Systems Software

Systems Software

1. Operating Systems

   • Manages operations
   • Held in permanent storage
   • Functions
     • User interface
     • Memory management
     • Interrupt service routines
     • Processor scheduling
     • Backing store management
     • Management of input and outputs

2. Memory Management

   • Paging
     • Memory divided into fixed size pages
   • Segmentation
     • Divides address space into varying length segments
       • Depends on program structure
   • Virtual Memory
     • Operating system moves inactive jobs out of disk to virtual memory

3. Interrupts

   • Signal to CPU
     • Software program
     • Hardware device
     • Internal clock
   • Triggered by timer
     • Indicate next process has processor time
       • Allows multi-tasking
   • Interrupt Service Routines

4. Scheduling

   • Round Robin
     • First In First Out
     • Each process given time slice
       • Process not complete or higher priority interrupt occurs
         • CPU given to next process
   • First Come First Served
     • Jobs processed in order they arrive
     • No priority system
   • Multi-Level Feedback Queues
     • Preference to short jobs
     • Preference to I/O processes
     • Separates processes into categories
       • Based on need for processor
   • Shortest Job First
     • Process with smallest estimated running time run next
     • Reduces number of waiting jobs
   • Shortest Remaining Time
     • Process with smallest estimated completion time run next
     • Reduces number of waiting jobs
     • Requires knowledge of how long job will take
       • User estimate

5. Types of Operating Systems

   • Distributed
     • Parallel processing system
     • Spreads load over multiple computer servers
       • Single job split into tasks
         • Each run on separate computer
   • Embedded
     • Found in hardware
     • Requirements vary
     • Minimal user interface
     • Input from sensors
       • Output to control devices
     • Limited amount of RAM
     • No permanent data storage devices managed
   • Multi-Tasking
     • Run on standalone computer
     • Run many jobs at the same time
   • Multi-User
     • Single supercomputer connected to dozens/hundreds of terminals
     • Each user gets slice of processor time
   • Real Time
     • Respond quickly to inputs
     • Deal with many inputs at the same time
     • Failsafe mechanisms
       • Detect if hardware component fails
         • Take appropriate action
     • Incorporate redundency
       • One component fails
         • Switch to back up hardware

6. BIOS

   • Gets computer started when turned on
   • Initialize and test system hardware components
   • Load operating system from hard disk into RAM

7. Device Drivers

   • Provides software interface to hardware devices
   • Operating systems access hardware without knowing hardware details
   • Hardware dependent
     • Operating system specific
   • Communicates via system bus

8. Virtual Machines

   • Software used to take on function of machine
     • Execute immediate code
     • Running operating system within another

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