OS Architecture

OS Architecture

1. Design

   • Largest, most complex system on computer
   • Goals and specification
     • User goals
       • Convient to use, easy to learn, fast, reliable, safe.
     • System goals
       • Easy to design, implement and maintain.
       • Flexible, reliable and efficient
     • Affected by choice of hardware and type of system
   • Policies and Mechanisms
     • Policy: what will be done? Mechanism: how it will be done?
       • Seperation important to allow maximum flexibility
   • Traditional UNIX
     • One big kernel
       • Consists of everything below system call interface ace and above physical hardware
       • Provides filesystem, CPU scheduling, memory management and OS functions

2. Strict layered

   • OS divided into a number of layers, each built on top of each other.
     • Bottom layer : hardware. Layer N : user interface
   • Modular
     • Layers selected so each uses the functions/services of lower levels.
       • Important for stability
         • Modern CPU have protected mode so transition between layer can be controlled by hardware
       • Hardware exceptions
         • Raised by CPU of any software attepemts to run instructions/access memory of high levels (privilege)
   • Simple construction
     • Layers have well defined functionality
     • Can be tested independently or altered with minimal impact to rest of OS

3. Modular Kernel

   • Seperation of modules logical
     • Kernel code runs in privileged address space which can cause problems
   • Kernel code can be added at runtime
     • OO approach
     • Each core component separate
     • Each module talks over known interface
     • Each loadable as needed in the kernel
   • Similar to layered architecture
     • More flexibilty
     • Doesn't need to be compiled in kernel binary

4. Microkernel

   • Moves as much as possible from kernel into less privileged user space (FS, device drivers)
     • E.G only small amount of FS operations need kernel specific functions (buffers)
   • Communication takes place between use modules using message passing
   • Advantages
     • Easier to develop extensions
     • Easier to port OS to new architecture
     • Reliable
     • Secure
   • Disadvantages
     • Performance overhead of user space to kernel space communication

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