1. IB Computer Science Topic 2.1.1
By: Mr. A. Lawson
IB Computer Science Topic 2.1.1

2. IB Computer Science Topic 2.1.1
Outline the architecture of a central processing unit (CPU) and the functions of the arithmetic logic unit (ALU) and the control unit (CU) and the registers within the CPU.
Key Terms:
CPU = Central Processing Unit
ALU = Arithmetic Logic Unit
CU = Control Unit
Registers = small temporary storage spaces inside the CPU

3. IB Computer Science Topic 2.1.1
Central Processing Unit
Video: How are CPUs made
CPU definition:
The key component of a computer system, which contains the circuitry necessary to interpret and execute programing instructions for the computer device.
CPU

4. IB Computer Science Topic 2.1.1
Examples of ALU’s & CU’s
Function of the ALU
Part of the CPU that does all the arithmetic (+/-) and logical (AND/OR) calculations
Sometimes an ALU is referred to a “core”, hence computers with dual core technology have two ALU’s to process two calculations simultaneously.

5. IB Computer Science Topic 2.1.1
Two functions of the CU:
It handles the loading of new commands into the CPU and the decoding of these commands.
Also, it directs the data flow and the operation of the ALU.
Computer Architecture:
In 1945, a scientist called Von Neumann suggested that a computer system be slip into a processor module (where the instructions are calculated) and a memory module (where instructions and data are stored).
We still use this model today: CPU and Primary Memory

6. IB Computer Science Topic 2.1.1
Special relationship between CPU & RAM
CPU needs it’s own “memory”
Registers: they are small, very fast circuits that store intermediate values from calculations or instructions inside the CPU.
There are many small units, but the four most important ones are the following:
MAR
MDR
Accumulator
Program Counter

7. IB Computer Science Topic 2.1.1
Program Counter & Accumulator
Program Counter holds the memory address of the next instruction.
Accumulator holds the intermediate results of the currently running instructions.
MAR (Memory Address Register)
MAR is connected to the address bus.
MAR contains a memory address.
MAR’s sole function contains the RAM address of the instruction the CPU wants next.

8. IB Computer Science Topic 2.1.1
MDR ( Memory Data Register)
MDR is connected to the data bus.
MDR holds data that will be written to the RAM or that was read from RAM.
Relationship between MAR & MDR: The MAR gives the address the data of the MDR will be read from or written to.

9. IB Computer Science Topic 2.1.2
Describe the Primary Memory.

10. IB Computer Science Topic 2.1.2
RAM = Random Access Memory
Contains the data and instructions the computer has loaded since starting up and everything the user has opened or loaded.
Is volatile = loses its contents if the power is lost
Has a special link to the CPU
ROM = Read Only Memory
Originally its contents were static (“read only”) and could not be changed – not true any more (“flash upgrades”).
Non-volatile = does not lose its contents if the power is lost
Stores the BIOS (Basic Input Output System) – a small program that allows the computer to know what to do to find the operating system to “boot” the computer after power is restored.
RAM
ROM
Volatile
Non-Volatile
Contains user’s programs and data that has been loaded since “booting up”
Contains the BIOS
Usually measured in Gigabytes (common capacities are 1GB, 2GB or 4GB)
Usually measured in Kilobytes (much smaller than RAM)

11. IB Computer Science Topic 2.1.2
Connections between RAM & CPU
Control Bus (bi directional)
Address Bus (uni directional-from CPU to RAM only)
Data Bus (bi directional)
Bus = a set of wires that connect two components in a computer system
Explain the use of cache memory.
IB Computer Science Topic 2.1.3

12. IB Computer Science Topic 2.1.3
Memory
Primary
Secondary
RAM
ROM
Cache
HDDs
Flash
A type of small, high-speed memory inside the CPU used to hold frequently used data, so that the CPU needs to access the much slower RAM less frequently
Definition: Cache

13. IB Computer Science Topic 2.1.3
Explain the machine instruction cycle. Simplified model:

14. Video's Tutorials to explain
IB Computer Science Topic 2.1.4
Simple Model #2
Video's Tutorials to explain
Video 1: Video 2:
IB Computer Science Topic 2.1.5
Identify the need for persistent storage

15. Types of storage Why do we need “storage”?
IB Computer Science Topic 2.1.5
Why do we need “storage”?
Types of storage
Processing done in CPU
* Has no storage
Stores results / data in RAM
* RAM is volatile (looses contents if power is lost)
Needs to be stored on persistent storage
* HDD/SSD

16. IB Computer Science Topic 2.1.6
Describe the main functions of an operating system
Hierarchy of Software
Software
System Software
Application Software
Operating System
Drivers / Libraries
Utilities
Games
Productivity
Specialist
Word processor
Spread sheet
Browser

17. A. User interface Functions of an operating system
IB Computer Science Topic 2.1.6
Functions of an operating system
Provides a user interface
Does memory management
Does peripheral management
Allows multi-tasking
Provides security
A. User interface
OS has to provide a link between the user and the computer hardware.
Types of user interfaces:
Graphical User Interfaces (GUIs) that have menus and icons, graphics, desktop style.
Command Line Interfaces (CLIs) where the user types in codes, data entry style.
Natural Language Interface (NLIs) where the user speaks to the interface.
Menu Based Interface (MBIs) which gives the user a selection of options.

18. B. Memory Management Examples of user interfaces
IB Computer Science Topic 2.1.6
Examples of user interfaces
GUI
CLI
MBI
NLI
B. Memory Management
Memory management is done by keeping track of storage devices (like HDD) and controlling which application has access to which area of memory (RAM).
Each location in memory can be read, modified, and written to by the OS. When the memory location is full the OS sends a confirmation message.
Similarly, the OS provides file management services by sorting out where data is stored on the disk drives and memory.
The OS allows users to organize files in folders as well as to copy and delete files.
Examples of memory management
File manager for managing files on HDD
Memory manager for RAM to make sure programs don’t use same memory space

19. C. Peripheral management
IB Computer Science Topic 2.1.6
C. Peripheral management
The OS coordinates the basic input and output systems (the BIOS).
Keyboard, mouse, monitor and printers are controlled through device drivers.
A device driver is a software program which allows hardware devices to be used by the OS. They act as translators between the devices and the computer system.
Peripheral management example