Operating systems can have different structures and architectures, including monolithic, layered, microkernel, and virtual machines. The original UNIX OS used a simple layered approach but most of the OS was in one large layer. Modern operating systems often use hybrid architectures combining elements from different structures. For example, Mac OS X uses a Mach microkernel for basic system management and a BSD kernel for additional services, while Android relies on the Linux kernel and uses middleware and apps layers.

1. Operating Systems
Structure of
Operating Systems

2. Structure/Organization/Layout of
OSs:
•Monolithic (one unstructured program)
•Layered
•Microkernel
•Virtual Machines
The role of
Virtualization

3. Monolithic Operating System

4. UNIX System Structure
UNIX – limited by hardware functionality, the original UNIX OS 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.

5. Traditional UNIX System Structure

6. UNIX Architecture Layers

7. UNIX System Structure
The original UNIX
OS used a simple
layered approach,
but almost all the
OS was in one big
layer, not really
breaking the OS
down into layered
subsystems
Simple Structure
- MS-DOS layer structure

8. UNIX System Structure
Layered Approach
This approach allows each layer to be developed and
debugged independently, with the assumption that all lower
layers have already been debugged and are trusted to
deliver proper services.
Another approach is to break the OS into a number of smaller
layers, each of which rests on the layer below it, and relies
solely on the services provided by the next lower layer

9. UNIX System Structure
A layered operating system
A layered operating system

10. UNIX System Structure
Hybrid Systems
Mac OS X
iOS
Android
Most OSes today
do not strictly
adhere to one
architecture, but
are hybrids of
several.

11. UNIX System Structure
Mac OS X
The Max OSX architecture
relies on the Mach
microkernel for basic
system management
services, and the BSD
kernel for additional
services. Application
services and dynamically
loadable modules ( kernel
extensions ) provide the
rest of the OS functionality:
The Mac OS X structure

12. UNIX System Structure
iOS
The iOS operating
system was developed
by Apple for iPhones
and iPads. It runs with
less memory and
computing power needs
than Max OS X, and
supports touchscreen
interface and graphics
for small screens:
Architecture of Apple's iOS

13. UNIX System Structure
Android
The Android OS
was developed
for Android
smartphones and
tablets by the
Open Handset
Alliance,
primarily Google.
Architecture of Google's Android