One of the most important aspects of operating systems is the ability to multi-program. A single program cannot, in general,keep either the CPU or the I/O devices busy at all times. Single users frequently have multiple programs running.Multiprogramming increases CPU utilization by organizing jobs(code and data)so that the CPU always has one to execute.
1. Monolithic Systems
The entire operating system executes with maximum privileges. The communication overhead inside the monolithic operating system is the same as the communication overhead inside any other software, considered relatively low.
CP/M and DOS are simple examples of monolithic operating systems. Both CP/M and DOS are operating systems that share a single address space with the applications.
2. Layered Systems
A layered design of the operating system architecture attempts to achieve robustness by structuring the architecture into layers with different privileges. The most privileged layer would contain code dealing with interrupt handling and context switching, the layers above that would follow with device drivers, memory management, file systems, user interface, and finally the least privileged layer would contain the applications.
MULTICS is a prominent example of a layered operating system, designed with eight layers formed into protection rings, whose boundaries could only be crossed using specialized instructions
3. Microkernel Systems
A microkernel design of the operating system architecture targets robustness. The privileges granted to the individual parts of the operating system are restricted as much as possible and the communication between the parts relies on a specialized communication mechanisms that enforce the privileges as necessary. The communication overhead inside the microkernel operating system can be higher than the communication overhead inside other software, however, research has shown this overhead to be manageable.
MACH is a prominent example of a microkernel that has been used in contemporary operating systems, including the NextStep and OpenStep systems and, notably, OS X. Most research operating systems also qualify as microkernel operating systems.
4. Virtualized Systems
Attempts to simplify maintenance and improve utilization of operating systems that host multiple independent applications have lead to the idea of running multiple operating systems on the same computer. Similar to the manner in which the operating system kernel provides an isolated environment to each hosted application, virtualized systems introduce a hypervisor that provides an isolated environment to each hosted operating system.
Hypervisors can be introduced into the system architecture in different ways.
• A native hypervisor runs on bare hardware, with the hosted operating systems residing above the hypervisor in the system structure. This makes it possible to implement an efficient hypervisor, paying the price of maintaining a hardware specific implementation.
• A hosted hypervisor partially bypasses the need for a hardware specific implementation by running on top of another operating system. From the bottom up, the system structure then starts with the host operating system that includes the hypervisor, and then the guest operating systems, hosted above the hypervisor.
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