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virtual memory is a section of a hard disk that's set up to emulate the computer's RAM.
A computer can address more memory than the amount physically installed on the system. This extra memory is actually called virtual memory and it is a section of a hard disk that's set up to emulate the computer's RAM.

Virtual Memory is a storage allocation scheme in which secondary memory can be addressed as thought it were part of main memory. Virtual memory is a memory management technique that is implemented using both hardware and software. It maps memory addresses used by a program, called virtual address, into physical addresses in computer memory. It gives an illusion to the programmer that programs which are larger in size than actual memory can be executed. Virtual memory can be implemented with demand paging.
The main visible advantage of this scheme is that programs can be larger than physical memory. Virtual memory serves two purposes. First, it allows us to extend the use of physical memory by using disk. Second, it allows us to have memory protection, because each virtual address is translated to a physical address.


Modern microprocessors intended for general-purpose unit, or MMU, is built into the hardware. The MMU's job is to translate virtual addresses into physical addresses.

Virtual memory also allows the sharing of files and memory by multiple processes with several benefits:

  • System libraries can be shared by mapping them into the virtual address space of more than one process. 
  • Processes can also share virtual memory by mapping the same block of memory to more than one process. 
  • Process pages can be shared during fork() system call, eliminating the need to copy all of the page of the original (parent) process. 


Concept of semaphore was devised by Dijkastra in 1965. Semaphore is an integer variable that is used to record number of weakups and had been saved. If it is equal to zero it indicates that no wakeup's are saved. A positive value shows that one or more wakeup's are pending. The DOWN operation (sleep) checks the semaphore to see if it is greater than zero. If it is, it decrements the value (UP) and continues. If the semaphore is zero the process sleeps. The UP operation (weakup) increments the value of the semaphore. If one or more proces were sleeping on that semaphore then one of the process is chosen and allowed to compute its DOWN. Checking and updating the semaphore must be done as an atomic action to avoid race conditions. Product Customer problem can be solved using semaphore as below.



 
#define N 100 /* number of slots in the buffer */
typedef int semaphore; /* semaphores are a special kind of int */
semaphore mutex = 1; /* controls access to critical region */
semaphore empty = N; /* counts empty buffer slots */
semaphore null = 0; /* counts full buffer slots */
void producer(void){
    int item; 
    message m;      // message buffer. 
    while(TRUE){
        item = produce_item(); // generate something to put in buffer 
        receive(consumer, &m); // Receive an empty message, if any 
        build_message(&m, item); // construct a message to send 
        send(consumer, &m);  // send item to consumer 
    }
} 
void consumer(void){
    int item, i; 
    message m; 
    for(i=0; i<N; i++)
        send(producer, &m);  // send N empty messages 
    while(TRUE){
        receive(produce, &m);  // get message containing item   
        item = extract_item(&m); // extract item from message
        send(producer, &m); //send back empty replay 
        consumer_item(term); //do something with the item
    }
}

The shell is an interface between user and the operating system

  • The operating system shell is the mechanism that carries out the system calls requested by the various parts of the system. 
  • Shell is not part of the operating system kernel. The shell is the part of operating systems such as UNIX and MS-DOS where we can type commands to the operating system and receive a response. 
  • Shell is also called the Command Line Interpreter (CLI) or the “C” prompt. Shell is the primary interface between a user sitting at his terminal and the operating system. Many shells exist, including sh, csh, ksh, and bash.
  • It starts out by typing the prompt, which tells the user that the shell is waiting to accept a command. If the user now types “date” command, the shell creates a child process and runs the date program as the child. 

  • While the child process is running, the shell waits for it to terminate. When the child finishes, the shell types the prompt again and tries to read the next input line. 
  • A more complicated command is: 
                cat file1 file2 file3 | sort > /dev/lp &
  •  This command concatenates three files and pipes them to the sort program. It then redirects the sorted file to a line printer. The ampersand “&” at the end of the command instructs UNIX to issue the command as a background job. This results in the command prompt being returned immediately, whilst another process carries out the requested work. 
  • Above command makes a series of system calls to the operating system in order to satisfy the whole request.



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. 

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). 

  • 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:


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