Topic 2.1.1
Outline the architecture of the central processing unit (CPU) and the functions of the arithmetic logic unit (ALU) and the control unit (CU) and the registers within the CPU.
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Key abbreviations
• CPU = Central Processing Unit
• ALU = Arithmetic Logic Unit
• CU = Control Unit
• MAR = Memory Address Register • MDR = Memory Data Register
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Simplified model: CPU, RAM Address Bus
Control Bus
Other registers
RAM (Primary Memory)
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Simplified model: CPU, I/O devices, Storage
I/O controller
Output device
Input device
Storage device
RAM (Primary Memory)
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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 ALUs to process two calculations simultaneously.
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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.
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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 registers, but the three most important ones are:
– Accumulator*
*Not assessed in this course
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MAR (Memory Address Register)
• MAR is connected to the address bus.
• MAR is contains a memory address.
• MAR’s sole function contain the RAM address of the instruction the CPU wants next.
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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.
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Busses are the connecting wires that connect the CPU to other devices, carrying instructions to/from components.
They are normally built into the motherboard.
The three most important busses are:
• Data bus (links RAM to CPU via MDR)
• Control bus (links RAM to CPU via CU)
• Memory bus (links RAM to CPU via MAR)
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Topic 2.1.2
Describe primary memory.
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Primary memory = RAM
As RAM is so important, it is often referred to as primary memory (even though it is actually only a branch of primary memory, alongside the cache and ROM).
In an exam/test, if you see memory, unless explicitly stated otherwise, it would normally be referring to RAM.
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RAM = Random Access Memory
• Contains the data and instructions the computer has loaded since starting up and everything the user has opened/loaded.
• Is volatile = loses its contents if power is lost
• Has a special link to the CPU (via busses)
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ROM = Read Only Memory
• Originally its contents were static (hence ‘read only’) and
could not be changed – not true any more (flash upgrades).
• Non-volatile = does not lose its contents if 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.
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Non-volatile
Contains user’s programs and data that has been loaded since ‘booting up’
Contains the BIOS
Usually upgradeable, can be increased
Usually part of motherboard, difficult to upgrade
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Topic 2.1.3
Explain the use of cache memory.
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Definition: cache
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
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Topic 2.1.4
Explain the machine instruction cycle
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Step 1: Fetching the instruction
The first step the fetch-execute cycle carries out is fetching the
instruction.
The CPU fetches this from the main memory (RAM) and stores it in the CPU temporary memory, called the registers.
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Step 2: Decoding the instruction Once the instruction has been fetched, the CPU will need to
understand the instruction to action it. This is called decoding.
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Step 3: Executing the instruction When the instruction has been decoded, the CPU can carry out
the action that is needed.
This is called executing the instruction. The CPU is designed to understand a set of instructions – the instruction set.
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Topic 2.1.5
Identify the need for persistent storage
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Why do we need storage?
Processing done in CPU
• Has no permanent storage (only registers)
Stores results/data in RAM
Needs to be stored on persistent storage
• RAM is volatile (looses contents if power is lost)
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Types of storage
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Topic 2.1.6
Describe the main functions of an operating system
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Functions of an operating system
A. Provides a user interface
B. Does memory management C. Doesperipheralmanagement D. Allows multi-tasking
E. Providessecurity
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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,
– Command Line Interfaces (CLIs) where the user types in codes,
– Natural Language Interface (NLIs) where the user speaks to the interface
– Menu Based Interface (MBIs) which gives the user a selection of options.
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Example of user interfaces
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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 organise files in folders as well as to copy and delete fi les.
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Example of memory management
Memory manager for RAM to make sure programs don’t use same memory space
File manager for managing files on HDD
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C. Peripheral management
• 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.
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Peripheral management example
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D. Multitasking
• The OS coordinates the working of different programs by allocating the CPU time between different programs based on time and priority of the software application.
• Each task running is given a slice of time, or a turn on the CPU.
• Each task has to wait its turn unless it is given a higher priority by the OS in which case it gets more or longer time slices.
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Multitasking example
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E. Security
• OS prevents unauthorised access.
• It ensures security of the system through usernames and passwords.
• The OS protects files from other users reading or writing files.
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Security examples
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Topic 2.1.7
Outline the use of a range of applications software
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Hierarchy of software
Operating System
System Software
Application Software
Drivers / Libraries
7 types…
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Common application software
• Word processors
• Spreadsheets
• Database Management Systems (DBMS) • Email clients
• Web browsers
• Computer Aided Design (CAD)
• Graphic Processing Software
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Word Processor (not Word!)
A program for storing, manipulating, and formatting text entered from a keyboard and providing a printout.
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Spreadsheet (not Excel!)
A program in which data is arranged in the rows and columns of a grid and can be manipulated and used in calculations.
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Database Management System
A database management system (DBMS) is system software for creating and managing databases. The DBMS provides users and programmers with a systematic way to create, retrieve, update and manage data.
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Email clients (not webmail like Gmail!)
A computer program used to access and manage a user’s email
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Web browser
A software application for retrieving, presenting, and traversing information resources on the World Wide Web.
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Computer Aided Design (CAD)
Programs that use computer systems to assist in the creation, modification, analysis, or optimization of a design.
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Graphic Processing Software
In computer graphics, graphics software or image editing software is a program or collection of programs that enable a person to manipulate visual images on a computer.
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