Computers - OS, utility software, languages, robust software, secondary storage

what is the operating system

program that acts as an interface between the hardware and other software in a computer system

enables efficient communication between them

provides interface for liveware

process management

modern computers are multitasking - can execute several processes concurrently

CPU can only execute one instruction at a time so it is up to OS to ensure that every process loaded into main memory gets a share of CPU time using scheduling algorithms

examples of process management

first in first out

shortest job first

round robin (each process assigned time slice)

memory management

when process loaded into main memory from secondary storage, OS allocates it a block of addresses (pages)

when computers memory full, OS frees up space by temporarily moving inactive processes into an area of hard drive (virtual memory)

uses paging algorithm to determine which processes to swap out

processes swapped back in again when it is their turn to use CPU

file management

OS responsible for managing and keeping track of files stored on computers hard drive

hierarchal tree structure with top node called root directory

user management

access control

user interface

GUI

CLI

access control

OS uses login names and password (and/or some form of biometric identification) to authenticate users and control who can go in

GUI

graphical user interface - has window, icons, menus, pointers WIMP

CLI

command line interface - only allows users to type in commands. mainly used by expert users who want more control over function of the computer

user interface

OS provides it to enable users to interact with computer

file permissions

part of user management

control who can do what with each file

4 levels of access: read, write(but cannot delete), execute (run files), delete(full access)

OS enforces permissions associated with each user

how does the OS allocate space on the hard drive to a file

file are broken up into blocks of a fixed size

each block is placed in an empty space on the disk

block may not be contiguous

left over space where a block is not complete is known as ‘slack space’

OS keeps track of starting point of each block and its sequence number for files larger than one block

data compression

reduces size of a file so it takes up less space in secondary storage and is faster to transfer across networks

repackages or removes some of files data to reduce its size

file repair

recovers data from and repairs files that have become corrupted

scans damaged file, extracts as much data from it as possible and stores it in a new, useable file

backup software

keeps a copy of files so that, if it gets lost or damaged, the most recent backup can be restored

backup copies of files are made at regular intervals and stored on a separate device in a different location to the live working environment (possibly on the cloud)

What is an audit trail?

helps improve accountability by keeping track of who made what changes and when during the development process

if a problem is found then it can be tracked back to its source and the code rolled back to a version before the flaw was introduced

part of robust software

version control software

used alongside audit trails, especially when a large team works on a complex project

uses data bases to keep track of every modification to the code

if a mistake is made, the code can be reverted to an earlier version

purpose of code reviews

to check that software adheres to agreed standards

to find any instances of inefficient code

to identify potential vulnerabilities

pert of robust software

what are code reviews

reviews carried out by other programmers or by specialised software that checks the code to make sure it meets a pre-defined set of rules.

can detect bugs and security issues and may suggest ways to fix them

examples of OS

Microsoft windows, Apple iOS, Linux

examples of low level languages

machine code and assembly language

what are low level languages

they work directly with a computers hardware

programs have to be translated into machine code before they can be executed by the CPU

what must programmers do for low level languages

A programmer must have detailed knowledge of a computers architecture in order to write a program

must be familiar with CPUs instruction set (binary instructions, opcodes)

uses of low level languages

device drivers, firmware for embedded systems

machine code

written in binary

thousands of 1s and 0s only for a simple program

very hard to write and debug

assembly language

uses mnemonics (short memorable keywords) to represent instructions

examples of high level languages

Java, Python

high level languages

use key words (print, if, return)

problem oriented - enable programmers to focus on their program’s logic rather than how it will be implemented on a computers hardware

must be translated before it can be executed by a CPU

single line of code requires multiple lines of machine code

what do high level languages have

come with libraries of ready-made functions, integrated development environments and editing tools to make it easier to write code

cons of low level languages

difficult and time consuming to use

have few tools to help with maintenance and debugging

machine specific - programs written in a low level language will not run on a computer with a different type of CPU

pros of low level languages

interact directly with the hardware, enabling memory to be used directly

pros of high level languages

programmer friendly

have tools that make maintenance and debugging easier

machine independent - will run on computers with different type of CPU

cons of high level languages

generally less memory efficient

types of secondary storage

magnetic

optical

solid-state

magnetic storage

contains a stack of circular, metal platters that spin at a high speed

surfaces of the platter are coated in a substance that can be magnetised

how to read and write data in magnetic storage

an electromagnet in the read-write head (each platter has one) magnetises the surface of the platter as north-south or south-north these represent 1 and o

the read-write head detects the magentic state of the platter

solid state storage

an electrical current is applied to a transistor, forcing the electrons through a barrier, trapping them on the other side in ‘pools’ .

full pool = 0, empty = 1

a small volatge is applied and if the pool is empty, the transistor turns on and a 1 is read out, if it is full, the transistor does not turn on and a 0 is read out

optical storage

a laser is used to burn the surface of a disk, changing its form to become more or less reflective. Reflective areas are called land and less reflective are called pits.

land = 1, pit = 0

data is stored as a series of lands and pits on a single track that spirals out from the centre of the disk

a laser beam is shone onto the surface of the disk. pit reflects more dimply than land

amount of light is detected by a light sensors and translated into 1s and 0s

advantages & disadvantages of magnetic storage

high capacity

fast data access

has moving parts that will eventually fail

noisy

advantages & disadvantages of optical storage

portable

disks are cheap

slow to access

prone to scratches

advantages & disadvantages of solid state storage

very fact data access

no moving parts

low power

quiet

relatively expensive

has a limited number of read-write cycles

examples of magnetic storage

tape drives, hard disk drives

examples of optical storage

CD, DVD, blu-ray

examples of slid state storage

USB sticks, SSD cards, mobile phones