OS Function + Structure

OS Function + Structure

1. OS Services

   • User interface
     • Command-line, GUI, Batch
   • Program Execution
     • Load, run, and end program normally and abnormally (indicating error)
   • I/O Operations
     • Input/Output with files or I/O devices
   • File-System Manipulation
     • Programs need to read/write files and directories, create/delete, search, permission management
   • Communications
     • e.g. between computers on a network, via shared memory or message passing (packets)
   • Error Detection
     • OS must be aware of errors in CPU and memory hardware, I/O devices etc.
     • OS must be able to handle errors
       • Debugging
   • Resource allocation
     • Multiple users + jobs can run concurrently
     • Scheduling algorithms considering speed of CPU
   • Accouting
     • Track which users use which kind of resources + how much
   • Protection and Security
     • Protect against unauthorised access to resources
     • Security to protect system from outsiders

2. Interrupts

   • Signal from hardware indicating need for attention/event in software needing change in execution
     • Processor saves its state, and executes interrupt handler

3. Dual-Mode Operation

   • Allows OS to protect itself and other system components
   • Modes
     • User Mode
       • a.k.a supervisor mode
     • Kernel Mode
       • a.k.a privileged mode
   • Mode bit
     • kernel(0)
     • user(1)
     • Provided by hardware
   • System Calls
     • Programming interface to services provided by OS
     • C/C++
     • Accessed via Application Program Interface (API)
     • Types
       • Process control
       • File Management
         • Includes creating, deleting, naming file etc. requiring memory space so must be done by OS
       • Device management
       • Information maintenance
         • Error detection
           • Errors can occur at hardware layers which the kernel at hardware level can only check
       • Communications
         • Message passing - User programs might not coordinate access to the network device

4. System Programs

   • Provide convenient environment for program development + execution
   • File Manipulation
   • Status information
   • File modification
   • Programming language support
   • Program loading + execution
   • Communications

5. Operating Systems Structure

   • Simple Structure (MS-DOS)
     • Most Functionality + Least space
     • Not divided into modules
     • Vulnerable to errant/malicious programs
     • UNIX OS
       • Systems Programs
       • Kernel
       • Everything BELOW system-call interface and ABOVE physical hardware
   • Layered Structure
     • Layer 0 = Hardware
       • Highest layer = User Interface
     • Each layer uses functions of ONLY lower layers
     • Easier debugging/maintenance
     • Example: "THE" System
       • Layer 0: Hardware
       • Layer 1: CPU scheduling
       • Layer 3: Device drivers
       • Layer 4: Buffering for I/O
       • Layer 5: User programs
   • Microkernel
     • Small core OS
       • Built up via independent user-level processes
         • Communication between user modules = message passing
           • Benefits
             • Easy to extend
             • Easy to port OS
             • Reliable
             • Secure
           • Detriments
             • Performance overhead
   • Modular
     • Modern OS
       • Object-oriented approach
       • Interfaces for communication
       • Loaded when needed in kernel

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