Operating-System Structure

Operating-System Structure

1. Simple Structure

• MS-DOS - Written to provide the most functionality in the least space

• Not divided into modules
• Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated

2. 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 of only lower-level layers

• #UNIX - limited by hardware functionality, the original UNIX operating system had limited structuring. The UNIX OS consists of two separable parts

• 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

UNIX System Structure

3. Microkernel System Structure

• Moves as much from the kernel into "user" space
• Communication takes place between user modules using message passing
• Benefits:
• Easier to extend a mocrokernel
• Easier to port the operating system to new architectures
• More reliable (less code is running in kernel mode)
• More secure
• Detriments:
• Performance overhead of user space to kernel space communication

Mac OS X Structure

Mac OS X is mixed structure (with Mach Microkernel and BSD kernel and kernel extension)

4. Modules

Solaris Modular Approach

• Most modern operating systems implement kernel modules
• Uses object-oriented approach
• Each core component is separate
• Each talks to the others over known interfaces
• Each is loadable as needed within the kernel
• Overall, similar to layers but with more flexible

Virtual Machines

Virtual Machines

• A virtual machine takes the layered approach to its logical conclusion. It treats hardware and the operating system kernel as though they were all hardware
• A virtual machine provides an interface identical to the underlying bare hardware
• The operating system creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory
• The resources of the physical computer are shared to create the virtual machines



The Java Virtual Machine

• CPU scheduling can create the appearance that users have their own processor
• Spooling and a file system can provide virtual card readers and virtual line printers
• A normal user time-sharing terminal serves as the virtual machine operator's console
• The virtual-machine concept provides complete protection of system resources since each virtual machine is isolated from all other virtual machines. This isolation, however, permits no direct sharing of resources
• A virtual-machine system is a perfect vehicle for operating-systems research and development. System development is done on the virtual machine, instead of on a physical machine and so does not disrupt normal system operation
• The virtual machine concept is difficult to implement due to the effort required to provide an exact duplicate to the underlying machine

VMware Architecture

참고  : Operating System Concepts by WILEY