16.1 Purposes of an operating system (OS)

• Optimise use of computer resources
  • Implements process scheduling to ensure efficient CPU use
  • Manages main memory usage
  • Optimises I/O, dictates whether I/O passes through CPU or not

• Hides the complexities of the hardware
  • UI allows users to interact with application programs
  • Automatically provides drivers for new devices
  • Provides file system, organises physical storage of files on disk
  • Provides programming environment, removing the need for knowledge of processor functions
  • Provides system calls/APIs, portability

• Kernel
  • Part of the operating system
  • Central component responsible for communication between hardware, software and memory

• Multitasking
  • More than one program can be stored in memory, but only one can have CPU access at any given time
  • Rest of the programs remain ready

• Process
  • A program being executed which has an associated Process Control Block in memory (a complex data structure containing all data relevant to the execution of a process)
  • Process states
    • Ready: New process arrived at the memory and the PCB is created
    • Running: Has CPU access
    • Blocked: Cannot progress until some event has occurred
    • Exit: When the process has finished execution

• Scheduling ensure that the computer system is able to server all requests and obtain a certain quality of service

• Interrupt:
  • Causes OS kernel to invoke ISR (Interrupt Service Routine), kernel may have to decide on priority, register values stored in PCB
  • Reasons: Errors, waiting for I/O, scheduler halts process

• Low-level scheduling: Allocate processor components to complete specific tasks

Low level scheduling algorithms

• Preemptive: Will stop the process that would have otherwise have continued to execute normally

1. First-come-first-served
   • Non-preemptive
   • FIFO (First In First Out) queue

2. Round-robin
   • Allocates time slice to each process
   • Preemptive
   • Can be FIFO queue
   • Does not prioritise

3. Priority-based
   • Most complex
   • Priorities re-evaluated on queue change
   • Priority calculation requires computation
   • Criteria for priority time
     • Estimated time of execution
     • Estimated remaining time of execution
     • Is the CPU/IO bound?
     • Length of time spent in waiting queue

• Paging
  • Process split into pages, memory split into frames
  • All pages loaded into memory at once

• Virtual memory
  • No need for all pages to be in memory
  • CPU address space thus larger than physical space
  • Address resolved by memory management unit
  • Benefits
    • Not all of the program has to be in the memory at once
    • Large programs can be run with or without large physical memory
  • Process
    • All pages on disk initially
    • More loaded into memory when process ready
    • Pages replaced from disk when needed
      • Can be done with FIFO queue or usage-statistics based algorithm

• Disk thrashing
  • Perpetual loading/unloading of pages due to a page from disk immediately requiring the page it replaced

16.2 OS structure

• OS has to be structured in order to provide a platform for resource management and the provision of facilities for users

• User mode is the one available for the user or an application program

• Privileged/kernel mode has the

16.3 Virtual machine

• Process interacts with software interface provided by the OS

• OS kernel handles interaction with actual host hardware

• Pros
  • Allows more than one OS to run on a system
  • Allows multiple copies of the same OS

• Cons
  • Performance drop from native OS
  • Time and effort needed for implementation is high

• Examples and usage
  • Used by companies wishing to use the legacy software on newer hardware and server consolidation companies
  • Virtualising machines allow developers to test applications on a multitude of systems without having to make expensive hardware purchases

16.4 Translation software

• Lexical analysis: The process of converting a sequence of characters to a sequence of tokens (strings with an assigned meaning)

• Syntax analysis: The process of double-checking the code for grammar mistakes (syntax errors)

• Code generation: The process by which an intermediate code is generated after syntax analysis

• Optimisation: A process in which the code is edited to make improvements in efficiency

• For interpreters:
  • Analysis and code generation run for each code line as above
  • Each line executed as soon as intermediate code generated

• BNF: Backus-Naur Form
  • Can be used as a basis for an algorithm
  • Can be defined recursively

• RPN: Reverse Polish Notation
  • Used for representing algebraic expressions
  • Operator placed after operands