Operating system 13 structure of os and layer design of dos

Design and Implementation of OS not “solvable”, but some approaches have proven successful

Internal structure of different Operating Systems can vary widely

Start the design by defining goals and specifications

Affected by choice of hardware, type of system

User goals and System goals
User goals – operating system should be convenient to use, easy to learn, reliable, safe, and fast
System goals – operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient

1. Operating System 13 Structure of OS and Layer design of DOS Prof Neeraj Bhargava Vaibhav Khanna Department of Computer Science School of Engineering and Systems Sciences Maharshi Dayanand Saraswati University Ajmer

2. Operating System Design and Implementation Design and Implementation of OS not “ solvable ” , but some approaches have proven successful Internal structure of different Operating Systems can vary widely Start the design by defining goals and specifications Affected by choice of hardware, type of system User goals and System goals User goals – operating system should be convenient to use, easy to learn, reliable, safe, and fast System goals – operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient

3. Operating System Design and Implementation (Cont.) Important principle to separate Policy : What will be done? Mechanism : How to do it? Mechanisms determine how to do something, policies decide what will be done The separation of policy from mechanism is a very important principle, it allows maximum flexibility if policy decisions are to be changed later (example – timer) Specifying and designing an OS is highly creative task of software engineering

4. Implementation Much variation Early OSes in assembly language Then system programming languages like Algol, PL/1 Now C, C++ Actually usually a mix of languages Lowest levels in assembly Main body in C Systems programs in C, C++, scripting languages like PERL, Python, shell scripts More high-level language easier to port to other hardware But slower Emulation can allow an OS to run on non-native hardware

5. Operating System Structure General-purpose OS is very large program Various ways to structure ones Simple structure – MS-DOS More complex -- UNIX Layered – an abstrcation Microkernel -Mach

6. Simple Structure -- MS-DOS 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

7. Simple Structure -- MS-DOS In MS-DOS, the interfaces and levels of functionality are not well separated. For instance, application programs are able to access the basic I/O routines to write directly to the display and disk drives. Such freedom leaves MS-DOS vulnerable to errant (or malicious) programs, causing entire system crashes when user programs fail. Of course, MS-DOS was also limited by the hardware of its era. Because the Intel 8088 for which it was written provides no dual mode and no hardware protection, the designers of MS-DOS had no choice but to leave the base hardware accessible

8. Simple Structure -- MS-DOS There are numerous commercial systems that do not have a well-defined structure. Frequently, such operating systems started as small, simple, and limited systems, and then grew beyond their original scope. MS-DOS is an example of such a system. It was originally designed and implemented by a few people who had no idea that it would become so popular. It was written to provide the most functionality in the least space, because of the limited hardware on which it ran, so it was not divided into modules carefully.

9. Structure of OS A system as large and complex as a modern operating system must be engineered carefully if it is to function properly and to be modified easily. A common approach is to partition the task into small components, rather than have one monolithic system. Each of these modules should be a well-defined portion of the system, with carefully defined inputs, outputs, and function. We have already discussed briefly the common components of operating systems. These components are interconnected and melded into a kernel.

10. Assignment Briefly Explain the Layered Architecture of DOS.