SLR04 - Operating Systems, Systems Software

What is the function of OS

• Provides interface between user and hardware

• Provides platform for application software

• Memory, File, User management

• Device drivers

• Interrupts

Multitasking

More than one program open & running same time, processor allocate small amount time each process, cycle between them

File management

• Filename extension tell OS which application to load files into

• OS may present logical structure of files into folders

• Allow user rename, delete, edit etc.

User management

• Allow multiple users log into same computer

• OS retains settings each user

• Manage access to files and programs

User Interface

WIMP - windows , icons, menus, pointer

Paging

• Memory divided into fixed size

• Program spilt into pages of same size to fit memory divisions

• Physical divisions

• In some cases it may be inefficient as program would be better kept together

Segmentation

• Different sizes & complete sections of programs

• Logical divisions

• Space large enough, program not split

• If not, program spilt

Paging and Segmentation Similarities & Differences

Virtual memory

• RAM full

• Program running in background held in secondary storage 

• Slower access speeds

• Program swapped with another when needed and brought to RAM

Interrupt

If other devices & applications require processor attention, need to signal processor they need attention

How are interrupts handled?

• Interrupt service routine is program with instructions needed to be fetched, decoded ,executed to carry out operations of interrupt

• This means contents of PC need to be changed to point to the address for the first instruction of the interrupt

How processor know continue previous executing program?

• When interrupt received, value held in register copied to stack in stack frame

• Interrupt executed

• Once complete, remove frame off top of stack allowing to:

  • Retrieve previous values for original program

  • Load them back into processor register

Interrupt priority

• Hardware

• User

• Software

• Timer

• I/O device

Scheduler manages…

• Which process execute next

• Length time next process can be executed for

Types of scheduler alogrithms

• FCFS

• SJF

• RR

• SRT

• MLFQ

FCFS

• First come first serve

• Processor executed in order arrived

SJF

• Shortest job first

• Pick process shortest time, runs unit finish

RR

• Round robin

• Each process allocated fixed amount time - time slice/quantum

SRT

• Shortest remaining time

• Similar to SJF

• SRT is pre-emptive, process can be suspended if higher priority process joins queue

MLFQ

• Multiple ready queues based on processing needs

• Gives preference to processes with short CPU bursts & processors with high I/O burst

Process blocking

• While program running, may require data from hard disk

• Slow so process blocked until input request serviced

• Next process can enter running state

• While this running, first process receive data needed

• Now needs to generate interrupt to let scheduler know it can re-join ready queue

Types of OS

• Multi-user operating system

• Distributed operating system

• Embedded operating system

• Real time operating system

Multi-user operating system

• Allow more than one person to use computer same time

• Manage user permissions and access rights when logging in

Distributed operating system

Combine processing power of multiple computers across network for single task

Real time operating system

• Designed for immediate data processing

• Can ensure tasks are processed in specific timeframes

Embedded operating system

• System software used to manage the device it is built into

• That has a specific task & hardware

• And is stored in the device’s ROM

What happen when computer turned on?

• BIOS (Basic input output system) - responsible for load OS when computer first turned on

• POST (Power on self test) - checks all hardware needed connected & working

• Bootstrap (Boot loader program) - used to load operating system kernel into memory which allow OS take over & boot rest

Device drivers

• A program that enables communication between OS and hardware devices

• Translating OS instructions to instructions peripherals can understand

Virtual machine

• Piece of software that behaves like a computer

• Entire operating systems running inside another operating system

Emulators

Trick software into thinking its running on different machine

Virtual servers

Physical server can run several virtual server spread over small num physical servers so if one servers stops working, load can be picked up by different server

Byte code / Intermediate code

• Halfway code that allows java code to be run on different devices

• Translated by JMV (Java Virtual Machine) on target device which then translates into specific machine code

Advantages and disadvantages of owning films that are streamed or downloaded on demand rather than owning a physical copy