1.2.1 Systems software

• Operating systems provide an interface between the user and computer
• Operating systems enable communication with the computer and perform low-level tasks involving memory and resource management
• Examples of popular desktop operating systems include Windows and macOS, while popular mobile phone operating systems include iOS and Android
• Operating systems are essential for memory management, resource management, file management, input/output management, interrupt management, utility software, security, and user interface
• Paging involves splitting memory into equal-sized pages that can be swapped between main memory and the hard disk
• Memory must be shared fairly between multiple programs through paging, segmentation, and virtual memory
• Virtual memory uses a section of the hard drive as RAM when main memory is insufficient, with unused program sections moved to virtual memory through paging
• Segmentation splits memory into logical divisions known as segments, representing the structure and flow of the program
• Interrupts are signals indicating a process needs attention, stored in a priority queue in an interrupt register
• Interrupts are checked at the end of each Fetch-Decode-Execute cycle, with the processor transferring contents to a stack to service the interrupt
• Interrupt Service Routines (ISR) are loaded into RAM to respond to interrupts
• First Come First Served processes jobs in chronological order, without considering job priority
• Shortest Job First orders jobs by completion time, suited for batch systems but requires knowing or calculating job durations
• Types of operating systems include Distributed, Embedded, Multi-tasking, Multi-user, and Real Time
• Scheduling algorithms include Round Robin, First Come First Served, Multi-level feedback queues, Shortest Job First, and Shortest Remaining Time
• Multilevel feedback queues use multiple queues ordered by priority, which can be challenging to implement
• Round Robin allocates equal time slices to jobs, potentially leading to longer job completion times and does not consider job priority
• Shortest Remaining Time orders jobs by time left for completion, risking processor starvation for longer jobs if short jobs are added
• Multi-user OS allows multiple users on one computer, Real Time OS performs tasks within a guaranteed time frame
• Virtual machines are software implementations of computer systems, used for running intermediate code and testing programs on different operating systems
• Virtual machines provide protection from malware, run incompatible software, and save time and money for testing programs
• Distributed OS runs across multiple devices to spread the load, Embedded OS performs specific tasks for a device, Multi-tasking OS allows tasks to run simultaneously
• BIOS is the first program that runs when a computer is switched on, responsible for key tests before loading the operating system into memory
• Device drivers allow the operating system to interact with hardware, specific to the computer's architecture and the installed operating system
• Intermediate code is independent of processor architecture and can be used across different machines and operating systems
• An application program is any other program written by a user or developer to perform some task on behalf of the user.
• Explain how an operating system deals with an interrupt. (3 marks)

  • Interrupt checked for at the start/end of each FDE cycle
  • If the interrupt is low priority then to the current process then the current process will continue
  • Flags are set to determine if interrupts are enabled/disabled
  • After interrupt complete, previous register values restored back from stack
  • Flag resets