1.2 Software

• What is an interrupt

  An external event that causes a change in the flow of instruction execution
• How are interrupts found and checked


  The CPU checks the interrupt line after every instruction is executed
• What happens if an interrupt is raised

  CPU suspends execution of current instruction

  Moves all contents of the PC to the system stack

  Calls ISR (interrupt service routine) to deal with the interrupt
• Facets of the operating system

  User Interface

  Security

  Memory management

  Network management

  Device management

  Utilities

  Interrupts

  Scheduling

  File management
• What is memory management used for

  To share the RAM between different processes to complete at the same time

  To reallocate memory where needed

  To handle virtual memory

  To optimise use of memory (paging and segmentation)
• What is paging

  Dividing RAM into fixed size blocks (frames)

  Programs are divided into fixed size pages

  Any page can fit into any frame

  A page table allocates pages to frames
• Benefits/detriments of paging

  Easy to swap pages between frames

  If page size is not same as frame size, waste of storage known as internal fragmentation
• What is segmentation

  Programs broken into varying sized blocks called segments, which is a logical division of the program

  A segment table maps segments to memory locations
• Benefits/detriments of segmentation

  No internal fragmentation

  Slower than paging due to logical placements

  Segments are not suited well for swapping

  Need to know beginning and end of segment
• What is virtual memory

  A section of secondary storage the OS uses as RAM

  Pages/Segments that have not been used recently are swapped to virtual memory
• What is disk thrashing

  A problem that can occur with virtual memory

  If pages and segments are being constantly swapped back and forth it is known as thrashing

  Makes computer feel very slow as secondary storage is slower than RAM
• What is a device driver

  Software that provides a bridge between the OS and the hardware so they can communicate

  Drivers are OS and hardware specific
• What is utility software

  Programs designed to help, analyse, configure, optimise or maintain a computer
• List of utilities

  Anti-virus

  Anti-malware

  Disk defragmenter

  Disk clean-up

  Compression

  Firewalls

  Backups
• Types of scheduling

  Round robin

  First come First Served

  Multi-level feedback queues

  Shortest job first

  Shortest remaining time
• What are the benefits of scheduling

  Improves efficiency (many jobs done quicker)

  Maximise processor utilisation

  Allocate time fairly

  Minimise process starvation

  Eliminates deadlock
• Pros and cons of first come first served (scheduling)

  Pros:

  Easy to implement

  No swapping so no overhead

  Cons:

  A more important process may have to wait for a less important one (could crash)

  Quick jobs may have to wait for long jobs

  Average waiting time may be long and is quite volatile
• Pros and cons of round robin(scheduling)

  
  

  Pros:

  Avoids process starvation

  Cons:

  Can be inefficient if the process is waiting for an input or output

  Overhead from swapping

  Performance is heavily based on time-slice selected
• How does round robin work (scheduling)

  Each task is designated a certain time-slice

  If not completed in that time it is paused and sent to the back of the queue
• Pros and cons of shortest job first (scheduling)

  Pros:

  Optimal as gives the shortest average waiting time of all scheduling types

  Cons:

  Uses guesstimates to decide job length

  Longer jobs never get executed if shorter jobs keep coming in
• How does shortest remaining time work (scheduling)

  Process with shortest remaining time takes priority

  Time left is evaluated at the end of every time-slice which causes pauses
• How do multi-level feedback queues work (scheduling)

  Number of queues each with different level of priority

  All processes start in top priority queue

  If a process uses its time allotment it is reduced a priority

  (to prevent processor starvation, after a certain amount of time all processes are increased a priority)
• Pros and cons of multi-level feedback queues (scheduling)

  Pros:

  A process that waits too long will be moved up a priority

  No prior knowledge of processes is needed

  Makes progress for long-running processes

  Cons:

  Moving processes between queues creates overheads

  Lots of variation in variables (How many queues, how large a time-slice)
• What is a Multi-tasking OS

  An OS where multiple processes can run on a single computer seemingly simultaneously

  Processor allots certain number of cycles to a process then switches

  Scheduling CPU time is necessary
• What is a multi-user OS

  Multiple users to utilise a computers resources at the same time

  Specially designed for sharing resources such as files and hardware

  Economical but have to share resources
• What is a real-time OS

  Designed to carry out action in a guaranteed amount of time

  The right answer too late is wrong

  Time critical safety systems run on this OS

  Low multitasking capabilities
• What is an embedded OS

  Operating systems built into different devices (washing machine)

  Limited functionality and reside in ROM

  Used to run efficiently on low powered CPUs

  Small size and cheap to code

  Not easy to update
• What is a distributed OS

  A collection of independent networked nodes that acct as if they were a single system

  Pros:

  Reduces the load on a single computer

  If a single node fails the others remain operational

  Cons:

  Security issues

  Overloading

  More difficult to administer many machines
• What is a virtual machine

  A self-contained operating environment that acts as if it is a separate computer

  Can run intermediate code or simulate a different OS
• Pros of a virtual machine

  Provides access to programs no longer supported

  Can use a different OS without having to use a separate computer

  App testing for different platforms

  Can use one large server for many different buisnesses
• Cons of a virtual machine

  Requires substantial processing power and memory

  Sharing resource can degrade performance of virtual and real machine

  Need antivirus for virtual machine
• What is bytecode

  Bytecode is platform independent

  If you write bytecode once you can run it anywhere

  Different languages can be compiled into java bytecode

  Can crash a virtual machine with faulty bytecode and the host computer will be fine
• What is bespoke software

  Software specially developed to suit the needs of the particular organisation
• What is closed source software

  Software that legally remains the property of the organisation or person who created it
• What is open source software

  The program's source code is freely available to edit by anyone
• What is system software

  Programs created to run the hardware or applications programs of the computer
• What is application software

  Programs designed for users rather than computers
• What is general purpose software

  Programs not made for a specific job and tend to do more than one task (word-processor, spreadsheet)
• What is special purpose software

  Software that has a specific purpose and usually completes a single task (CAD, calculator)
• Pros of open source

  Source code can be compiled to work on different platforms

  Potentially a community of developers to keep improving code

  Potentially a community to learn from

  Potentially less vulnerabilities as lots of people can view source code