Computer Organization
IB Computer Science Topic 2
Outline the four phases of the machine
instruction cycle. [8 marks]
Draw a block diagram showing the basic
components of the CPU and how they
interact with memory. [8 marks]
Machine Instruction Cycle Overview
Fetch
next instruction
read from
memory and
loaded into CIR,
program counter
incremented.
Decode
Store
Instruction decoder
translates instruction
for ALU.
Results of calculations
written to memory.
Execute
Instruction is
executed, ALU
performs required
actions.
Components of the CPU
 You need to be able to reproduce this diagram and discuss
everything in it
Control
Unit
Address Bus
Registers
Arithmetic
Logic
Unit
MAR
MDR
Memory
[01]
[02]
[..]
[FF]
Data Bus
The Fetch Phase

The PC (Program Counter) stores the address of the next instruction in memory.

The contents of the PC are copied to the MAR and the PC is incremented.

The address is put onto the address bus.

The data at that address (i.e., the next machine code instruction) is read into the MDR.

The data returns on the data bus.
The Decode Phase

The instruction carried by the MDR is copied to the IR (Instruction Register)

The instruction is translated into a format that the ALU can understand.
The Execute Phase

The ALU carries out the operation, e.g. ADD, MUL, CMP, JMP, STO, etc.
The Store Phase (optional)

This is almost exactly the same as the Fetch phase, except that the control bus carries a
"write" signal instead of a "read" signal. Therefore, instead of reading the data stored at
the MAR address into the MDR, the data stored in the MDR is written to the location at
the MAR address.
Registers
 Registers are the smallest and fastest unit of memory in a computer
system, and are used to store information that the CPU is working
on at that exact moment.
 Registers you need to know about:
 Program Counter: Stores the address of the next instruction
 Memory Address Register: Stores the address (i.e., the memory
address) that is to be read or written.
 Memory Data Register: Stores the data that has been read (from
memory) or is to be written (to memory).
 Instruction Register (aka Current Instruction Register): Stores the
current (machine code) instruction being processed.
 Accumulator: A set of general purpose registers used for storing
temporary intermediate results of the CPU's calculations.
Assembly Language
mov ax, [1000h]
(from http://simple.wikipedia.org/wiki/Assembly_language)
// read (load) a 2-byte object from address 4096
// (0x1000 in hexadecimal) into a 16-bit register
// called 'ax':
mov bx, ax
// move (copy) the value in register ax into register bx
sub bx, 100
// subtract 100 from bx, store the result back into bx
jge continue
// if the result of the previous calculation is
// greater than or equal to zero, then
// jump to label ‘continue ’
mov ax, 100
// put the value 100 into register ax
continue:
mul ax, ax
// multiply the value in ax times the value in ax,
// and store the result back in ax
Questions
 Draw a block diagram showing the basic components of the CPU and
how they interact with memory. [8 marks]
 Explain why the address bus is unidirectional and the data bus is
bidirectional. [4 marks]
 Outline the four phases of the machine instruction cycle. [8 marks]
 Compare the roles of the Memory Address Register and the Memory
Data Register. [4 marks]
Operating Systems
 Operating System: Software (a collection of programs) that controls
the execution of programs. May provide services such as resource
allocation, scheduling, input/output control, and data management.
 How many operating systems can you name?
 Main functions of an operating system:
 Input/Output (IO) control
 File management
 Software/hardware interface
 Memory management
 User interface
 Scheduling
 Program execution control
 Security
Operating Systems and Security
 User management: login with password
 System of privileges, determining which user or process can do
what (e.g., delete files, shut down the system, etc.).
 Permissions for files and directories (e.g., read-only, etc.)
 Memory management: keeping one process’s (or one user's)
memory space separate from another’s
 Built-in firewall (a program, or set of programs, that protects the
resources of a private network from users of other networks.)
Types of Memory
 Primary Memory
 Registers: Smallest, fastest memory, used to hold data that the CPU is
currently working on.
 Cache: Small, fast memory, used to hold very frequently used data.
 RAM: Random Access Memory. Holds data and instructions from running
programs.
 ROM: Non-volatile memory, used to hold the operating system's
bootstrap loader (small start-up program) or BIOS (Basic Input/Output
System).
 Secondary Memory
 Disk storage
 Magnetic tape
 USB flash drives
Operating Systems and Memory Management
 Multi-tasking environment: keeping the memory
space of each process safe from other running
processes (see Security)
 Multi-user environment: keeping the memory
space (primary and secondary) of each user safe
from other users (see Security)
 Operating systems allocate and deallocate
memory for each process
 Paging: Dividing virtual memory up into equalsized blocks (pages)
Paging allows OSs to allocate non-contiguous
chunks of memory to the same process, thus
reducing fragmentation problems
 Virtual memory: The use of secondary memory
as if it were primary memory.
By creating virtual memory, the OS makes it
easier for programs to reference memory
because they don’t need to worry about the
complications of the underlying physical structure
of memory and disk (another example of
abstraction)
Applications
(memorize this slide)
An application is a computer program that allows a user to perform some
useful activity. Examples are:
 Word processor: Text document management
 Spreadsheet: Mathematical and financial calculations
 Database: Organisation of information
 Email: Electronic mail
 Web browser: Viewing HTML pages
 Graphics processing: Editing, cropping and enhancing images
 Computer-Aided Design (CAD): Creating and editing designs for
engineering or manufacturing
 Note that you are not allowed to use proprietary names in your exam.
So you say "spreadsheet software" and not "MS Excel", etc.
Common Features of Applications
The IB Guide refers to features:
"Including toolbars, menus, dialogue boxes, graphical user interface GUI)
components.
Students should understand that some features are provided by the application
software and some by the operating system."
Operating systems make services available to applications:
 I/O (input/output) operations
 File system manipulation (reading/writing files)
 Frameworks of GUI components
 Program execution
 Hardware interface
 Error detection
All applications need these services, so it makes sense to centralise their provision.
They make the development of software simpler because potentially complex
interaction with hardware is avoided.
Questions
 Outline the main functions of an operating system. [4 marks]
 Explain the advantages of virtual memory. [4 marks]
 In what ways can an operating system improve information security
in a computer? [4 marks]
Binary Representation
Definitions
 Bit: The smallest amount of data that can be represented. Short for
"binary digit". Represented as either a 0 or 1.
 Byte: 8 bits
 Denary/Decimal: The base 10 counting system. The normal placevalue counting system in which a new column is created when a
power of 10 is reached.
 Hexadecimal: The base 16 counting system. Requires six new
symbols in addition to 0-9, which are A-F. Since 24 = 16, one Hex
digit can be used to represent 4 bits, and two Hex digits can be used
to represent a byte.
Data Representation
 The Guide mentions: Strings, integers, characters, colours.
 The most basic point is that more bits allows more different values to be
stored.
0
1
00
01
10
11
000
001
010
011
100
101
110
111
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
00000
00001
00010
00011
00100
00101
00110
00111
01000
01001
01010
01011
01100
01101
01110
01111
10000
10001
10010
10011
10100
10101
10110
10111
11000
11001
11010
11011
11100
11101
11110
11111
No bits No diff
values
1
2 = 21
2
4 = 22
3
8 = 23
4
16 = 24
5
32 = 25
n
= 2n
More bits means:
• More different values
• Data is larger
Raster images
 Also called bitmaps, raster images use a
grid of points of colour
 But how many different colours can one
point be?
 Colour depth is the number of bits per
pixel (generally "bit depth")
 (The number of pixels makes a difference
too.)
 Obvious tradeoff between picture quality
vs file size
Questions
An image file is made up of a grid of 200 x 300 pixels and has a colour depth of 8
bits.

How many different colours can each pixel represent? [2 marks]

What is the size of the file? [2 marks]

A friend argues that if you increase the colour depth of this file, you will improve its
quality. To what extent is he correct? [4 marks]
RGB (see http://www.rapidtables.com/web/color/RGB_Color.htm )

What does RGB stand for?

How many colours can RGB represent?

What are the RGB values for:
 white?
 black?

What colour is:
 888888?
 00FFFF?
Why is ASCII vs Unicode relevant to this topic? Explain one advantage and one
disadvantage of Unicode over ASCII.
(see http://stackoverflow.com/questions/2241348/what-is-unicode-utf-8-utf-16 )
Logic Gates
The Guide mentions
 AND, OR, NOT, NAND, NOR, XOR
 Which of these are binary operators and which are unary operators?
 Construct truth tables for each.
 Look at the logic diagram below. Construct a truth table for X given the
inputs A, B and C.
A
AND
B
NOT
XOR
C
X
Redraw each of these diagrams using
fewer gates
A
NOT
AND
B
X
NOT
NOT
A
AND
OR
B
AND
NOT
X