1. Operating system structures can be simple, layered, or use a microkernel approach. Simple structures like MS-DOS do not have well-defined layers, while layered structures break the OS into smaller, more modular components. Microkernel structures remove nonessential components from the kernel into user-level programs. 2. Modern OSes often use a hybrid approach, combining elements from different structures. They may also use loadable kernel modules to dynamically link additional services. 3. A task control block (TCB) contains the state of a thread of execution in an OS, and typically includes the program counter, CPU registers, stack, I/O assignments, timers, and kernel data structures.

1. Operating System Structures
Types
1. Simple Structure
Such structure doesn’t have well-defined structure. Systems that uses such structure are
simple and for limited systems. An example for this structure is MS-DOS. You can see figure
of MS-DOS structure below:
Hardware also affects such structures. Traditional UNIX also used simple structure because had
limited hardware. You can see figure of such structure below:

2. 2. Layered Approach
With proper hardware support, operating system can be broken in pieces that are smaller and
more appropriate and can maintenance better. You can see the figure of layered structure
below:

3. A typical operating system layer— for example, layer M— consists of data structures and a
set of routines that can be invoked by higher-level layers.
3. Microkernel
When the UNIX became larger, the kernel of the UNIX also became larger and was hard to
manage. In the mid-1980s, researcher at Carnegie Mellon University developed an operating
system called Mach. Mach that modularized the kernel using the microkernel approach. In
this approach, all nonessential component removed from kernel and implemented as system
or user-level program. You can see figure of approach below:
4. Modules
Perhaps the best approach for design of the current operating system is loadable kernel
modules. Here, the kernel has a set of components and links in additional services via
modules, either at boot time or during run time. You can see structure of Solaris loadable
modules as below:

4. 5. Hybrid Systems
In most operating systems, they combine different structures, resulting in hybrid systems that
have performance, security, and usability issues. Examples of such structures:
MAC OS X

5. iOS
Android

6. VxWorks
- Structure:

7. - System Call:
- TCB
a thread of execution; that is, the task's program counter
the CPU registers and (optionally) floating-point registers
a stack for dynamic variables and function calls
I/O assignments for standard input, output, and error
a delay timer
a time-slice timer
kernel control structures
signal handlers
debugging and performance monitoring values
- End of Task
taskDelete() API

8. References
[1] A. Silberschatz, Operating System Concepts, 2012.
[2] "vxworks_kernel_programmers_guide_6.2," [Online]. Available:
https://www.uio.no/studier/emner/matnat/fys/FYS4220/h11/undervisningsmateriale/laboppgaver-
rt/vxworks_kernel_programmers_guide_6.2.pdf.
[3] "TCB in VxWorks," [Online]. Available: http://comp.os.vxworks.narkive.com/0awyxysO/tcb-in-
vxworks.