16 - System software and virtual machines

• Important tasks of an operating system is to hide the complexities of the hardware from the users by using GUI interfaces and device drivers
• The two types of multitasking operating systems are preemptive and non-preemptive
• Low level scheduling - decides which process should next get the use of CPU times
• Process schedular - Process priority depends on its category
• Process states refer to running, ready and blocked
• The conditions when changing from one process to another is

  running state --> ready state

  ready state --> running state

  running state --> blocked state

  blocked state --> ready state
• Memory management - determines which processes should be in main memory and where they should be stored (optimisation)
• Single/contiguous allocation - All of the memory is made available to a single allocation
• Paging - The memory is split up into partitions of a fixed size. Physical memory blocks are known as frames and fixed sized logical memory blocks are known as pages.
• Segmentation - Logical address space is broken up into variable size memory partitions called segments. Segments from logical memory are loaded into physical memory
• Differences between paging and segmentation:

  • A page is a fixed size block of memory
  • A segment is a variable size block of memory
  • The process of paging is invisible to the user
  • The process of segmentation is visible to the user
• Multitasking - managing the execution of many programs that appear to run at the same time
• Paging - reading/writing same sized blocks of data from/to secondary storage when required
• Interrupt handling - Transferring control to another routine when a service is required
• Scheduling - managing the processes running on the CPU
• Virtual memory - using secondary storage to stimulate additional main memory
• Disk thrashing - an issue occurring when the hard drive is overworked by transferring information between the system memory and the virtual memory. This swapping leads to a very high rate of hard disk head movements
• A virtual machine is the emulation of an existing computer system
• pros on virtual machines:

  There are security benefits

  Cost savings due to not needing to buy extra hardware
• Cons on virtual machines:

  Costly to maintain

  Cannot emulate some hardware
• Explain how an interpreter executes a program without producing a complete translated version of it:

  An interpreter examines source code one statement at a time. Check each statement for errors. If no error is found the statement is executed. If an error is found this is reported and the interpreter halts. Interpretation has to be repeated every time the program is run
• The 4 stages of compilation:

  lexical analysis

  syntax analysis

  code generation

  optimisation
• Lexical analysis: the first stage in the process of compliation. It removes unnecessary characters and tokenises the prgram
• Syntax analysis: the second stage in the process of compilation. The output from the lexical analysis is checked for syntax errors
• Code generation: the third stage in the process of compilation. This stage produces an object program
• optimisation: the fourth stage in the process of compilation. The creation of an efficient object program
• Explain why RPN is used to carry out the evaluation of expressions:

  RPN reads from left to write and doesn't use brackets
• What data structures can be used to evaluate an expression in RPN:

  Stack

  binary tree
• Outline the purpose of syntax analysis:

  Checks for grammar errors and syntax errors.

  A parse tree is produced
• Outline the reasons why an operating system may need to use virtual memory:

  Virtual memory is used when RAM is running low such as when a computer is running many processes at once.
• Explain how an expression stored in RPN can be evaluated:

  The RPN value reads from left to right one item at a time. Items are pushed onto the stack until an operator is found. This process is iterated.
• Justify the use of a linked list instead of an array to implement a stack:

  A linked list is a dynamic structure. It allows more efficient editing using pointers
• Explain how a compiler makes use of a stack when translating recursive programming code:

  The compiler code must produce object code to push values onto a stack with each recursive call and pop values off the stack after the base case is reached.