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Types of Operating System

Mainframe Operating Systems:

A mainframe with 1000 disks and thousands of gigabytes of data is not unusual. Mainframes are normally used as web servers, servers for large-scale electronic commerce sites etc.
The operating systems for mainframes typically offer three kinds of services: batch, transaction processing, and time sharing.
A batch system is one that processes routine jobs without any interactive user present. For example, claim processing in an insurance company
Transaction processing systems handle large numbers of small requests per second; for example, check processing at a bank or airline reservations.
Timesharing systems allow multiple remote users to run jobs on the computer at once, such as querying a big database.
These functions are closely related: mainframe operating systems often perform all of them. An example mainframe operating system is OS/390, a descendant of OS/360

Personal Computer Operating Systems:

Job of personal computer operating system is to provide a good interface to a single user. They are widely used for word processing, spreadsheets, Internet access etc.
Personal computer operating systems are so widely known to the people who use computers but only few computer users knows about other types of operating systems.
Common examples of PC operating systems are Windows 2008, Windows 2007, the Macintosh operating system, Linux, Ubuntu etc.

Server Operating Systems:

Server operating systems run on servers, which are very large personal computers, workstations, or even mainframes.
They serve multiple users at once over a network and allow the users to share hardware and software resources. Servers can provide print service, file service, or Web service.
Internet providers run many server machines to support their customers and Web sites use servers to store the Web pages and handle the incoming requests. Some Examples of typical server operating systems are UNIX and Windows 2007 server, Sun Solaris etc.

Real-Time Operating Systems:

Another type of operating system is the real-time system. These systems are characterized by having time as a key parameter.
Deadline slip may cause huge disaster sometimes. Two Types: hard real-time system, soft real-time
If the action absolutely must occur at a certain moment (or within a certain range), we have a hard real-time system. For example, if a car is moving down an assembly line, certain actions must take place at certain instants of time, if a welding robot welds too early or too late, the car will be ruined
Another kind of real-time system is a soft real-time system, in which missing an occasional deadline is acceptable. Digital audio or multimedia systems fall in this category.

Time Sharing Systems:

Time sharing is a technique which enables many people, located at various terminals, to use a particular computer system at the same time.
Time-sharing or multitasking is a logical extension of multiprogramming. Processor's time which is shared among multiple users simultaneously is termed as time-sharing. In Time-Sharing Systems objective is to minimize response time.
Multiple jobs are executed by the CPU by switching between them, but the switches occur so frequently. Thus, the user can receives an immediate response.

History of Operating System

In this section we take a brief look at the history of operating system which is almost the same as looking at the history of computers.

First Generation (1945-1955)

During second second world war many people were developing automatic calculating machines. These first generation computers filled entire rooms with thousands of vacuum tubes.
They did not have an operating system, they did not even have programming languages and programmers had to physically wire the computer to carry out their intended instructions. The programmers also had to book time on the computer as a programmer had to have dedicated use of the machine.

Second Generation (1955-1965)

Vacuum Tubes provide very unreliable and a programmer, wishing to run his program, could quite easily spend all his/her time searching for an replacing tubes that had blown.
Development of Transistor : Now, instead of programmers booking time on the machine, the computers were under submitted on punched cards that were placed onto a magnetic tape. This tape was given to the operators who ran the job through the computer and delivered the output to the expectant programmer.
Concept of batch-processing (jobs): Instead of submitting one job at a time, many jobs were placed onto a single tape and these were processed one after another by the computer. The ability to do this can be seen as the first real operating system.

Third Generation (1965-1980)

IC (Integrated circuit) as a replacement for transistors : The third generation saw the start of multi programming. This is the computer could give the illusion of running more than one task at a time.
When one job had to wait for I/O request, another program could use the CPU. The concept of multi-programming led to a need for a more complex operating system.
Another feature of third generation machine was that they implemented spooling. This allowed reading of punch cards onto disc as soon as they were brought into the computer room. This eliminated the need to store the jobs stored to disc, thus allowing programs that produced output to tun at the speed of the disc, and not the printer.
Up until these point programmers were used to giving their job to an operator and watching it run.
This concept led to the concept of time sharing. This allowed programmers to access the computer from a terminal and work in an interactive manner.
Obviously, with the advent of multi programming, spooling and time sharing, operating system had to become a lot more complex in order to deal with all these issues.

Fourth Generation (1980-present):

The late seventies saw the development of Large Scale Integration (LSI). This led directly to the development of the personal computer (PC).
These computers were (originally) designed to be single user, highly interactive and provide graphics capability. One of the requirements for the original PC produced by IBM was an operating system and, Bill Gates supplied MS-DOS on which he made his fortune.
In addition, mainly on non-Intel processors, the UNIX operating system was being used.
Mainly, we can say that Graphical User Interface (GUI) became popular in 4th generation computers.

Fifth Generation (Sometime in the future):

We can notice that each generation have been influenced by new hardware. The fifth generation of computers may be the first that breaks with this tradition and the advances in software will be as important as advances in hardware.
Able to interact with humans in a way that is natural to us. No longer will we use mice and keyboards but we will be able to talk to computers in the same way that we communicate with each other.
In addition, we will be able to talk in any language and the computer will have the ability to convert to any other language. Computers will also be able to reason in a way that imitates humans. Advances need to be made in AI (Artificial Intelligence).
It is also likely that computers will need to be more powerful. Maybe parallel processing will be required. Maybe a computer based on a non-silicon substance may be needed to fulfill that requirement (as silicon has a theoretical limit as to how fast it can go).

What is Segmentation? (Operating System)

Paged virtual memory is one-dimensional in which virtual addresses goes from 0 to some maximum address
A segment is a logical entity. It might contain a procedure, or an array, or a stack, or a collection of scalar variables, but usually it doesn’t contain a mixture of different types.
Segmentation is the mechanism that provides the machine with multiple completely independent address spaces. Thus segmentation provides two-dimensional address space. Each segment consists of linear sequence of addresses, from 0 to some maximum

To specify an address in this segmented or 2-dimensional memory, the program must supply a 2-part virtual address: 1) segment number(s) and 2) an address within the segment called offset (s).
Virtual address to physical address translation process is shown in figure below:

Deadlock Detection and Recovery (Operating System)

If a system does not employ either a deadlock-prevention or a deadlock-avoidance algorithm, then a deadlock situation may occur. In this environment, the system must provide:
An algorithm that examines the state of the system to determine whether a deadlock has occurred.
An algorithm to recover from the deadlock.

Algorithm for detecting deadlock:

For each node, N in the graph, perform the following five steps with N as the starting node.
Initialize L to the empty list, designate all arcs as unmarked.
Add current node to end of L, check to see if node now appears in L two times. If it does, graph contains a cycle (listed in L), algorithm terminates.
From given node, see if any unmarked outgoing arcs. If so, go to step 5; if not, go to step 6.
Pick an unmarked outgoing arc at random and mark it. Then follow it to the new current node and go to step 3.
If this is initial node, graph does not contain any cycles, algorithm terminates. Otherwise, dead end. Remove it, go back to previous node, make that one current node, go to step 3.

Recovery From Deadlock
When a detection algorithm determines that a deadlock exists, there are two options for breaking a deadlock. One solution is simply to abort one or more processes to break the circular wait. The second option is to preempt some resources from one or more of the deadlocked processes.

Process Termination: To eliminate deadlocks by aborting a process, we use one of two methods. In both methods, the system reclaims all resources allocated to the terminated processes.

Abort all deadlocked processes
Abort one process at a time until the deadlock cycle is eliminated –overhead, a deadlock-detection after terminating a process

Resource Preemption: To eliminate deadlocks using resource preemption, we successively preempt some resources from processes and give these resources to other processes until the deadlock cycle is broken. Some issues need to be addressed are:

Selecting a victim: Some process will have to rolled back to break deadlock. Select that process as victim that will incur minimum cost.
Rollback: Determine how far to roll back the process

Total rollback: Abort the process and then restart it. More effective is to roll back the process only as far as necessary to break deadlock

Starvation: Starvation happens if same process is always chosen as victim. To avoid this, we include the number of rollbacks in the cost factor.

Critical Section (Operating System)

One way to avoid race conditions is not to allow two processes to be in their critical sections at the same time.
Critical section is the part of the process that accesses a shared variable.
That is, we need a mechanism of mutual exclusion. Mutual exclusion is the way of ensuring that one process, while using the shared variable, does not allow another process to access that variable.

Race Condition (Operating System)

Race conditions are situations where two or more processes reads or writes some shared data at the same time and the final result is incorrect.
Final result may be different according to order of completion of competing processes.
let us consider a simple but common example: a print spooler. When a process wants to print a file, it enters the file name in a special spooler directory.
Another process periodically checks to see if there are any files to be printed, and if there are, it prints them and then removes their names from the directory.
Consider the situation given in figure below, where in and out are two shared variables.

Process A reads value of in(i.e. 7) but before inserting the name of the document to be printed in index 7, the CPU decides to schedule process B.
Now, process B also reads in, and also gets 7. Process B now continues to run and stores the name of its file in slot 7 and updates in to be an 8.

After some time process A runs again. It looks at value of in stored in its local variable and finds a 7 there, and writes its file name in slot 7.Then it increments value of in and sets in to 8.The printer daemon will not notice anything wrong, but process B will never receive any output. Situations like this are called race conditions.