Computers

Unfinished. These only include Year 10 content, sorry.

Modern computer architecture

Von Neumann architecture

Stored program concept - data and instructions in RAM

(Parts of) CPU

Central Processing Unit

• Control unit

• ALU

• Registers

Control unit

Sends and receives signals to / from other hardware components

• Program counter register

• Instruction register

RAM

Random Access Memory

Primary, volatile storage

Hard disk

Magnetic storage medium

Secondary, non-volatile storage

Registers

Immediate access storage

Examples:

• Program counter - holds memory address of the next instruction

• Memory address register - holds the address of what is needed to read / write to RAM

• Memory data register - holds data fetched from RAM

• Instruction register - holds instruction being executed

• Accumulator - holds intermediate ALU results

Address

First memory element that stores an instruction / data

Steps to process data

Fetch: get data and instructions from RAM using address / data bus, store in registers

Decode: control unit looks up instruction in the instruction set

Execute: instructions carried out on data

Clock chips

Vibrating crystal that determines the speed of the CPU

Buses

Physical connections between the CPU and other components

Data bus

Bidirectional

Reads and writes data to memory

Control bus

Bidirectional

Carries signals that control CPU components

Address bus

Unidirectional

Carries the address of a location in memory

CPU bandwidth

The number of unique memory addresses that can be accessed

Primary storage

First place of storage

E.g. RAM, ROM, registers, cache

Secondary storage

Second place of storage

E.g. hard disk, various media

Secondary storage benefits

• Store data during power off

• Retrieve data when power is back on

• Read and edit data during power off

• Higher storage capacity

Optical media

Flat, reflective surface with pits and lands burned in by lasers - laser is directed at it, straight back up = 0, angled reflection = 1

E.g. DVD, CD, Blu-ray

Low capacity, slow data access, portable

Magnetic media

Magnetise parts as North = 1 and South = 0

E.g. hard disk, tape

High capacity, quick data access, moving parts, defragmentation

Solid state media

Electric current forces electrons into pools - full pool = 0, empty pool = 1

E.g. USB flash drive, SSD cards

Medium capacity (expensive), very quick access, no moving parts, no defragmentation

Cloud storage

Storing data on remote servers

• Accessible anywhere

• Backups / security managed by the business

• Additional storage easily accessed

• Requires internet access

Types of software

• System: programs that allow a computer to function

• Application: programs that the users interact with the most

Operating System

Provides a platform for other software to interact with the computer hardware

Operating system tasks

• Process management

• File management

• User management

• Peripheral management

OS file management

Uses a hierarchy called a ‘directory’

Breaks files into blocks and allocates an empty sector on the hard disk for them

• Slack space is often left

Required sectors calculation

Ceiling [ file size in bytes / block size in bytes ]

Levels of acces

System administrator has full access - only user who can access the root directories

• Read

• Write

• Execute

• Delete

Authentication is used to prove you are the account holder

OS process management

Allocates CPU / hardware time to each process waiting to be executed

Multitasking means making multiple processes available at once

Scheduling algorithms

• First In First Out - processes in the order that items arrive

• Shortest Job First - processes items that are the closest to completed

• Round-robin - process are allocated time slices and if the process is not finished by the end of it, it goes to the back of the queue

  • High priority tasks are given more time slices

Paging algorithms

Memory is shared between processes as series of addresses called pages

These algorithms manage virtual memory

Virtual memory

RAM may become full when many processes are running

Paging algorithms then move pages to the hard disk temporarily and back to RAM when ready to be executed

Peripheral devices

Input / output devices

Drivers (software) provide an interface between the device and OS

User interface

The point where the user and the sytem interact

UI types

• GUI: uses WIMP format, more common, easier to use

• CLI: more challenging but run faster and offer more control over a computer / server / network’s function

Utility software types

Adds functionality / improves performance / maintains hardware and software /performs a specific management task

• File repair - fixes corrupted / inaccessible files

• Backup and recovery - creates and stores file copies

• Data compression - reduces file sizes

• Disk defragmentation - rearranges data on the disk to be more continuous

• File encryption

• Anti-malware - detects and removes malware

Low-level programming languages

• Sits close to computer hardware

• Binary codes or mnemonics that represent binary

• e.g. machine code / assembly code

High-level programming languages

• Abstracted from hardware- don’t need to understand how the computer works

• uses English- like statements

• Requires a translator from source code to machine code

• e.g. python, java, javascript, c, c++

Low and high level language advantages

Low-level:

• Tune code for speed of execution

• Hard to learn and read

• Need to know CPU architecture

• Best for:

  • Efficiency

  • No translator

  • Embedded / real-time systems

  • Device drivers and parts of OS

High-level:

• Easier and quicker to learn / write

• More choice of development tools

• Don’t need to know architecture

• Best for:

  • Portability

  • OS

  • Games / apps / office productivity

Translator

Translates source code into machine code

E.g. assembler, interpreter, compiler

Interpreter

Translator that converts and executes each instruction immediately (one-by-one)

Compiler

Translator that converts and executes the whole program at once

Interpreter and compiler advantages / disadvantages

Interpreter:

• Cross-platform

• Simpler to test / easier to debug

• Better interactivity

• Require less memory

• Executor requires interpreter

• Often slower

• Public source code

Compiler:

• Ready to run

• Often faster / optimised code

• Private source code

• Not cross platform

• Inflexible

• Extra step

• Require lots of memory

Robust software

Software that can handle many kinds of errors without crashing

Common issues in code

• User / logic / syntax / runtime errors

• Hardware faults

• Missing peripherals

Vulnerability

A flaw in code that introduces security issues

Bad coding practices

• Poorly planned design

• Quick fixes (may cause future problems and ignore current ones)

• Not using industry coding standards

• Poorly structured code

• Insufficient testing

Fixes to bad coding practice

• Code reviews

  • Can reduce vulnerabilities - check if it fits standards / has no flaws

• Audit trails

  • Check who made changes - restore code to pre-flaw and improve accountability

Embedded systems

Simple computers on a circuit board, usually responsible for a dedicated task

Sensor gets data —> processor processes it —> corresponding output

E.g. switching lights, keeping track of time, receiving button / sensor input

Embedded system characteristics

• Single purpose

• Battery powered - low power consumption and small physical size

• Low cost and maintenance

• Real time / environment responsive

Firmware

Very small programs written for embedded systems

Fixed inputs / outputs and do not need to be modified once installed