OS Introduction

Major functionality of OS.

1. Resource manager: When there are multiple user in a system then OS manages the resource like governing how much hardware to be provided to which user, so that system is not loaded.
2. Process management: OS manages how to execute all the multiple process running at same time.
   • Here we use various CPU scheduling.
   • in this OS decides which process gets time. This function is called process scheduling. An operating system does the following activities for process management:
     • Keeps track of processor and states of process. The program responsible for this task is known as traffic controller.
     • Allocates the processor (CPU) to a process.
     • De-allocates processor when a process in no longer required.
3. Storage management (HD): Basically includes secondary devices. It manages how to store permanent data in memory.
   • Storage management is done by "file mangement".
4. Memory management (RAM): managing how much memory of RAM to allocate or deallocate to different process as RAM is limited.
   • It refers to management of primary memory.
   • Main memory is a large array of bytes where each byte has its own address.
   • Main memory provides a fast storage that can be accessed directly by the CPU. For a program to be executed, it must be in the main memory.
   • Operating system keeps track of primary memory, i.e. what part of it are in use by whom, what part are not in use.
   • In multiprogramming, the OS decides which process will get memory when and how much.
   • Allocates the memory when a process requests it to do so.
   • De-allocates the memory when a process no longer needs it or has been terminated.
5. Device management: an operating system manages device communication via their respective device drivers.
   • OS does the following things:
     1. Keep tracks of all devices. The program responsible for this task is known as the I/O controller.
     2. Allocates the device in the most efficient way.
     3. De-allocates devices.
6. File management: a file system is normally organized into directories for easy navigation and usage. These directories may contain files and other directories.
   • O/S does ↓
     1. Keeps track of information, location, user, status, etc. The collective facilities often known as file system.
     2. Decides who gets the resources.
7. Security and privacy: practising authentication
   • By means of password and similar other techniques, it prevents unauthorized access to programs and data.
8. Control over system performance: records the delay between a request for a service and response from the system.
9. Job accounting: keeps track of time and resources used by various jobs and users.
10. Error detection aids: production of dumps, traces, error messages and other debugging and error detecting aids.
11. Co-ordination between other software and users: it involves managing the integration and utilization of compilers, interpreters, assemblers, and other software applications among the various users of computer systems.

Goals of OS

• Convenience: user can operate computer without knowing assembly language.
  • easy to use.
  • user friendly
• Efficient: able to perform task easily.
• Ability to evolve: ability to improve.
  • permit development, testing, sercurity and updation.

Properties or characteristics of OS:

• Reliability: reliability is that a system does exactly what it is designed to do.
  • At software level OS must be more reliable on detecting virus or malicious code as they take control of the system for their own purposes by exploiting design or implementation errors in the operating system's defenses.
• Availability: it is the percentage of time that the system is usable. Some system crashes frequently, losing the user's work. Both reliability & availability are desirable.
  • Availability is affected by two factors: the frequencry of failure called the mean time to failure (MTTF) and the time it takes to restore a system to a working state after failure (for example, to reboot), the MTTR (mean time to repair).
  • Availability can be improved by increasing the MTTF or reducing the MTTR.
• Ease of management: OS must be easy to manage as there are lots of help & tutorials are available to rectify the erros.
• Security: as OS has administrative user that can make changes to the computer but for other user to make changes he has to enter the password of administration.
• Associated utilities: OS must have lots of associated utilities available like windows OS has lots of version and according to their version lots of free utilities are available like MS Office + MS SQL + Developing Languages.
• Cost: OS cost should be accordance of its features like windows has around $100 where DOS is free of charge as so is LINUX.
• Support for the users: OS should have a help section for users if any error or problem occurs, like Windows has a proper help section and websites, whereas Linux has books in pdf format available and DOS has websites and a forum for help as DOS is not very popular.

The First Generation (1930 to early 1950s)

• When the first electronic computer was developed in 1940, it was created without any operating system.
• In early times, users have full access to the computer machine and write a program for each task in absolute machine language.
• The programmer can perform and solve only simple mathematical calculations during the computer generation, and this calculation does not require an operating system.

The Second Generation (1955 - 1965)

• The first operating system (OS) was created in the early 1950s and was known as GMOS. General Motors has developed OS for the IBM computer.
• The second-generation operating system was based on a single stream batch processing system because it collects all similar jobs in groups or batches and then submits the jobs to the operating system using a punch card to complete all jobs in a machine.
• At each completion of jobs (either normally or abnormally), control transfer to the operating system that is cleaned after completing one job and then continues to read and initiates the next job in a punch card. After that, new machines were called mainframes, which were very big and used by professional operators.

The Third Generation (1965 - 1980)

• During the late 1960s, operating system designers were very capable of developing a new operating system that could simultaneously perform multiple tasks in a single computer program called multiprogramming.
• The introduction of multiprogramming plays a very important role in developing operating systems that allow a CPU to be busy every time by performing different tasks on a computer at the same time.
• During the third generation, there was a new development of minicomputer's phenomenal growth starting in 1961 with the DEC PDP-1. These PDP's leads to the creation of personal computers in the fourth generation

The Fourth Generation (1980 - Present Day)

• The fourth generation of operating systems is related to the development of the personal computer. However, the personal computer is very similar to the minicomputers that were developed in the third generation. The cost of a personal computer was very high at that time; there were small fractions of minicomputers costs.
• A major factor related to creating personal computers was the birth of Microsoft and the Windows operating system. Microsoft created the first window operating system in 1975. After introducing the Microsoft Windows OS, Bill Gates and Paul Allen had the vision to take personal computers to the next level. Therefore, they introduced the MS-DOS in 1981; however, it was very difficult for the person to understand its cryptic commands.
• Today, Windows has become the most popular and most commonly used operating system technology. And then, Windows released various operating systems such as Windows 95, Windows 98, Windows XP and the latest operating system, Windows 7. Currently, most Windows users use the Windows 10 operating system. Besides the Windows operating system, Apple is another popular operating system built in the 1980s, and this operating system was developed by Steve Jobs, a co-founder of Apple. They named the operating system Macintosh OS or Mac OS.

Switching between User Mode and Kernel Mode

• A process gets executed in user mode and kernel mode in its lifetime. In the user mode, the process has limited access. On the other hand, the kernel mode is the privileged mode in which the process has free access to system resources such as memory, hardware etc.
• Services such as hardware I/O can be executed by accessing kernel data in kernel mode. Anything linked with I/O hardware management, process management and memory management needs a process to get executed in Kernel mode.
• In Kernel mode, a process gets the power to access any device and memory, but any kind of crash in kernel mode brings down the entire system. However, if there is any crash in user mode, it brings down only the faulty process.
• The kernel offers SCI (System Call Interface), which acts as entry points for the user processes to enter into kernel mode. System calls are the only method by which a process can go from user mode to kernel mode.

Why is switching required?

Two primary reasons for switching between user mode and kernel mode are

1. If every process were to run in a single mode only, it would end up with Microsoft's concern in the earlier versions of Windows. If any process were competent enough to exploit a vulnerability, then that process could control the system.
2. Some conditions are identified as a trap, a system fault or an exception usually caused by an exceptional condition like invalid memory access, division by zero etc. Such a trap situation can crash the whole operating system if the process is in kernel mode. A process running in user mode that comes across a trap situation crashes the user-mode process only.

Therefore, switching overhead is essential for a more stable and secure system.

Difference between User Mode and Kernel Mode

Single user operating system

• A single-user operating system is a type of operating system developed and intended for use on a computer or similar machine that will only have a single user at any given time.
• This type of OS is typically used on devices like wireless phones and two-way messaging devices.
• A single task operating system can only run one program or application at a time. So it is not as useful for a computer or other device intended to run multiple programs at once.
• This is where a multitasking single-user operating system is used instead. A multitasking OS can run multiple applications and programs at once.
• This is often used on computers where someone may want to navigate the internet, run a graphics editing program, play music through a media playing program, and type in notes in a simple word processing program all at the same time.
• A single task OS could not do this, but the multitasking systems can handle all these processes.

Multi-User OS

• A multi-user operating system is an operating system that permits several users to access a single system running to a single operating system.
• Here systems are frequently quite complex, and they must manage the tasks that the various users connected to them require.
• Users will usually sit at terminals or computers connected to the system via a network and other system machines like printers
• A multi-user operating system varies from a connected single-user operating system in that each user accesses the same operating system from different machines.
• The main goal of developing a multi-user operating system is to use it for time-sharing and batch processing on mainframe systems. This multi-user operating system is now often used in large organizations, the government sector, educational institutions like large universities, and on servers side such as Ubuntu Server or Windows Server. These servers allow several users to access the operating system, kernel, and hardware at the same time.
• It is usually responsible for handling memory and processing for other running programs, identifying and using system hardware, and efficiently handling user interaction and data requests. It's specially important for an operating system, a multi-user operating system because several users rely on the system to function properly at the same time.

Batch OS

• In the 1970s, Batch processing was very popular. In this technique, similar types of jobs were batched together and executed in time. People were used to having a single computer which was called a mainframe.
• In Batch operating system, access is given to more than one person; they submit their respective jobs to the system for the execution.
• The system put all of the jobs in a queue on the basis of first come first serve and then executes the jobs one by one. The users collect their respective output when all the jobs get executed.
• The user of this type of OS does not interact with the computer directly.
• Each user prepares his job on an offline device like puch cards and submits it to the computer operator.
• There is an operator which takes similar jobs having the same requirement and group them into batch.

• The purpose of this OS was mainly to transfer control from one job to another as soon as the job was completed.
• It contained a small set of programs called the resident monitor that always resided in one part of the main memory. The remaining part is used for servicing jobs.

Multiprogramming OS

• Multiprogramming is an extension to batch processing where the CPU is always kept busy. Each process needs two types of system time: CPU time and IO time.
• In a multiprogramming environment, when a process does its I/O, the CPU can start the execution of other process. Therefore, multiprogramming improves the efficiency of the system.

Multiprocessing OS

• In multiprocessing, parallel computing is achieved.
• There are more then one processors present in the system which can execute more than one process at the same time.
• This will increase the throughput of the system.

Distributed OS

• A distributed system is a model where distributed applications are running on multiple computers linked by a communications network.
• Sometimes it is also called loosely coupled systems because in which each processor has its own local memory and processing units.
• LOCUS and MICROS are some examples of distributed operating systems.

Difference between Distributed and parallel system:

Real-Time OS

• A real-time operating system is a type of operating system designed to serve real-time applications that process data as it arrives.
• It completes a task within a specific time.
• The logical result of computation and the time required to produce the result determine the correctness of the system output.
• It includes methods for real-time task scheduling
• It is primarily used on embedded systems.
• It is highly useful for timing applications or activities that are performed within a particular time limit. It uses strict time limits to drive task execution in an external environment.

Applications of RTOS

• Radar gadget.
• Missile guidance.
• Traffic manipulate gadget.
• Autopilot travel simulators.

Types of RTOS

• Hard Real-Time OS: All critical tasks must be completed within the specified time duration, i.e., within the given deadline. Not meeting the deadline would result in critical failures such as damage to equipment or even loss of human life.
• Soft Real-Time OS: It accepts a few delays via the means of the Operating system. In this kind of RTOS, there may be a closing date needed for a particular job, but a delay for a small amount of time is acceptable. So, cut off dates are treated softly via means of this kind of RTOS.

Simple Structure

• There are many operating systems that have a rather simple structure.
• These started as small systems and rapidly expanded much further than their scope.
• A common example of this is MS-DOS. It was designed for a niche amount for people, there was no indication that it would become so popular.

OS structure

• It is better that operating systems have a modular structure, unlike MS-DOS. That would lead to greater control over the computer system and its various applications. The modular structure would also allow the programmers to hide information as required and implement internal routines as they see it without changing the outer specifications.

Layered Structure

• One way to achieve modularity in the OS is the layered approach.
• In this, the bottom layer is the hardware and the topmost layer is the user interface.
• An image demonstrating the layered approach is as follows ↓

Monolithic kernel Structure

• The monolithic operating system is a very basic operating system in which file management, memory management, device management, and process management are directly controlled within the kernel.
• The kernel can access all the resources present in the system.
• In monolithic systems, each component of the operating system is contained within the kernel. Operating systems that use monolithic architecture were first time used in the 1970s.
• Example: LINUX.

Micro Kernel Structure

• In this structure the services like file sharing, scheduling, kernel services etc. have their particular address allotted for them which results in the reduction of the size of kernel and the operating system.
• Example: Minix, Symbian OS.

Program execution

• To execute a program, several tasks need to be performed. Both the instructions and data must be loaded into the main memory.
• In addition, input-output devices and files should be initialized, and other resources must be prepared.
• The Operating structures handle these kinds of tasks. The user now no longer should fear the reminiscence allocation or multitasking or anything.

Control Input/Output devices

• As there are numerous types of I/O devices within the computer system, and each I/O device calls for its own precise set of instructions for the operation.
• The Operating System hides that info with the aid of presenting a uniform interface. Thus, it is convenient for programmers to access such devices easily.

Program Creation

• The Operating system offers the structures and tools, including editors and debuggers, to help the programmer create, modify, and debugging programs.

Error detection and response

• An Error in a device may also cause malfunctioning of the entire device.
• These include hardware and software errors such as device failure, memory error, division by zero, attempts to access forbidden memory locations, etc.
• To avoid error, the operating system monitors the system for detecting errors and takes suitable action with at least impact on running applications.

While working with computers, errors may occur quite often. Erros may occur in the:

• Input/Output devices: For example, connection failure in the network, lack of paper in the printer, etc.
• User program: For example - attempt to access illegal memory location, divide by zero, use too much CPU time, etc.
• Memory hardware: For exmple, memory error, the memory becomes full, etc.

To handle these errors and other types of possible errors, the operating system takes appropriate action and generates message to ensure correct and consistent computing.

Accounting

• An Operating device collects utilization records for numerous assets and tracks the overall performance parameters and responsive time to enhance overall performance.
• These personal records are beneficial for additional upgrades and tuning the device to enhance overall performance.

Security and protection

• Operating device affords safety to the statistics and packages of a person and protects any interference from unauthorized users.
• The safety feature counters threats, which are published via way of individuals out of doors the manage of the running device.

For example:

• When a user downloads something from the internet, that program may contain malicious code that may harm the already existing programs. The operating system ensures that proper checks are applied while downloading such programs.
• f one computer system is shared amongst a couple of users, then the various processes must be protected from another intrusion. For this, the operating system provides various mechanisms that allow only those processes to use resources that have gained proper authorization from the operating system. The mechanism may include providing unique users ids and passwords to each user.

File management

• Computers keep data and information on secondary storage devices like magnetic tape, magnetic disk, optical disk, etc. Each storage media has its capabilities like speed, capacity, data transfer rate, and data access methods.
• For file management, the operating system must know the types of different files and the characteristics of different storage devices. It has to offer the proportion and safety mechanism of documents additionally.

Communication

• The OS manages the exchange of data and programs among different computers connected over a network. This communication using message passing and shared memory.

Making a system call

• Now system calls are directly available and used in high level languages like C & C++. So it has become easy to use system calls in programs.
• For a programmers, system calls are same as calling a procedure or function.
• The difference between a system call and a normal function call is that a system call enter a kernel but a normal function call doesn't.

Executing the system call

• There is a sequence of steps to execute a system call.
• For this, there is the need to pass various parameters of system call to the OS.
• For passing these parameters to the OS, three methods are used as follow :
  1. Register method, where in the parameters are stored in registers of the CPU.
  2. If parameters are not in number, compared to the size of registers, a block of memory is used and the address of that block is stored in register.
  3. Stack method, where in parameters are pushed onto stack and popped of by the OS.

Sequence in which system call is executed

1. In the user program when the system call is executed, first of all, its parameters are pushed onto the stack and later on saved in processor registers.
2. The corresponding library procedure for the system call is executed.
3. There is a particular code for every system call by which the kernel identifies which system call function or handler needs to be executed. Therefore library procedure places the system call number in the processor register.
4. Then the library procedure traps to the kernel by executing interrupt instruction, with this interrupt execution, the user mode switches to kernel mode by loading program status word (PSW) register to 0.
5. The hardware saves the current contents of CPU registers, so that after executing the system call, the execution of the rest of the program can be resumed.
6. The kernel identifies the system call by examining its number and dispatches the control to the corresponding system call handler.
7. The system call handler executes.
8. On completion of system call handler, the control is returned to the user program and it resumes its execution.

Why do you need system calls in OS?

• It is must require when a file system wants to create or delete a file.
• Network connections require the system calls to sending and receiving data packets.
• If you want to read or write a file, you need system calls.
• If you want to access hardware devices, including a printer, scanner, you need a system call.
• System calls are used to create and manage new processes.

Types of system calls

Sysmtem calls vs library function

1. A system call is a request made by a program to the operating system kernel to perform an operation on its behalf, such as creating a new process, reading or writing a file, or allocating memory. System calls are implemented by the operating system and are executed in kernel mode, which provides the necessary privileges to access system resources.
2. On the other hand, a library function is a pre-written block of code that performs a specific task and is packaged with the application. Library functions are executed in user mode and provide a higher-level abstraction than system calls. For example, the C standard library provides functions such as printf() and scanf() for performing input and output operations, which internally make system calls to access system resources.

• System calls are typically used for low-level operations that require direct access to system resources, while library functions provide a higher-level interface that simplifies common tasks and abstracts away system-specific details.

Examples of Windows and Unix system calls

Answer ↓

• While the primary goal of an operating system is to make efficient use of computing hardware, there may be situations where it is appropriate for the operating system to "waste" resources. One such situation is when the operating system is providing a high level of security or reliability. In this case, the operating system may need to use more resources than necessary to ensure that the system is secure and reliable.
• For example, if an operating system is running a critical application, it may need to allocate more resources to that application to ensure that it runs smoothly and does not crash. This may mean that other applications running on the system may have less resources available to them, but the priority is to ensure that the critical application is able to function as required.
• Another example is when the operating system is running in a virtualized environment. In this case, the operating system may need to allocate more resources to the virtual machines running on it to ensure that they are isolated from each other and have enough resources to function properly. This may mean that the host operating system itself has less resources available to it, but the priority is to ensure that the virtual machines are able to function as required.
• Despite the apparent waste of resources in these situations, it is important to remember that the operating system is still serving a purpose. By allocating more resources to certain tasks, the operating system is ensuring that the overall system is functioning as required, which can lead to increased security, reliability, and performance. Therefore, while it may seem wasteful at first glance, such a system is not truly wasteful as it is still serving a valuable purpose in the larger context of the system.

Answer ↓

• A user may be better off using a time-sharing system rather than a PC or a single-user workstation under the following circumstances:
  1. Limited hardware resources: If the user has limited access to computing hardware, such as memory, storage, or processing power, a time-sharing system can provide access to shared resources that are not available on their personal device. This can be particularly beneficial for complex tasks that require more resources than a user's personal device can provide.
  2. Collaborative work: If the user needs to collaborate with other users on a project or task, a time-sharing system can provide a shared environment where multiple users can work on the same project simultaneously. This can be particularly beneficial for tasks that require multiple users to work on the same file or program.
  3. Remote access: If the user needs to access computing resources from a remote location, a time-sharing system can provide remote access to computing resources via the internet or other network connections. This can be particularly beneficial for users who travel frequently or work remotely.
  4. Software availability: If the user needs access to specialized software or applications that are not available on their personal device, a time-sharing system can provide access to shared resources that have the required software or applications. This can be particularly beneficial for users who need access to specialized software for a limited period of time or for infrequent use.
  5. Cost-effectiveness: If the user does not have the financial resources to purchase or maintain their own personal device, a time-sharing system can provide access to computing resources at a lower cost than purchasing and maintaining their own personal device. This can be particularly beneficial for users who have limited financial resources or who only need access to computing resources for a limited period of time.

Answer ↓

• In a multi-programming and time-sharing environment, several users share the system simultaneously, which can result in various security problems. Two such problems are:
  1. Unauthorized access: In a shared system, it is possible for users to gain unauthorized access to data or resources that they are not authorized to access. This can happen if a user gains access to another user's account or if a user exploits a vulnerability in the system to gain access to resources that they should not be able to access. Unauthorized access can lead to data breaches, theft of intellectual property, and other security incidents.
  2. Data confidentiality: In a shared system, it is possible for users to access or view data that they are not authorized to view. This can happen if a user gains access to another user's files or if a user is able to intercept network traffic and view sensitive data in transit. Data confidentiality is important for protecting sensitive information, such as personal identifying information, financial data, and intellectual property.
  • To mitigate these security problems, it is important to implement appropriate security measures, such as access control mechanisms, encryption, and monitoring and auditing tools. Users should also be educated on best practices for maintaining security in a shared environment, such as choosing strong passwords, logging out of shared systems when not in use, and reporting any suspicious activity to system administrators.
• Ensuring the same degree of security in a time-shared machine as in a dedicated machine can be challenging, but it is possible to achieve a high level of security in both environments with appropriate security measures.
  • In a dedicated machine, there is typically only one user who has access to the system, which can make it easier to enforce security policies and control access to resources. In contrast, in a time-shared environment, multiple users share the same system, which can make it more difficult to enforce security policies and control access to resources. Additionally, in a time-shared environment, there is a greater risk of unauthorized access or data breaches, since multiple users have access to the same resources.
  • To ensure a high degree of security in a time-shared machine, it is important to implement appropriate security measures, such as:
    1. Access control mechanisms: To prevent unauthorized access, access control mechanisms such as passwords, encryption, and multi-factor authentication can be implemented. User accounts should be created with appropriate access levels and permissions, and users should only be granted access to the resources they need to perform their tasks.
    2. Encryption: To protect sensitive data, encryption should be used to ensure that data is protected when it is transmitted or stored on the system.
    3. Monitoring and auditing tools: To detect and prevent security incidents, monitoring and auditing tools can be used to track user activity and identify any suspicious behavior.
    4. Regular security updates and patches: To prevent known vulnerabilities from being exploited, regular security updates and patches should be applied to the system.
  • While it may be more challenging to ensure the same degree of security in a time-shared machine as in a dedicated machine, by implementing appropriate security measures, it is possible to achieve a high level of security in both environments.