evolution of operating systems designs

8/3/2019 Evolution of Operating Systems Designs

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Evolution of Operating Systems

Designs/What is an Operating System?


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This necessitated that the program be able to tell in some manner which type of file it was

reading on the tape. A similar system, IBSYS was IBM's operating system for the 7094. Loadingthe operating system required that a tape containing the operating system be loaded first in order

to start the operating system. Early operating systems did not have the bootstrap mechanism

which all modern operating systems use to initialize the computer, so start up of a computer was

a long and involved process.

By the early 1960's it became obvious that computer companies were having to support twocompletely different types of computers, 1401 like character based computers, and word-oriented

7094 like scientific computers. Maintenance and training on these completely different systems

was problematic so IBM invented System/360 and created a range of products that used thiscommon architecture. The IBM 360 was the first computer line to use small scale integrated

circuits, and thus offered a major cut in price over earlier solid state machines. This radically

expanded the market and the power of the machines.

OS360 was written to act as the operating system on all of the System360 machines from the

smallest input processor to the largest number cruncher type processor. It was made up ofliterally thousands of lines of code written by hundreds of programmers, and maintenance wasproblematic since almost every time a bug was fixed a new one would be generated. Despite the

impossibility to maintain such a massive monolithic program it was a distinct improvement on

previous second generation operating systems, and became quite popular.

One of the concepts that it popularized was the idea of multi-computing, where instead of

waiting for a single job, the computer could be running multiple jobs simultaneously. Thiseliminated waiting periods while the operators mounted the tapes, since the computer could be

processing a different program while it waited. One problem with combining business computer

operating systems with number crunching system operating systems, was that the size of the jobs

involved required different strategies. Business jobs were simple, and required less time per job,and as a result mounting and unmounting tapes took up 80 to 90 percent of the processing time.

While a large number cruncher was still expensive enough that small jobs were not economical

so mounting tapes was more like 10 percent of the job, and not a significant problem. One wayof getting around this was to partition the memory, so that each job could have its own partition.

This went along with the Multi-programming concept, because it meant that more than one job

could run at the same time on the same computer without interfering with the memory of otherjobs.

Along with this concept came the idea of running the card reader from one partition, the number

crunching in another, and the output to the printer from a third, eliminating the satellite 1401type computers, and reducing the wear and tear on tapes from mounting stress. While third

generation units were good for large runs they were essentially still batch systems and as

programs began to become more complex, managing the card stacks for a program becameunwieldy. This paved the way for timesharing systems where each user had his own terminal,

and they shared a central mainframe.

Timesharing didn't become popular until late in the third generation when the hardware for

protection mechanisms became widespread. At that time, the idea of a Computing Utility began


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to be worked on, the idea being that a single mainframe would supply timesharing opportunities

to the whole Boston Area. Out of this concept the first 4th generation operating systemMULTICS was born. Of course the idea was based on the small scale integration, and the

development of LSI and then Later VLSI (Very large scale Integration) eventually replaced

whole rooms of computers with a small desktop unit. As a result of LSI (Large Scale Integration)

it became possible to build a whole computer system on a single rack. These Mini-computersbegan to compete with IBM for smaller businesses, and because their financing terms were not

as pecuniary their ability to compete made them replacements for the small and midsized

system360 components. It was not surprising by the 80's to find that a timesharing system usedmini-computers as their front ends to handle terminal multiplexing and had a mainframe as the

main computing mechanism.

In the 1970's Ken Thompson designed UNIX as a stripped down one user version of MULTICS

based on a PDP-7 Minicomputer. Since he published the source code, it became a popular

operating system and was used extensively by Universities and small businesses. By the 1990's itwas so popular that more software was being written for it, by outsiders than was written by

ATT, the then license holder for it. Legal battles between ATT and Universities as to who ownedthe rights to the new software involved in Unix V5 resulted in two things, the sale of theoperating system to SCO and the development of an alternative kernel by Linus Torvalds thatcould run the by then public domain software produced by the universities because it met the

POSIX standard which had been developed by IEEE.

It was this defiant rewrite of an operating system kernal that created the Open software

movement that has powered the internet revolution. Most operating systems today have gained

by the work of the Open Software Movement because their internet software's sophistication,depends on ideas pioneered and spread through free software.

For most purposes today, an operating system is best defined as the software that comes on anoperating system install CD or DVD. This definition works well for users and operating system

vendors. It has even been argued in court.

A definition of an operating system now might look like the following:

A System

A set of principles and concepts and all of the components which obey these principles.

An Operating SystemA system of software which serves as a base for its users' programmatic computing

needs.

By programmatic we mean that the software is used primarily by programs, as opposed tointeractively by the user directly.

So C++ for example is notan operating system since it fails to provide many critical functions ofcomputation. Without execution of programs, scheduling of processes, and many other functions

provided to C++ by system software it would be impossible for C++ to run on top of bare

hardware. Basic, Lisp, Smalltalk, and Java in contrast can sometimes be considered operating


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systems. Although routinely hosted on top of other OSes, they have sometimes been run directly

on top of exotic special-purpose hardware.

Moving onto principles, UNIX for example has as its principles "everything is a file, referred to

by a descriptor" and "programs act as filters". All of the little programs and utilities that obey

these principles are an integral part of the operating system whereas the X Window System,which does not obey these principles, is not.

In contrast, Plan 9, a derivative of Unix, has as its principle "everything is a filesystem" and

many more functions are part of the operating system in Plan 9 than in Unix. In particular, Plan

9's windowing system is an integral part of the operating system.

[edit] A note on consistency

While an operating system is a set of principles and the software that obeys them, it is often thecase that parts of the operating system disobey the principles. This is the case when the

principles aren't expressed clearly and/or they are inadequate.

In the case of Unix and Plan 9, processes are neither filestreams nor filesystems. It is impossible,

for example, to duplicate a process by copying it. So we see that the principles "everything is afilestream / filesystem" are inadequate. Note that just because something is a filter doesn't

prevent it from being a filestream; the concept of filestreams is simply inadequate.

The result of inadequate principles is that development proceeds at random, following the path of

least resistance. The end result of such development cannot be characterized by any design

principles at all, and has abrogated any principles the system began with. This explains both

modern Unix and C++. Such systems are only perceived to be cohesive in that their incoherence

is sharply distinguished from other systems.

[edit] How to distinguish different operating systems?

For academic purposes, a good test of the differences between two operating systems is ease of

porting software. If two operating systems are such that porting software from one to another isdifficult because the concepts are different, then the OSes are relatively unalike. At the other

extreme, if two platforms are such that no effort is required to "port" from one to another, then

the two are relatively similar. Because there are legal reasons why we can't call two different

products produced by different manufactorers the same, we call them Compatible.

Usually two systems that are compatible are so because they follow a single protocol called a

standard. Organizations like IEEE, and ISO, publish these standards, and different companiescan build their project to the standard even though they have proprietary elements at a lower

level. For instance Linux and Unix share the IEEE POSIX standard so they are compatible at the

interface level that standard defines.
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By this definition, many common operating systems can be lumped together and considered to

be similar or Compatible. OS/2 and Windows are similar. UNIX and BSD are Similar, Linux isCompatible.

The minimum distance between two distinct operating systems is probably that between Plan 9

and Unix. Despite their differences, both were created by the same person to achieve the exactsame goals. And despite the time interval between Unix and Plan 9, it is obvious that the

designers felt that many of the UNIX design decisions were still good. This all goes to show thatPlan 9 and Unix are as close as two operating systems can ever get while remaining distinct.

Note that this test of the distinctiveness of operating systems has an analog used in biology toestablish the distinctness of species. Biology recognizes species as distinct when they can no

longer freely interbreed. For our purposes, operating systems are distinct when they can no

longer freely share code.

evolution of operating systems designs

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