or true/false
DATATYPES

A variable is something that can change and can only hold one bit of data at
one time, a variable must be a sign. For example a box is a good example of
a variable as you can change that amount of 1 thing or what that thing is.
VARIABLE

Parentheses
Module
Function
Operator
Concatenate
comment
Key words

Module=orange
Function=purple
Integer=black
Variable=black
String=green
Comment=red
colours

A function is a block of organisation, reusable code that is used to perform a
single, related action.
They are used for when you need to run the same piece of code multiple
times.
The parentheses ( )
Function e.g. print

A python file generally contains definitions of variables, functions and
classes.
module

+ - / * > <
If x>2
Print “x is higher than two!”
Else:
print “x is less that three.”
Operations

It is the correct computer science
term for joining two things
together
concatenate

Commenting is one thing that separate an average programmer from a good
programmer. If you type a hash (#) it will turn red and you type comments.
It will not be noticed by python and will be ignored.
commenting

1 Bit = 1 bit
4 bits = 1 nibble
8 bits = 1 byte
1024 bytes = 1 kilobyte
1024 kilobytes = 1megabyte
1024 megabytes = 1 gigabyte
1024 gigabytes = 1 terabyte

Binary digit
= BIT
What is a BIT

Computers cant work with our everyday denary.
Computers use ‘base -2’ number system which is ‘binary’ it : 0, 1
Computer numbers

Go into excel.
Type in an = sign
Then type in
Type in your number e.g. 0101001
Then close bracket
ENTER
Converting binary to denary

Go into excel.
Type in an = sign
Then type in
Type in your number e.g. 45
Close bracket
ENTER
Denary to binary

1 and 1 = 10
1 and 0 = 1
0 and 0 = 0
How can you add these two binary
numbers?
1001
+0101

Hexadecimal digits represent four binary digits (bits), and
the primary use of hexadecimal notation is a human-
friendly representation of binary-coded values in
computing and digital electronics.
One hexadecimal digit represents a nibble, which is half
of a byte (8 bits)
Hex, or hexadecimal, is a number system of base 16.This
number system is especially interesting because in our
casually used decimal system we have only 10 digits to
represent numbers.
As hex system has 16 digits, the extra needed 6 digits are
represented by the first 6 letters of English alphabet.
Hence, hex digits are:
0,1,2,3,4,5,6,7,8,9 and A, B, C, D, E, F.
HEXADECIMAL

So a single Hexadecimal digit can
show 16 different values instead of
the normal 10 like this:

Go into excel.
Type in an = sign
Type in a
Enter number e.g. 62
Close bracket
ENTER
CONVERTING DENARYTO
HEXADECIMAL

American Standard Code for Information Interchange.
is a code for representing English characters as numbers, with each letter
assigned a number from 0 to 127.
Most computers use ASCII codes to represent text, which makes it possible
to transfer data from one computer to another.
ASCII

http://www.ascii-code.com/
THE ASKII CODE

The standard ASCII character set uses just 7 bits for each character.
There are several larger character sets that use 8 bits, which gives them 128
additional characters.
The type of text you can use is limited to the standard Latin alphabet
This means that you cannot represent text in languages other than those
based solely on the standard Latin alphabet. Mostly just English.
Limitations

A Digital Logic Gate is an electronic device that
can process multiple bits at once and can make
logical decisions based on the different
combinations of the digital signals it receives.
DIGITAL LOGIC GATE

Two men approach the stile, can they
get through to the next field?
A man and a dog approach the stile,
can they both get through?
Two dogs approach the stile, can they
get through to the other field?
CONSIDORTHIS STILE…

Most logic gates have two inputs and one output
The inputs it can read are either 1s (high) or 0s (low)
There are seven basic logic gates: AND, OR, XOR, NOT, NAND, NOR, and
XNOR.
Rather than men and dogs, consider
binary…
Input A
= 1
Input B
= 0
Output C
= 0
Logic
Gate

These are the basic gates: AND, OR
AND NOT

The AND gate is so named because, if 0 is called "false" and 1 is called "true,"
the gate acts in the same way as the logical "and" operator in Python.
The output is "true" when both inputs are "true." Otherwise, the output is
"false."
AND gate
1 1 1
1 0 0
0 1 0
0 0 0

AND gate
If one is off the gate is not
true and will not be
activated

The OR gate gets its name from the fact that it behaves like the Python
command "or."
The output is "true" if either or both of the inputs are "true." If both inputs
are "false," then the output is "false.“
OR gate
X=A or B

OR gate
only 1 has to be activated for
the gate to be true, whatever
doorbell you press it will still
sound the same bell

The NOT gate, to differentiate it from other types of electronic inverter
devices, has only one input. It reverses the logic state.
NOT gate
X=Not A

Individual logic gates can be connected together to form logic circuit
diagrams, or larger logic gate functions.
Logic Circuit Diagrams

A B C D E F
0 0 0 0 1 1
0 0 1 0 0 0
0 1 0 0 1 1
1 1 1 1 0 1
1 0 1 0 0 0
1 0 0 0 1 1

Memory is the way the computer stores data
Data is stored in bits (binary digits)
Memory comes in the form of RAM or ROM
What is memory?

Main memory of the computer
Short-term computer memory
Stores data before it is saved to a file
Standard amount is 2GB
RAM is volatile
Important to save work as your going along so
you do not loose anything
It does NOT automatically save
RAM – Random Access Memory

Desktop computers can normally hold more than one slot of RAM

Long term memory
Secondary storage choice
Examples:
• CDs
• DVDs
ROM – Read Only Memory

A form of long term memory (ROM)
Can be written to and read from (a bit like RAM)
Much slower than RAM
Non volatile
Hard disk drives

A computer’s processor searches for instructions that
are stored in the RAM memory of the computer. If
those instructions are not stored in the RAM
memory, they will have to be transferred from the
hard disk to the RAM memory - the well-known
process of "loading" a program.
So, a greater amount of RAM memory means that
more instructions fit into that memory and,
therefore, bigger programs can be loaded at once.
How RAM affects a computer’s performance

Explain the difference between RAM and
ROM
Research what DRAM is and explain it
Write a definition of volatile and non-
volatile memory
Research and explain the term latency
Task – Update revision guide

ROM is memory that cannot be changed by a program or user. ROM retains
its memory even after the computer is turned off. For example, ROM stores
the instructions for the computer to start up when it is turned on again.
RAM
RAM is a fast temporary type of memory in which
programs, applications and data are stored. Here are some examples of
what's stored in RAM:
the operating system
applications
the graphical user interface (GUI)
If a computer loses power, all data stored in its RAM is lost.
Difference between RAM and ROM

Dynamic random-access memory (DRAM) is a type of random-access
memory that stores each bit of data in a separate capacitor within an joined
circuit.
DRAM

Volatile memory contrary to non-volatile memory, is computer
memory that requires power to maintain the stored information; it retains
its contents while powered, but when power is interrupted stored data is
immediately lost.
VOLATILE

Non-volatile memory, non volatile memory, NVM or non-volatile
storage is computer memory that can get back stored information even
when not powered. Examples of non-volatile memory include read-only
memory, flash memory, ferroelectric RAM (F-RAM), most types of
magnetic computer storage devices (e.g. hard disks, floppy disks,
and magnetic tape), optical discs, and early computer storage methods such
as paper tape and punched cards.
NON-VOLATILE

Latency is the time it takes for a computers
memory to respond after a request has been
made
E.g. imagine you are in your room and your
parents call you for dinner. The length of time it
takes you to respond is Latency
RAM has less latency than a hard drive because it
is Solid State (has no moving parts)
Latency

Sometimes a part of the hard drive can be used
to store RAM. This is known as Virtual Memory
It causes more latency in the system but
increases the size of a computers memory
By adding Virtual Memory you can increase the
size of memory on a computer e.g. a computer
with 1GB of actual memory + 1GB of virtual
memory has 2GBs of memory it an use.
Virtual memory

The CPU cannot fetch the data it needs directly
from the Hard Drive because the hard drive is at
least 50 times slower. Fast memory is therefore
required, the fastest type of memory is called
Cache.
CACHE MEMORY
CPU Cache RAM
Disk
Storage
Virtual
Memory

RAM is faster than a hard drive but often has a bit
rate of 128 or 256
Most CPUs now run at over 2GHz so RAM is still
too slow for a processor
So the Cache’s ‘memory controllers’ have to
predict what piece of data the CPU will need next.
They then collect it from RAM and add it to their
high speed memory.
This speeds up the systems performance
Why do we need Cache when we have
RAM?

Explain the benefit of virtual memory
What is a computers Cache memory used
for?
Update your revision Guides with these
exam questions

Answer this question in approx. 500 words
Some suggestions to help:
phase-change memory (PCM)
magnetoresistive RAM (MRAM)
resistive RAM (RRAM)
How are changes in memory leading to
innovative computer design?

The processing is carried out by a range of
hardware such as a graphics card, a sound card,
the operating system and the processor on the
motherboard known as the CPU. The CPU is
seen as the brain of the computer, translates all
computer commands from binary to text we can
understand. Also tells all of the computer
components what to do.
What is a computers CPU?
CPU = Central Processing unit

Often referred to as ‘the computer’s brain’
It’s purpose is to process data
It does this by performing functions such as
searching or sorting data and calculating and
decision making using the data.
Whenever you carry out a task on a computer the
processor carries out all the data-processing
associated with that task.
- e.g. design a spreadsheet, write an email, play a game or search
the internet
What is a computers CPU?

Having two processors is like carrying out two
jobs at once.
This is quicker and more efficient
Technology has continued to advance with the
invention of quad-core and even hexa-core
processors
Dual-Core Processors

BIOS stands for Basic Input/Output
System. This is the first place the processor
goes to when a computer is switched on.
This is accessed even before the Operating
System is loaded. It is used to access initial
instructions for booting a computer.
BIOS

The CPU undertakes the instructions it receives from programs in what is
called a cycle
If we go back to the making tea and toast example: if you worked faster
you could produce the breakfast quicker, the addition of a faster kettle
would help even more.
Not only does the CPU have multiple cores but it also has a speed. This
speed is measured in how many cycles it can perform per second.
The name given to one cycle per second is a hertz.
How does a CPU work?

A CPU that processes one million cycles per
second is said to have a speed of one megahertz
(MHz) and one billion cycles per second would
be one gigahertz (GHz).
The faster the processor, the more processes it
can complete but that results in the processor
getting hotter and requiring more power.
How does a CPU work?

A computer accesses 3 different levels of cache:
Level 1- is always build into the CPU.
Level 2 - is built in separately on an external microchip stored on a motherboard.
However, modern CPUs have the L2 cache built in. The faster the CPU, the larger
the L2 cache needs to be. There is no fixed size for L2 cache but a standard
amount at the moment is 512KB, it can be as high as 2MB.
Level 3 – is the name given to the extra cache which is built into motherboards
between the processor and the main memory.
The size of Cache can affect
performance

Cache predicts what data will be needed next
The larger the cache the greater the amount of
potentially needed data, therefore, the faster the
computer
The size of Cache can affect
performance

The same concept of cache is used on the
internet.
When you visit a website the details of it are
‘cached’ on your computer.
The next time the web page is accessed, the
computer will load what has been cached on the
computer and then fetch any missing data from
the internet.
This makes loading a webpage much faster.
Other uses for cache

Pseudocode is a detailed yet readable description of what a computer
program or algorithm must do, expressed in a formally-styled natural
language rather than in a programming language.
Pseudocode is used when planning/designing code structures in a similar
way to flow charts
What is Pseudocode?

Variables
Variables are assigned in Pseudocode using arrows to represent =
E.g.
customer_name “Steven”
Customer_age 45

Pseudocode
The first thing we do when designing a program is to decide on a name for the
program.

Pseudocode
The first thing we do when designing a program is to decide on a name for the
program.
Let’s say we want to write a program to calculate interest, a good name for the
program would be CalculateInterest.
Note the use of CamelCase.

Pseudocode
So we start the program as:
PROGRAM CalculateInterest:
And in general it’s:
PROGRAM <ProgramName>:

Pseudocode
Our program will finish with the following:
END.
And in general it’s the same:
END.

Pseudocode
So the general structure of all programs is:
PROGRAM <ProgramName>:
<Do stuff>
END.

Pseudocode
When we write programs, we assume that the computer executes the program
starting at the beginning and working its way to the end.
This is a basic assumption of all algorithm design.
We call this SEQUENCE.

Pseudocode
In Pseudo code it looks like this:
Statement1;
Statement2;
Statement3;
Statement4;
Statement5;
Statement6;
Statement7;
Statement8;

Pseudocode
For example, for making a cup of tea:
Organise everything together;
Plug in kettle;
Put teabag in cup;
Put water into kettle;
Wait for kettle to boil;
Add water to cup;
Remove teabag with spoon/fork;
Add milk and/or sugar;
Serve;

Pseudocode
Or as a program:
PROGRAM MakeACupOfTea:
Plug in kettle;
Put teabag in cup;
Add water to cup;
Add milk and/or sugar;
Serve;
END.

Pseudocode
What if we want to make a choice, for example, do we want to add sugar or not to
the tea?
We call this SELECTION.

Pseudocode
So, we could state this as:
IF (sugar is required)
THEN add sugar;
ELSE don’t add sugar;
ENDIF;

Pseudocode
Or to check which number is biggest:
IF (A > B)
THEN Print A + “is bigger”;
ELSE Print B + “is bigger”;
ENDIF;

Pseudocode
Adding a selection statement in the program:
Plug in kettle;
Put teabag in cup;
Add water to cup;
Add milk;
THEN add sugar;
ELSE do nothing;
ENDIF;
Serve;
END.

Pseudocode
What if we need to tell the computer to keep doing something until some
condition occurs?
Let’s say we wish to indicate that the you need to keep filling the kettle with water
until it is full.
We need a loop, or ITERATION.
Loops are the same as iteration

Pseudocode
So, we could state this as:
WHILE (Kettle is not full)
DO keep filling kettle;
ENDWHILE;

Pseudocode
Or to print out the numbers 1 to 5:
A = 1;
WHILE(A > 5)
DO Print A;
A = A + 1;
ENDWHILE;

Pseudocode
Plug in kettle;
Put teabag in cup;
WHILE (Kettle is not full)
DO keep filling kettle;
ENDWHILE;
Add water to cup;
Add milk;
THEN add sugar;
ELSE do nothing;
ENDIF;
Serve;
END.

Re-cap –Theory 
Computer programming is about creating a set of instructions
to complete a specific task.
These are the fundamental building blocks of any program and
any true programing language will contain all of these.
1. Sequence – the ORDER in which statements are executed.
2. Selection – the use of logic to select the statement to be
executed.
3. Iteration – the use of repetition (to prevent typing the
same code out many times a loop structure is used. For
example While or For).

Flowchart
Turn A/C off
(process)
False
Turn on A/C
(process)
True
Temp > 25
(decision)
End
Start
Temp
(input)
The flow chart is
made up of an
input, decision and
the alternative
processes or
actions.
You cannot
have to yes’
or two no’s,
(true, false)
with a
diamond

start
Can you
see in the
dark?
no
yes
stop
Is the lamp
plugged in
yes
no
Plug in
lamp
Is the
bulb
Brocken
Fix bulb

• In general, an array is a number of items arranged in some
specified way - for example, in a list or in a three-dimensional
table.
• An array is a data structure that contains a group of elements.
Typically these elements are all of the same data type, such as an
integer or string.
• Arrays are commonly used in computer programs to organize
data so that a related set of values can be easily sorted or
searched.
• This is like a variable but you can store more than one item.
• Arrays in python are called lists
Definition of Arrays

Arrays in Python
• The name given to Arrays in Python is Lists or Tuples
• In other programming languages Lists and Arrays are separate things
• Tuples are the same as Lists but they are Immutable
• Lists can be changed tuples can never be changed.

Creating Lists in Python
• A list is assigned in the same way as a variable but the square brackets are used to
hold the values that need to be stored.
• Commas separate each item in the list
Name_of_list = [“item1”, “item2”, “item3”, “item4”]
• Brackets show where the list starts and finishes.

Printing a list using an index
Think of the index of a book – what is its purpose?
In programming the index of a list is the location of an item in a list which is denoted
using a number
Items in a list are given an index number starting from 0
countries = [“France”, “Italy”, “Spain”, “Belgium”]

Adding to a list
A list can be added to by using the append method of the list object
countries.append(“Great Britain”)
Note: square brackets are used to assign the list but parenthesis are used to call a
method within the list object e.g. append

Removing from a list
An item in a list can be removed in the same way as append, however, the
method ‘remove’ is used:
countries.remove(“France”)
France will now be removed when you print the list

Lists must be declared at the beginning of a programme
Empty list =

Operating Systems and Utility
Programs
Explain the need for the following functions of an operating
system:
• user interface - UI
• memory management
• peripheral management page 2 on PM
• multi-tasking
• security
Describe the purpose and use of common utility programs for:
• computer security
• system maintenance

USER INTERFACE
• a user interface (UI) brings structure to the
interaction between a user and the computer
• Most leading computer companies have invested
time and money in developing graphical user
interfaces (GUI)
• Windows 8 and Apple’sOSx are examples of
current operating systems which contain GUIs
• It's important to remember that the user interface
is a program or set of programs that sits as a layer
above the operating system itself

Memory Management
• Memory management is the
functionality of an operating system
which handles or manages primary
memory
• The memory management function
keeps track of the status of each
memory location, either allocated or
free
• It checks how much memory is to be
allocated to processes. It decides
which process will get memory at
what time.
• It tracks when memory is freed or
unallocated and updates the status.

Peripheral management
What are peripherals?
“able to be attached to and used with a
computer, though not an integral part of it.”

Peripheral management
Device management controls
peripheral devices by sending
them commands in their own
proprietary language.
The software routine that
knows how to deal with each
device is called a "driver," and
the OS requires drivers for the
peripherals attached to the
computer.
When a new peripheral is
added, that device's driver is
installed into the operating
system

Multi-tasking
Examples of
operating
systems

Security
If a computer program is run by unauthorized user then he/she may
cause severe damage to computer or data stored in it. So a computer
system must be protected against unauthorized access, malicious
access to system memory, viruses, worms etc.
These are some of the areas a computers operating system can
handle:
Unauthorised user – username/password to login and confirm your
identity
ProgramThreats – Torjan horse, trap door, logic bomb, virus
SystemThreats - System threats refers to the misuse of system
services and network connections to put user in trouble.

Trojan Horse - Such programs steal user login credentials and stores them
to send to malicious user who can later on login to computer and can access
system resources.
Trap Door Trap doors, also referred to as backdoors, are bits of code
embedded in programs by the programmer(s) to quickly gain access at a
later time, often during the testing or debugging phase. If an unscrupulous
programmer purposely leaves this code in or simply forgets to remove it, a
potential security hole is introduced. Hackers often plant a backdoor on
previously compromised systems to gain later access.
Logic Bombs are small programs or sections of a program triggered by
some event such as a certain date or time, a certain percentage of disk
space filled, the removal of a file, and so on. For example, a programmer
could establish a logic bomb to delete critical sections of code if she is
terminated from the company
Virus - Virus as name suggest can replicate themselves on computer
system. They are highly dangerous and can modify/delete user files, crash
systems.

Utility Software
Computer security
Disk organisation
continued
System maintenance

Computer security
Research to find the answer – What do these three
types of software do?
antivirus
firewall
Spyware protection

Disk Organisation
Formatting – Before a hard disk can be used, it needs
to be prepared so that files and applications can be
added to it.
Disk formatting is the initial part of the process in
preparing a hard disk drive for its first use. As part of
the disk formatting process ‘partitions’ are created.
These are know to us as ‘drives’ and are assigned
letters to represent them e.g. C: , D: , E:
File Transfer Windows Explorer is an example of a
graphical utility software that enables you to view
files, copy them and delete them.

Disk Organisation continued…
Defragmentation –You may notice after a
while that your computer begins to run more
slowly. There could be many reasons for this
but one of the key reasons is that many of
your files and folders become fragmented or
scattered over time.
It is therefore a good idea to ‘defrag’ your
hard disk.
All computers have a defragmenter utility
pre-installed which will reorganise the hard
drive by putting pieces of related data back
together so that files are stored next to each
other. This often frees up more space on the
hard disk.

System maintenance
System information – The operating stores information
about hardware such as make, clock speed, no. of
processors etc.
System clean-up tools – A good disk clean-up tool will
remove temporary files, old shortcuts and old installation
files.
Automatic Updating – the software that you used,
whether it be games or office applications, are constantly
being redeveloped by the authors of the software. Most
software is updated automatically using the operating
system which checks the internet for any changes to
software versions and automatically downloads and
installs the update if it exists.

Antivirus or anti-virus software (often abbreviated as AV), sometimes
known as anti-malware software, is computer software used to prevent,
detect and remove malicious software. Antivirus software was originally
developed to detect and remove computer viruses, hence the name.
A firewall is a program or device that acts as a barrier to keep destructive
elements out of a network or specific computer. Firewalls are configured (in
hardware, software, or both) with specific criteria to block or prevent
unauthorized access to a network.
Anti-spyware (AS) software are programs that attempt to remove or block
spyware – malicious programs that try and collect small pieces of
information without the computer owner’s knowledge and may change
settings, make your computer run slowly or affect other programs installed.
Antivirus, firewall, anti-spyware

values= [A, 2, 3, 4, 5, 6, 7, 8, 9, J, Q, K]
Suit= [“hearts”, “diamonds”, “spades”, “clubs”]
VALUES AND SUITS FOR C.A

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