Operating System Structure

MOST OS TAKE A HYBRID APPROACH IN USING MIXING DIFFERENT APPROACHES

Various ways to structure OS:

• Simple structure – MS-DOS

• More complex – UNIX

• Layered – an abstraction

• Microkernel – Mach

Original UNIX

• UNIX – limited by hardware functionality, the original UNIX operating system had limited structuring.

• The UNIX OS consists of two separable parts
  • Systems programs
  • The kernel:
    • Consists of everything below the system-call interface and above the physical hardware
    • Provides the file system, CPU scheduling, memory management, and other operating system functions; a large number of functions for one level

Linux approach

Layered Approach

• The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.

• With modularity, layers are selected such that each uses functions (operations) and services if only lower-level layers

Micro Kernels

• Moves as much from the kernel into user space

• Mach is an example of microkernel

• Communication takes place between user modules using message passing

Pros:

• Easier to extend a microkernel

• Easier to port the operating system to new architectures

• More reliable (less code is running in kernel mode)

• More secure

Cons:

• Performance overhead of user space to kernel space communication

Loadable Kernel Modules (LKMs)

• OOP approach

• Each core component is separate

• Each talks to the others over known interfaces

• Each is loadable as needed within the kernel

• This is how linux is made

Mac OS and IOS structures

Darwin OS structure

Android