History Of Computer Operating Systems

OPERATING SYSTEMS ASSIGNMENT # 1

History of Operating Systems & Directory Structure of Linux Submitted By:

Abdul Basit Roll# 925 BS(CS) 3rd Semester Submitted To:

Miss Mobeen

CONTENTS:

History of Computer Operating Systems • • • • • •

Stacked Job Batch Systems (mid 1950s-mid 1960s) Spooling Batch Systems (mid 1960s-late 1970s) Multiprogramming Systems (1960s-present) Timesharing Systems (1970s-present) Personal Computers Real-Time, Multiprocessor, and Distributed/Networked Systems

Directory Structure of LINUX • • • • • • • • • • • • • •

“/” “ /bin ” “ /boot ” “ /dev ” “ /etc ” “ /home ” “ /lib ” “ /mnt ” “ /opt ” “ /root ” “ /sbin ” “ /tmp ” “ /usr ” “ /var ”

History of Computer Operating Systems Stacked Job Batch Systems (mid 1950s - mid 1960s) A batch system is one in which jobs are bundled together with the instructions necessary to allow them to be processed without intervention. Often jobs of a similar nature can be bundled together to further increase economy The basic physical layout of the memory of a batch job computer is shown below: -------------------------------------| | | Monitor (permanently resident) | | | -------------------------------------| | | User Space | | (compilers, programs, data, etc.) | | | --------------------------------------

The monitor is system software that is responsible for interpreting and carrying out the instructions in the batch jobs. When the monitor started a job, it handed over control of the entire computer to the job, which then controlled the computer until it finished. A sample of several batch jobs might look like: $JOB user_spec purposes $FORTRAN source program cards $LOAD $RUN data cards $EOJ $JOB user_spec $LOAD application $RUN data $EOJ

;

identify

the

user

for

accounting

; load the FORTRAN compiler ; load the compiled program ; run the program ; end of job ; identify a new user

Often magnetic tapes and drums were used to store intermediate data and compiled programs. 1. Advantages of batch systems o move much of the work of the operator to the computer

increased performance since it was possible for job to start as soon as the previous job finished 2. Disadvantages o turn-around time can be large from user standpoint o more difficult to debug program o due to lack of protection scheme, one batch job can affect pending jobs (read too many cards, etc) o a job could corrupt the monitor, thus affecting pending jobs o a job could enter an infinite loop o

As mentioned above, one of the major shortcomings of early batch systems was that there was no protection scheme to prevent one job from adversely affecting other jobs. The solution to this was a simple protection scheme, where certain memory (e.g. where the monitor resides) were made off-limits to user programs. This prevented user programs from corrupting the monitor. To keep user programs from reading too many (or not enough) cards, the hardware was changed to allow the computer to operate in one of two modes: one for the monitor and one for the user programs. IO could only be performed in monitor mode, so that IO requests from the user programs were passed to the monitor. In this way, the monitor could keep a job from reading past it's on $EOJ card. To prevent an infinite loop, a timer was added to the system and the $JOB card was modified so that a maximum execution time for the job was passed to the monitor. The computer would interrupt the job and return control to the monitor when this time was exceeded.

Spooling Batch Systems (mid 1960s - late 1970s) One difficulty with simple batch systems is that the computer still needs to read the the deck of cards before it can begin to execute the job. This means that the CPU is idle (or nearly so) during these relatively slow operations. Since it is faster to read from a magnetic tape than from a deck of cards, it became common for computer centers to have one or more less powerful computers in addition to there main computer. The smaller computers were used to read a decks of cards onto a tape, so that the tape would contain many batch jobs. This tape was then loaded on the main computer and the jobs on the tape were executed. The output from the jobs would be written to another tape which would then be removed and loaded on a less powerful computer to produce any hardcopy or other desired output. It was a logical extension of the timer idea described above to have a timer that would only let jobs execute for a short time before interrupting them so that the monitor could start an IO operation. Since the IO operation could proceed while the CPU was crunching on a user program, little degradation in performance was noticed.

Since the computer can now perform IO in parallel with computation, it became possible to have the computer read a deck of cards to a tape, drum or disk and to write out to a tape printer while it was computing. This process is called SPOOLing: Simultaneous Peripheral Operation OnLine. Spooling batch systems were the first and are the simplest of the multiprogramming systems. One advantage of spooling batch systems was that the output from jobs was available as soon as the job completed, rather than only after all jobs in the current cycle were finished.

Multiprogramming Systems (1960s - present) As machines with more and more memory became available, it was possible to extend the idea of multiprogramming (or multiprocessing) as used in spooling batch systems to create systems that would load several jobs into memory at once and cycle through them in some order, working on each one for a specified period of time. -------------------------------------| Monitor | | (more like a operating system) | -------------------------------------| User program 1 | -------------------------------------| User program 2 | -------------------------------------| User program 3 | -------------------------------------| User program 4 | --------------------------------------

At this point the monitor is growing to the point where it begins to resemble a modern operating system. It is responsible for: • • • • •

starting user jobs spooling operations IO for user jobs switching between user jobs ensuring proper protection while doing the above

As a simple, yet common example, consider a machine that can run two jobs at once. Further, suppose that one job is IO intensive and that the other is CPU intensive. One way for the monitor to allocate CPU time between these jobs would be to divide time equally between them. However, the CPU would be idle much of the time the IO bound process was executing.

A good solution in this case is to allow the CPU bound process (the background job) to execute until the IO bound process (the foreground job) needs some CPU time, at which point the monitor permits it to run. Presumably it will soon need to do some IO and the monitor can return the CPU to the background job.

Timesharing Systems (1970s - present) Back in the days of the "bare" computers without any operating system to speak of, the programmer had complete access to the machine. As hardware and software was developed to create monitors, simple and spooling batch systems and finally multiprogrammed systems, the separation between the user and the computer became more and more pronounced. Users, and programmers in particular, longed to be able to "get to the machine" without having to go through the batch process. In the 1970s and especially in the 1980s this became possible two different ways. The first involved timesharing or timeslicing. The idea of multiprogramming was extended to allow for multiple terminals to be connected to the computer, with each inuse terminal being associated with one or more jobs on the computer. The operating system is responsible for switching between the jobs, now often called processes, in such a way that favored user interaction. If the context-switches occurred quickly enough, the user had the impression that he or she had direct access to the computer. Interactive processes are given a higher priority so that when IO is requested (e.g. a key is pressed), the associated process is quickly given control of the CPU so that it can process it. This is usually done through the use of an interrupt that causes the computer to realize that an IO event has occurred. It should be mentioned that there are several different types of time sharing systems. One type is represented by computers like our VAX/VMS computers and UNIX workstations. In these computers entire processes are in memory (albeit virtual memory) and the computer switches between executing code in each of them. In other types of systems, such as airline reservation systems, a single application may actually do much of the timesharing between terminals. This way there does not need to be a different running program associated with each terminal.

Personal Computers The second way that programmers and users got back at the machine was the advent of personal computers around 1980. Finally computers became small enough and inexpensive enough that an individual could own one, and hence have complete access to it.

Real-Time, Multiprocessor, and Distributed/Networked Systems A real-time computer is one that execute programs that are guaranteed to have an upper bound on tasks that they carry out. Usually it is desired that the upper bound be very small. Examples included guided missile systems and medical monitoring equipment. The operating system on real-time computers is severely constrained by the timing requirements. Dedicated computers are special purpose computers that are used to perform only one or more tasks. Often these are real-time computers and include applications such as the guided missile mentioned above and the computer in modern cars that controls the fuel injection system. A multiprocessor computer is one with more than one CPU. The category of multiprocessor computers can be divided into the following sub-categories: •

•

shared memory multiprocessors have multiple CPUs, all with access to the same memory. Communication between the the processors is easy to implement, but care must be taken so that memory accesses are synchronized. distributed memory multiprocessors also have multiple CPUs, but each CPU has it's own associated memory. Here, memory access synchronization is not a problem, but communication between the processors is often slow and complicated.

Related to multiprocessors are the following: •

•

networked systems consist of multiple computers that are networked together, usually with a common operating system and shared resources. Users, however, are aware of the different computers that make up the system. distributed systems also consist of multiple computers but differ from networked systems in that the multiple computers are transparent to the user. Often there are redundant resources and a sharing of the workload among the different computers, but this is all transparent to the user.

Goals of Operating System 

Job Management Job management controls the order and time in which programs are run and is more sophisticated in the mainframe environment where scheduling the daily work has always been routine. IBM's job control language (JCL) was developed decades ago. In a desktop environment, batch files can be written to perform a sequence of operations which can be scheduled to start at a given time.



Task Management Multitasking, which is the ability to simultaneously execute multiple programs, is available in all operating systems today. Critical in the mainframe and large server environment, applications can be prioritized to run faster or slower depending on their purpose. In the desktop world, multitasking is necessary just for keeping several applications open at the same time so you can bounce back and forth among them. See multitasking.



Data Management Data management keeps track of the data on disk, tape and optical storage devices. The application program deals with data by file name and a particular location within the file. The operating system's file system knows where that data are physically stored (which sectors on disk) and interaction between the application and operating system is through the programming interface. Whenever an application needs to read or write data, it makes a call to the operating system.



Device Management Device management controls peripheral devices by sending them commands in their own proprietary language. The software routine that knows how to deal with each device is called a "driver." The operating system contains all the drivers for the peripherals attached to the computer. When a new peripheral is added, that device's driver is installed into the operating system.



Security Multiuser operating systems provide password protection to keep unauthorized users out of the system. Large operating systems also maintain activity logs and accounting of the user's time for billing purposes. They also provide backup and recovery routines for starting over in the event of a system fail.

Linux Directory Structure One of the best ways to come to terms with a new operating system is to get familiar with the way that the system organizes its files. Files are usually stored in directories which are arranged in a hierarchal tree structure. This is commonly called the directory structure. Here is an example of a common Linux directory structure represented by the KDE desktop. This should look similar to what you would see with most other visual operating systems. Linux uses "The Filesystem Hierarchy Standard". There will be slight variations but a lot of what you see here is common to many Unix-based systems. Most Linux distributions such as Red Hat, Mandrake, Suse, Debian, etc, use this file system, or at least something very close to it. It should be noted, you can build your own Linux system any way that you want (Free software by definition). There is no strict requirement that you use any particular structure, however, other users will have difficulty using your system and it could turn into a maintenance nightmare very quickly. Some of the directories shown here will be of little interest to many Linux users. For most users, other then root, their primary concern will be their home directory which can be structured any way that is convenient. Windows users will notice that there are no hard drive distinctions. The directory structure shown actually represents a system which has 3 hard drives. With Unixbased systems, drives are not shown. File systems are mounted to particular drives but for the user, the actual implementation is transparent. This approach allows files to be presented to the user in a more centralized view even though the files may actually be spread out among several hard drives and partitions. The result is better security and protection against system crashes and data loss. Lets take a look at each directory and what it is used for: "/" This is a mandatory directory. It is the beginning of the filesystem and includes all of the directories beneath it. If you type cd / and then ls you will see a listing of all directories on the system (That you have permission to see). This should be similar to the graphic above. /bin This is a mandatory directory. This directory contains the binaries that are used in single-user systems. For multi-user systems these binaries are usually stored in the /usr/bin directory. When you type a command such as ls or chmod it is usually directed to one of these two directories where the program exists.

/boot This is a mandatory directory. This directory stores the files which are used for the system startup except for the configuration and map installer. Frequently the kernel is stored here especially if more then one kernel is installed. /dev This is a mandatory directory. The device files, sockets, and named pipes are stored here. /etc This is a mandatory directory. This directory, pronounced "et-see", holds the configuration files for the system. It is divided into many subdirectories. /home This is an optional but widely used directory. The other variation on the /home directory is to use a subdirectory in the /var directory. This is where users will do most of their work. Each user is given their own directory in the /home directory which is theirs to organize and use as they choose. Frequently web server document roots are located in the /home directory. (ex. /home/public_html or /home/www/public_html) The /home directory is designed to host dynamically changing files and usually occupies one of the larger partitions on a hard disk. /lib This is a mandatory directory. Shared libraries needed at bootup or which need to be run by top level commands are stored here. Libraries which support users are usually stored in the /usr/lib directory. /mnt This is an optional but very popular directory. This directory contains mount points for external storage devices. To access a floppy disk drive you cd to mnt/floppy. Once an external drive is accessed, its file system is mounted to the host system in the /mnt directory. /opt This is an optional directory. It is a directory intended to contain software packages which are added to the original system. On my system it is present, but empty. /proc This is an optional but widely used directory. It contains a virtual filesystem which is created and used by the currently running kernel. It is deleted when the system is shut down. Frequently, monitoring programs use the /proc directory to obtain information on currently running processes and other environmental information. /root This is an optional but widely used directory. It is often created to eliminate clutter from the "/" directory. It contains configuration files for the root user. /sbin This is a mandatory directory. This directory was originally a place to store static binaries. It has been expanded to include administrative binaries which are used by the root user only.

/tmp This is a mandatory directory. This directory is used by programs to store temporary files. Files which are located here are often flushed on reboot or flushed periodically. /usr This is a mandatory directory. The /usr directory is designed to store static, sharable, read-only data. Programs which are used by all users are frequently stored here. Data which results from these programs is usually stored elsewhere (often /var). /var This is a mandatory directory. This directory stores variable data like logs, mail, and process specific files. Most, but not all, subdirectories and files in the /var directory are shared. This is another popular location for web server document roots.

REFERENCES http://www.personal.kent.edu/~rmuhamma/OpSystems/Myos/osHistory.htm http://www.math-cs.gordon.edu/courses/cs322/lectures/history.html http://www.osdata.com/kind/history.htm http://en.wikipedia.org/wiki/History_of_operating_systems http://doc.vic.computerbank.org.au/tutorials/linuxdirectorystructure/ http://www.geocities.com/sunnylug/lindir.html http://linux.omnipotent.net/article.php?article_id=9027 http://www.dynamic-apps.com/linux_directories.jsp http://www.comptechdoc.org/os/linux/usersguide/linux_ugfilestruct.html http://www.cae.wisc.edu/site/public/?title=lindir http://www.tuxfiles.org/linuxhelp/linuxdir.html