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There are mainly three types of operating system architecture models :

    1. Monolithic OS :
      • Used in older but smaller operating systems.
      • Operating system is divided into procedures. Each procedure have a particular task and all procedures have common data structures and system tables. Each procedure can call other procedures. Resulting in high degree of inter-process dependency.
      • Very little importance is given to data hiding.
      • A small change in a procedure can fail other procedures. Therefore, it is very difficult to extend the OS and maintenance is also very difficult.
      • The degree of portability is also very low. Since every procedure talks to hardware.
      • Application programs runs in user mode and OS functions run in system mode.
      • Monolithic OS
    2. Layered OS :
      • Various components are implemented as a collection of logical layers.
      • Each layer consists of related modules.
      • Order of layer depends on OS designer.
      • Lower layer provides service to upper layer. Upper layer calls lower layer but not vice versa.
      • Advantages :
        • Each layer can be developed or enhanced independently.
        • Debugging is easier. Add lower layer first and then upper layer to find the bug.
        • Degree of portability is high because interaction with hard is carried out by only one layer
      • Disadvantage :
        • The OS becomes slow due to layers.
        • OS cannot handle misbehaving layer because all runs in same mode.
      • Layered OS
    3. MicroKernel OS (Client Server Model) :
      • Most essential components are included in very small sized microkernel.
      • Microkernel handles tasks like process management, inter process communication, virtual memory, device management.
      • All other components of OS are called servers e.g. file server, network server, display server, process server etc. All these runs in user mode. Any of the server can request a service from other server.
      • These are easily portable.
      • Microkernel OS