Operating System Design Approaches

1. Simple Structure

• some operating systems do not have well defined structures. Often these started as simple systems and grew beyond their original scope.
• 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.

1.1. UNIX System Structure

• UNIX: limited by hardware functionality, the original UNIX operating system has limited structure.
• The Unix OS consists of two separated system parts
• system programs
• the kernel (everything below the system call interface and above the physical hardware)
• provide the file system. CPU scheduling, memory management, and other operating system functions
• A large number of functions for one level

2. Layered Approach

• The operating system  is divided into a number of layers (levels), each build on top of low 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 users functions (operations) and services of only lower-level layers
• Simplifies debugging and system verification

3. Modular Approach

• Modular kernel
• the kernel has a set of core components
• dynamic links in additional services either during boot time or during run-time
• common in modern implementations of Unix such as Linux and Solaris
• Moves as much as possible from kernel into "user space"
• Communication takes space between users modules using "message passing"

• Benefits
• easier to extend
• more reliable (less code is running in kernel mode)
• convenient for distributed architectures
• security
• Many modern OS are designed as microkernels
• apple MAC OS (based on Mach OS)
• Many SmartPhone OS
• Android (L4 Microkernel family)
• IPhone OS (based on Mach)