Computer Science 1.2

UNFINISHED ( FINISHED INTERRUPTS MOVE TO SCHEDULING )

Function of operating systems

The user needs to interact with hardware to produce an output. So, the operating system is used to allow interaction with the hardware.

Provides an interface for the user to use

Provides a platform on which application software can run.

Application software

Created to allow the user to perform various functions to interact with the operating system to interact with the hardware

usually involves a user interface so that the user is interacting with something familiar and easy to use.

Utility programs

built into the operating system that help to maintain the computer system including compression software, encryption software, and defragmentation software, and antivirus software

Memory/Resource management

a core function of the operating system which allows for programs to simultaneously run in main memory and virtual memory. With their data being managed

File management

Data is stored in files.
File extensions tell the operating system which application to load the file with

The operating system may present a logical structure of files in folders and allow the user to rename, delete, copy and move files.

The functions of operating systems are:

file management, memory / resource management, interrupt handling, user management / security, user interfaces

User management

allows multiple users to log into the same comptuer.

the operating system will retain settings for each user such as icons, backgrounds, and other computer settings

each user may have different access levels and rights to files and programs

a client-server network may impose a fixed or roaming profile for a user and manage login request to the network.

User interfaces

Allow the user to interact with the operating system which interacts with the hardware to produce an output.

GUIs, CLIs

WIMP - Windows, Icons, Menus, Pointers

Visual, Interactive, Intuitive, Optimised for mouse and touch gesture

Fixed / Roaming Profiles

Fixed profile means that each user will have the exact same settings

Roaming profile means that each user can have different settings and when they log into other computers, those settings follow them.

Memory Management strategies

There are two main strategies of memory management, both of which involve dividing the memory up into smaller sections:

Paging

Segmentation

Paging

Pages are a fixed size and are made to fit sections of memory

Pages are physical divisions

Programs are split up to fit into a given number of pages

Paging takes no account of how it splits the program, only that it splits it into fixed-sized pages

It could separate the instructions inside a looping condition so they are in different pages meaning that different parts of RAM would be jumped to which isn’t very efficient

It is better to keep those instructions together in memory

Segmentation

Segments are different sizes and are COMPLETE SECTIONS of a program / of programs.

Segments are logical divisons meaning the program is divided into complete parts and places them in the available free space leaving two smaller areas of free space

Paging and Segmentation Similarities

Both allow programs to run despite insufficient memory

Pages and segments are both stored on the disk

Pages and segments are transferred into memory when needed

Paging and Segmentation Differences

Pages are fixed size. Segments are different sizes

Pages are made to fit sections of memory. Segments are complete segments of a program

Pages are physical divisions, Segments are logical divisions.

Virtual memory

when the size of the programs being executed is larger than the available space in RAM, virtual memory is used.

Virtual memory is a section of secondary storage dedicated to run like RAM. It is slower than RAM but faster than secondary storage.

Data of programs that are not currently being used when the RAM is full is moved to virtual memory so that the program being executed can run in RAM which is faster.

When the data in virtual memory is needed again, it can be swapped out for a program in main memory that is not currently being used.

What is an interrupt

While the processor is fetching, decoding and executing instructions, other devices and applications may require the processor’s attention. This is when interrupts are used to signal to the processor that they require attention.

Fetch decode execute cycle becomes Fetch, decode, execute, check for new interrupts

What is the interrupt service routine

The processor needs to stop executing its current program to run code for the interrupt. This code is called the interrupt service routine.

The interrupt service routine is a program with a set of instructions that need to be fetched, decoded, and executed to carry out the operations of the interrupt. This means that the contents of the program counter need to be changed to point to the address for the first instruction of the interrupt.

What is the stack

When an interrupt is received, the values held in the registers are copied into the stack. These values are pushed into the stack in a stack frame saving them for later retrieval.

once the interrupt is completed, the stack frame is popped off the top of the stack allowing the processor to retrve the previous values for the original program, load them back into the registers, and carry on execution of the original program where it was left off.

the stack is a data structure in memory.

What happens when there is an interrupt

The currently running program is suspended, and the data in registers is pushed to the stack in a stack frame. The processor then handles the fetch, decoding, and executing of the interrupt service routine.

What happens if there is a higher priority interrupt while an interrupt is running

The interrupt service routine that was currently running is suspended and data stored in registers is pushed onto the top of the stack it then starts executing the higher priority interrupts’s service routine.

Once the higher priority interrupt service routine has completed execution, the frame of the previous program (ISR) is popped off the top of the stack retrieving the values for the program, loading them back into registers and continuing execution of the program from where it was left off.

After the first interrupt is handled, we pop the frame off the top of the stack for the original program so that the original data values are retrieved and put into registers and the program continues to execute where it was left off.

Example of hardware interrupt

Power supply failure, power button pressed, power down command

Example of User interrupt

Moving the mouse, clicking / tapping an icon to open a program, keyboard presses

Examples of software interrupt

illegal instructions encountered, arithmetic overflow, new log on request

Example of timer interrupt

data logging programs reading an input sensor every second, screen recording applications

Example of I/O interrupts

signal the completion of a data transfer to/from a device

printer ink supply notification

What is scheduling? What is a schedular?

A schedular manages which process to execuite next and the length of time the next process can execute for

How does scheduling work?

Processes move into the process ready queue given a time slice

Once the program currently executing in the CPU is finished, the next process will move into the CPU to be run

from here it will either finish execution and leave the system completely, get blocked as it requires an input or output command (cannot continue until more data is received), or run out of time - each process is given a certain amount of time it can run for - after which it is suspended and moved to the back of the ready queue

What is a scheduling algorithm

An algorithm that decides the order in which processes in the process ready queue run

First come first serve (FCFS)

Processes are executed in the order that they arrive

If a process takes a long time, the others behind simply have to wait or leave the CPU before they can be processed

NON PRE-EMPTIVE

Shortest job first (SJF)

Picks the process that takes the shortest amount of time and runs them until they finish

for this to work, the scheduler needs to know how long each process will take

(organises the process ready queue in order of largest → shortest assuming cpu is on the right.

NON PRE-EMPTIVE

Round robin (RR)

Each process is allocated a fixed amount of time known as a time slice / quantum.

if the process is not complete before the end of its time slice, it returns to the back of the ready queue

if time slice is 5, process A (10 ticks needed) will enter the running state and execute for 5 ticks before it is suspended and sent to the back of the queue. It will re enter the queue with 5 ticks remaining because 10-5=5

time slice values are global (if 5 then all processes run for 5 ticks)

PRE-EMPTIVE

what is a time slice

time slice is the amount of ticks the CPU will execute on each process in round robin

time slice values are global meaning if it is 5, ALL processes will run for 5 ticks

what is cpu burst time / execution time / processing time

The total number of CPU ticks required for a process to complete execution

total CPU time needed for a process to finish

Shortest remaining time (SRT)

Shortest remaining time allows for higher priority processes to go to the front of the ready queue

e.g. if process A runs for 4 ticks and becomes 6 and process B joins with 5 ticks process B will go to the front of the queue.

Shortest job first but allows for the shortest remaining time process to go to the front of the queue

PRE-EMPTIVE

Process Blocking

While a process is running, if it requires data from the hard disk, it will be blocked until this data has been received through an input request. The next process will go to run in the cpu while the first process is still waiting to receive the data

once the data is received, the process must generate an interrupt to let the scheduler know it is ready and can rejoin the ready queue

Multi level feedback queues (MLFQ)

a pre emptive CPU scheduling algorithm used used by operating systems

balances responsiveness for short/interactive tasks and fairness for longer CPU-bound tasks

Multiple priority queues, new processes start in the highest priority queue

processes can move down if they use too much CPU time

processes may move up if they do not use all of their allocated time slice or frequently wait for inputs/outputs

higher priority queues are always serviced first

designed to adapt to process behaviour dynamically

uses multiple process ready queues with different priorities. the higher priority queues are always services first. processes move between queues based on their cpu behaviour

PRE-EMPTIVE

what is starvation? how is it solved?

when a process waits and never gets CPU time because other processes keep getting priority

priority boosting solves this - if a process doesnt get CPU time in a while, priority is boosted

advantages of MLFQ

adapts dynamically to process behaviour

gives short or interactive tasks higher priority

reduces waiting time for priority processes

disadvantages of MLFQ

more complex to implement

lower priority tasks may starve.

harder to tune parameters like number of queues, time slice

pre-emptive meaning

a process running in the CPU can be stopped at any time

it does not have to finish its allocated time slice

allows higher priority or interactive processes to run immediately

Multi tasking operating systems

we expect to have more than one application open at once

in single-processor systems, each active program is scheduled to receive a tiny time slice in quick rotation to give the impression that they are operating at the same time

modern processiors with mutliple cores have to handle many processes at the same time

Multi user operating system

allows more than one person to use a computer at the same time

manages the user’s permissions and access rights when they log on

the server operating system software will handle requests of mutliple people using different computers on a network at the same time

Distributed operating system

combine the processing power of multiple computers across a network for a single tasks

in distributed computing, the operating system controls and coordinates the computers presenting them to the user as if they were a single system

e.g. one server handles querying of items, one handles the basket.

as the system becomes more busy with more requests, additional servers may automatically join the system - load-balancing requests without the user knowing

embedded operating systems

embedded operating systems tend to run on dedicated hardware so they run with maximum efficiency using low powered processors and very little memory

HIGHLY specialised for the task they are designed for

real time operating systems

in safety-critical environments, such as self-drive cars and missile systems

processes have to be guaranteed to execute within a known time frame

ALOT of redundancy is built into these systems so they can handle sudden increases in input

processors in real time operating systems rarely run at full capacity

overlapping classifications

all these classifications of operating systems can be combined together - overlapping

What is the BIOS

The basic input output system

responsible for loading the operating system when the computer first turns on

it first checks that all hardware is connected and working using a POST (power on self test)

what is a bootloader / bootstrap

a program that is used to load the operating system kernel into main memory

the operating system then takes over and boots up the rest of the system

where are initial startup instructions stored?

stored on ROM

where are BIOS settings stored and why?

stored in flash memory so they can be changed and are retained when there is no power

what is a device driver

software that tells the operating system how to communicate with a device

the device driver translates the operating system’s instructions into a series of instructions that a specific piece of hardware will understand

the computer has to output to a wide range of devices with different manufacturers and standards

What is a virtual machine

a program that has the same functionality as a physical computer

a virtual machine is hosted using a portion of the host pc’s hardware and can run any OS and any programs without affecting the host computer - completely isolated. the machine runs as if it were the host computer.

what can VMs be used for

testing applications on phones/consoles that were built on PC originally

server technology in VMS

VMs can be used to support a large number of virtual servers spread over a small number of powerful physical servers

4 physical servers runnign 6 virtual servers for example

as demand on a network increases, additional servers can be created and the load is balanced across different physical devices

if one server stops working, the others can pick up the load and continue working as if nothing happened

intermediate code

with traditional programming, you have to use the language of each device or a suitable compiler for each device

java gets around this by compiling its code into bytecode or intermediate code

this code is then translated by a Java Virtual Machine (JVM) running on the target device which then translates into specific machine code.

this makes the code highly portable between devices

what is hardware

physical parts of a computer system and its related devices both internal or external

what is software

general term covering all computer programs. most software falls into broad categories such as application, system, and utility software

what are teh two types of software

system software

application software

what is system software

operating systems, utility software

operating = windows, linux, macos, ios, android

utility = antivirus, defragmentation, backup, compression, firewall

what are the types of application software

presentation, word processing, spreadsheet, web browser, communication, email, photo editing, video editing, database, graphics manipulation

what is a generic application

does not ahve a very specific or narrow puirpsoe - can be used to carry out different tasks and meet multiple needs

generic applications such as word processors tend to be the most commonly installed and used products as they allow the user to be productive in many different ways

what is specific software

it has close to one specific purpose - making it essential for specific tasks such as database management

only tends to be installed on systems where there is a direct need for it.

what is DTP

desktop publishing (software)

like publisher

whjat is utility software

software designed to keep your computer safe, keep it running efficiently, provide you with useful tools to manage files and applications

examples of utility software

backup, compression, defragmentation, antimalware, file management, device drivers, file repair

File repair

files can become corrupt or damaged

this software attempts to correct these issues to restore the file to its original working state

file repair is often built into certain applications as well as being a separate dedication software tool found in most operating systems

backuip software

backups can be manual, automatic, or scheduled

incremental or full backups

you can backup to different media or an off site cloud system

backup software often comes with operating system but can also be purchased separately

full backup

takes all files and takes a copy of them all and puts them in a different media in a different location

incremental backup

scans to see what has changed since the last backup and only copies those files which have changed and adds them to the previous BASE full backup

Compression software

compression reduces the size of a file so it takes up less space and downloads and uploads faster over the internet

compressed files must be extracted before they can be read

data is either lost, reducing the quality of the file e.g. image or video OR represented in a different way using binary, retaining the original data in a new, compressed format

Defragmentation software

reorganises files on a hard disk, putting fragments of files and free space back together. This process reduces the movement of the read/write head across the disk which speeds up file access and improves the life span of the hard disk

solid state drives should not be defragmented as they have no moving parts. It also reduces the drive’s lifespan

Anti malware software

keeps the computer and files safe from malware such as viruses, trojans, worms, spyware, ransomware

most operating systems come with malware protection already built in, configured and turned on

there are also companies who specialise in dedicated anti malware programs.

open source software

users can modify and distribute the software

can be isntalled on any number of computers

support is provided by the community

users have access to the original source code

may not be fully tested

adv and disadv of open source for users

software is free, wide community modification to suit ur needs

can be poorly supported, some features might not be well tested

adv and disadv of open source for creators

gets their software out there to a wider community

little to no financial gain

closed source software

protected by the copyright design and patents act

users cannot modify the software

usually paid for and licensed per user or per comptuer

supported by developers who do not release source code

tested by developers prior to release, although it may run beta programs to allow people early access

adv and disadv of closed source for users

well supported, tested, professional built product

cost or ongoing subscription fee

adv and disadv of closed source for creators

advantages:

they receive an income for their product

constant demand from community for more features and improvements

disadvantages:

piracy issues

what is source code

we pick a programming language and write source code - it is descriptive and easy for us to understand, read, maintain and debug

however, computers cannot understand it so we translate it into machine code which is a pure binary form of source code so that computers can execute it

what is translation

the process of converting source code into machine code

different types of translators

assembler, compiler, interpreter

what is an assembler

translates assembly codfe mnemonics directly into machine code which can be executed on the specific processor architecture

how are high level languages translated

compilers or interpreted

what is an interpreter

takes one line of source code, translates it then executes it

what is a compiler / compilation

compilation takes the entire source code and translates it to produce a machine code output

the compiler translates the source code into object code then a linker program adds any libraries that are needed. The finished machine code can be produced as an executable file or an intermediate stage can be completed creating intermediate code which can be interpreted to convert to machine code.

advantages of assembler

memory efficient, speed of execution is faster, hardware oriented, requires fewer instructions to accomplish the same result

disadvantages of assembler

takes a lot of time to code or write the program as it is more complex

lack of portability between computers of different makes

advantages of compiler

no need for translation at run time

speed of execution is faster

code is usually optimised

original source code is kept secret

disadvantages of compiler

the program will not run with syntax errors - can be more difficult to write the code

code needs to be recompiled when the code is changed

designed for a specific type of processor

advantages of interpreter

easy to write source code as the program will always run only stopping when a syntax error is found

code does not need to be recompiled when code is changed

it is easy to try out commands when the program has paused after finding an error ( compared to recompiling ) making interpreted languages easy for beginner programmers to learn to write code