CSC204 Operating Systems I Notes PDF

CSC204: Operating Systems I Introduction to Operating Systems Introduction to Operating System An operating system acts as an intermediary between the user of a computer and computer hardware. The purpose of an operating system is to provide an environment in which a user can execute programs in a...

CSC204: Operating Systems I Introduction to Operating Systems Introduction to Operating System An operating system acts as an intermediary between the user of a computer and computer hardware. The purpose of an operating system is to provide an environment in which a user can execute programs in a convenient and efficient manner. An operating system is a software that manages the computer hardware. The hardware must provide appropriate mechanisms to ensure the correct operation of the computer system and to prevent user programs from interfering with the proper operation of the system. Operating System Definition: An operating system is a program that controls the execution of application programs and acts as an interface between the user of a computer and the computer hardware. A more common definition is that the operating system is the one program running at all times on the computer (usually called the kernel), with all else being application programs. An operating system is concerned with the allocation of resources and services, such as memory, processors, devices, and information. The operating system correspondingly includes programs to manage these resources, such as a traffic controller, a scheduler, memory management module, I/O programs, and a file system. Functions of Operating system Operating system performs three functions: 1. Convenience: An OS makes a computer more convenient to use. 2. Efficiency: An OS allows the computer system resources to be used in an efficient manner. 3. Ability to Evolve: An OS should be constructed in such a way as to permit the effective development, testing and introduction of new system functions at the same time without interfering with service. Operating system as User Interface – 1. User 2. System and application programs 3. Operating system 4. Hardware Every general-purpose computer consists of the hardware, operating system, system programs, and application programs. The hardware consists of memory, CPU, ALU, and I/O devices, peripheral device, and storage device. System program consists of compilers, loaders, editors, OS, etc. The application program consists of business programs, database programs. Page 1 https://eng.libretexts.org/@go/page/82836  Figure : Conceptual view of a computer system 2.1.1 Every computer must have an operating system to run other programs. The operating system coordinates the use of the hardware among the various system programs and application programs for various users. It simply provides an environment within which other programs can do useful work. The operating system is a set of special programs that run on a computer system that allows it to work properly. It performs basic tasks such as recognizing input from the keyboard, keeping track of files and directories on the disk, sending output to the display screen and controlling peripheral devices. OS is designed to serve two basic purposes: 1. It controls the allocation and use of the computing System’s resources among the various user and tasks. 2. It provides an interface between the computer hardware and the programmer that simplifies and makes feasible for coding, creation, debugging of application programs. The Operating system must support the following tasks. The task are: 1. Provides the facilities to create, modification of programs and data files using an editor. 2. Access to the compiler for translating the user program from high level language to machine language. 3. Provide a loader program to move the compiled program code to the computer’s memory for execution. 4. Provide routines that handle the details of I/O programming. I/O System Management One of the important jobs of an Operating System is to manage the operations of various I/O devices including mouse, keyboards, touch pad, disk drives, display adapters, USB devices, Bit-mapped screen, LED, Analog-to-digital converter, On/off switch, network connections, audio I/O, printers etc. The I/O system of an OS works by taking I/O request from an application software and sending it to the physical device, which could be an input or output device then it takes whatever response comes back from the device and sends it to the application. Components of I/O Hardware I/O Device Device Driver Device Controller I/O Device: I/O devices such as storage, communications, user-interface, and others communicate with the computer via signals sent over wires or through the air. Devices connect with the computer via ports, e.g. a serial or parallel port. A common set of wires connecting multiple devices is termed a bus. I/O devices can be divided into two categories: Page 2 https://eng.libretexts.org/@go/page/82836  Block devices − A block device is one with which the device driver communicates by sending entire blocks of data. For example, Hard disks, USB cameras, Disk-OnKey etc. Character devices − A character device is one with which the device driver communicates by sending and receiving single characters (bytes, octets). For example, serial ports, parallel ports, sounds cards etc Device Driver: Device drivers are software modules that can be plugged into an OS to handle a particular device. Operating System takes help from device drivers to handle all I/O devices. Device Controller: The Device Controller works like an interface between a device and a device driver. I/O units (Keyboard, mouse, printer, etc.) typically consist of a mechanical component and an electronic component where electronic component is called the device controller. History of Operating system Operating system has been evolving through the years. Following Table shows the history of OS. Generation Year Electronic device used Types of OS Device First 1945-55 Vacuum Tubes Plug Boards Second 1955-65 Transistors Batch Systems Third 1965-80 Integrated Circuits(IC) Multiprogramming Fourth Since 1980 Large Scale Integration PC Page 3 Starting with the Basics Processor The processor is an important part of a computer architecture, without it nothing would happen. It is a programmable device that takes input, perform some arithmetic and logical operations and produce some output. In simple words, a processor is a digital device on a chip which can fetch instruction from memory, decode and execute them and provide results. Basics of a Processor – A processor takes a bunch of instructions in machine language and executes them, telling the processor what it has to do. Processors performs three basic operations while executing the instruction: 1. It performs some basic operations like addition, subtraction, multiplication, division and some logical operations using its Arithmetic and Logical Unit (ALU). 2. Data in the processor can move from one location to another. 3. It has a Program Counter (PC) register that stores the address of next instruction based on the value of PC. A typical processor structure looks like this. Figure 2.2.1 : Von Neumann Architecture. Basic Processor Terminology Control Unit (CU) A control unit (CU) handles all processor control signals. It directs all input and output flow, fetches the code for instructions and controlling how data moves around the system. Arithmetic and Logic Unit (ALU) The arithmetic logic unit is that part of the CPU that handles all the calculations the CPU may need, e.g. Addition, Subtraction, Comparisons. It performs Logical Operations, Bit Shifting Operations, and Arithmetic Operation. Main Memory Unit (Registers) 1. Accumulator (ACC): Stores the results of calculations made by ALU. 2. Program Counter (PC): Keeps track of the memory location of the next instructions to be dealt with. The PC then passes this next address to Memory Address Register (MAR). 3. Memory Address Register (MAR): It stores the memory locations of instructions that need to be fetched from memory or stored into memory. Page 4  4. Memory Data Register (MDR): It stores instructions fetched from memory or any data that is to be transferred to, and stored in, memory. 5. Current Instruction Register (CIR): It stores the most recently fetched instructions while it is waiting to be coded and executed. 6. Instruction Buffer Register (IBR): The instruction that is not to be executed immediately is placed in the instruction buffer register IBR. Input/Output Devices – Program or data is read into main memory from the input device or secondary storage under the control of CPU input instruction. Output devices are used to output the information from a computer. Buses – Data is transmitted from one part of a computer to another, connecting all major internal components to the CPU and memory, by the means of Buses. Types: 1. Data Bus (Data): It carries data among the memory unit, the I/O devices, and the processor. 2. Address Bus (Address): It carries the address of data (not the actual data) between memory and processor. 3. Control Bus (Control and Timing): It carries control commands from the CPU (and status signals from other devices) in order to control and coordinate all the activities within the computer. Memory Memory attached to the CPU is used for storage of data and instructions and is called internal memory The internal memory is divided into many storage locations, each of which can store data or instructions. Each memory location is of the same size and has an address. With the help of the address, the computer can read any memory location easily without having to search the entire memory. when a program is executed, it’s data is copied to the internal memory and is stored in the memory till the end of the execution. The internal memory is also called the Primary memory or Main memory. This memory is also called as RAM, i.e. Random Access Memory. The time of access of data is independent of its location in memory, therefore this memory is also called Random Access memory (RAM). I/O Modules The method that is used to transfer information between main memory and external I/O devices is known as the I/O interface, or I/O modules. The CPU is interfaced using special communication links by the peripherals connected to any computer system. These communication links are used to resolve the differences between CPU and peripheral. There exists special hardware components between CPU and peripherals to supervise and synchronize all the input and output transfers that are called interface units. Mode of Transfer: The binary information that is received from an external device is usually stored in the memory unit. The information that is transferred from the CPU to the external device is originated from the memory unit. CPU merely processes the information but the source and target is always the memory unit. Data transfer between CPU and the I/O devices may be done in different modes. Data transfer to and from the peripherals may be done in any of the three possible ways 1. Programmed I/O: is the result of the I/O instructions written in the program's code. Each data transfer is initiated by an instruction in the program. Usually the transfer is from a CPU register and/or memory. In this case it requires constant monitoring by the CPU of the peripheral devices. 2. Interrupt- initiated I/O: using an interrupt facility and special commands to issue an interrupt request signal whenever data is available from any device. In the meantime the CPU can proceed processing other programs. The interface meanwhile keeps monitoring the device. When it is determined that the device is ready for a data transfer it initiates an interrupt request signal to the CPU. Upon detection of an external interrupt signal the CPU momentarily stops the task it was processing, and services program that was waiting on the interrupt to process the I/O transfer> Once the interrupt is satisfied, the CPU then return to the task it was originally processing. 3. Direct memory access( DMA): The data transfer between a fast storage media such as magnetic disk and main memory is limited by the speed of the CPU. Thus we can allow the peripherals directly communicate with each other using the memory buses, removing the intervention of the CPU. This type of data transfer technique is known as direct memory access, or DMA. During DMA the CPU is idle and it has no control over the memory buses. The DMA controller takes over the buses to manage the transfer directly between the I/O devices and the memory unit. Page 5 The Role of the Operating System The role of the operating system The operating system underpins the entire operation of the modern computer. Abstraction of hardware The fundamental operation of the operating system (OS) is to abstract the hardware to the programmer and user. The operating system provides generic interfaces to services provided by the underlying hardware. In a world without operating systems, every programmer would need to know the most intimate details of the underlying hardware to get anything to run. Worse still, their programs would not run on other hardware, even if that hardware has only slight differences. Multitasking We expect modern computers to do many different things at once, and we need some way to arbitrate between all the different programs running on the system. It is the operating systems job to allow this to happen seamlessly. The operating system is responsible for resource management within the system. Many tasks will be competing for the resources of the system as it runs, including processor time, memory, disk and user input. The job of the operating system is to arbitrate these resources to the multiple tasks and allow them access in an orderly fashion. You have probably experienced when this fails as it usually ends up with your computer crashing (the famous "blue screen of death" for example). Standardised Interfaces Programmers want to write programs that will run on as many different hardware platforms as possible. By having operating system support for standardised interfaces, programmers can get this functionality. For example, if the function to open a file on one system is open() , on another is open_file() and on yet another openf() programmers will have the dual problem of having to remember what each system does and their programs will not work on multiple systems. The Portable Operating System Interface (POSIX) is a very important standard implemented by UNIX type operating systems. Microsoft Windows has similar proprietary standards. Security On multi-user systems, security is very important. As the arbitrator of access to the system the operating system is responsible for ensuring that only those with the correct permissions can access resources. For example if a file is owned by one user, another user should not be allowed to open and read it. However there also need to be mechanisms to share that file safely between the users should they want it. Operating systems are large and complex programs, and often security issues will be found. Often a virus or worm will take advantage of these bugs to access resources it should not be allowed to, such as your files or network connection; to fight them you must install patches or updates provided by your operating system vendor. Performance As the operating system provides so many services to the computer, its performance is critical. Many parts of the operating system run extremely frequently, so even an overhead of just a few processor cycles can add up to a big decrease in overall system performance. The operating system needs to exploit the features of the underlying hardware to make sure it is getting the best possible performance for the operations, and consequently systems programmers need to understand the intimate details of the architecture they are building for. In many cases the systems programmers job is about deciding on policies for the system. Often the case that the side effects of making one part of the operating system run faster will make another part run slower or less efficiently. Systems programmers need to understand all these trade offs when they are building their operating system. Page 6 Operating System Organisation Operating System Organisation The operating system is roughly organised as in the figure below. Figure 4.1. The Operating System The organisation of the kernel. Processes the kernel is running live in userspace, and the kernel talks both directly to hardware and through drivers. The Kernel The kernel is the operating system. As the figure illustrates, the kernel communicates to hardware both directly and through drivers. Just as the kernel abstracts the hardware to user programs, drivers abstract hardware to the kernel. For example there are many different types of graphic card, each one with slightly different features. As long as the kernel exports an API, people who have access to the specifications for the hardware can write drivers to implement that API. This way the kernel can access many different types of hardware. The kernel is generally what we called privileged. As you will learn, the hardware has important roles to play in running multiple tasks and keeping the system secure, but these rules do not apply to the kernel. We know that the kernel must handle programs that crash (remember it is the operating systems job arbitrate between multiple programs running on the same system, and there is no guarantee that they will behave), but if any internal part of the operating system crashes chances are the entire system will become useless. Similarly security issues can be exploited by user processes to escalate themselves to the privilege level of the kernel; at that point they can access any part of the system completely unchecked. Monolithic v Microkernels One debate that is often comes up surrounding operating systems is whether the kernel should be a microkernel or monolithic. The monolithic approach is the most common, as taken by most common Unixes (such as Linux). In this model the core privileged kernel is large, containing hardware drivers, file system accesses controls, permissions checking and services such as Network File System (NFS). Since the kernel is always privileged, if any part of it crashes the whole system has the potential to comes to a halt. If one driver has a bug it can overwrite any memory in the system with no problems, ultimately causing the system to crash. Page 7 A microkernel architecture tries to minimise this possibility by making the privileged part of the kernel as small as possible. This means that most of the system runs as unprivileged programs, limiting the harm that any one crashing component can influence. For example, drivers for hardware can run in separate processes, so if one goes astray it can not overwrite any memory but that allocated to it. Whilst this sounds like the most obvious idea, the problem comes back two main issues 1. Performance is decreased. Talking between many different components can decrease performance. 2. It is slightly more difficult for the programmer. Both of these criticisms come because to keep separation between components most microkernels are implemented with a message passing based system, commonly referred to as inter-process communication or IPC. Communicating between individual components happens via discrete messages which must be bundled up, sent to the other component, unbundled, operated upon, re- bundled up and sent back, and then unbundled again to get the result. This is a lot of steps for what might be a fairly simple request from a foreign component. Obviously one request might make the other component do more requests of even more components, and the problem can multiply. Slow message passing implementations were largely responsible for the poor performance of early microkernel systems, and the concepts of passing messages are slightly harder for programmers to program for. The enhanced protection from having components run separately was not sufficient to overcome these hurdles in early microkernel systems, so they fell out of fashion. In a monolithic kernel calls between components are simple function calls, as all programmers are familiar with. There is no definitive answer as to which is the best organisation, and it has started many arguments in both academic and non- academic circles. Hopefully as you learn more about operating systems you will be able to make up your own mind! Modules The Linux kernel implements a module system, where drivers can loaded into the running kernel "on the fly" as they are required. This is good in that drivers, which make up a large part of operating system code, are not loaded for devices that are not present in the system. Someone who wants to make the most generic kernel possible (i.e. runs on lots of different hardware, such as RedHat or Debian) can include most drivers as modules which are only loaded if the system it is running on has the hardware available. However, the modules are loaded directly in the privileged kernel and operate at the same privilege level as the rest of the kernel, so the system is still considered a monolithic kernel. Virtualisation Closely related to kernel is the concept of virtualisation of hardware. Modern computers are very powerful, and often it is useful to not think of them as one whole system but split a single physical computer up into separate "virtual" machines. Each of these virtual machines looks for all intents and purposes as a completely separate machine, although physically they are all in the same box, in the same place. Page 8 Some different virtualisation methods. This can be organised in many different ways. In the simplest case, a small virtual machine monitor can run directly on the hardware and provide an interface to the guest operating systems running on top. This VMM is often often called a hypervisor (from the word "supervisor"). In fact, the operating system on top may have no idea that the hypervisor is even there at all, as the hypervisor presents what appears to be a complete system. It intercepts operations between the guest operating system and hardware and only presents a subset of the system resources to each. This is often used on large machines (with many CPUs and much RAM) to implement partitioning. This means the machine can be split up into smaller virtual machines. Often you can allocate more resources to running systems on the fly, as requirements dictate. The hypervisors on many large IBM machines are actually quite complicated affairs, with many millions of lines of code. It provides a multitude of system management services. Another option is to have the operating system aware of the underlying hypervisor, and request system resources through it. This is sometimes referred to as paravirtualisation due to its halfway nature. This is similar to the way early versions of the Xen system works and is a compromise solution. It hopefully provides better performance since the operating system is explicitly asking for system resources from the hypervisor when required, rather than the hypervisor having to work things out dynamically. Finally, you may have a situation where an application running on top of the existing operating system presents a virtualised system (including CPU, memory, BIOS, disk, etc) which a plain operating system can run on. The application converts the requests to hardware through to the underlying hardware via the existing operating system. This is similar to how VMWare works. This approach has many overheads, as the application process has to emulate an entire system and convert everything to requests from the underlying operating system. However, this lets you emulate an entirely different architecture all together, as you can dynamically translate the instructions from one processor type to another (as the Rosetta system does with Apple software which moved from the PowerPC processor to Intel based processors). Performance is major concern when using any of these virtualisation techniques, as what was once fast operations directly on hardware need to make their way through layers of abstraction. Intel have discussed hardware support for virtualisation soon to be coming in their latest processors. These extensions work by raising a special exception for operations that might require the intervention of a virtual machine monitor. Thus the processor looks the same as a non-virtualised processor to the application running on it, but when that application makes requests for resources that might be shared between other guest operating systems the virtual machine monitor can be invoked. Page 9 This provides superior performance because the virtual machine monitor does not need to monitor every operation to see if it is safe, but can wait until the processor notifies that something unsafe has happened. Covert Channels This is a digression, but an interesting security flaw relating to virtualised machines. If the partitioning of the system is not static, but rather dynamic, there is a potential security issue involved. In a dynamic system, resources are allocated to the operating systems running on top as required. Thus if one is doing particularly CPU intensive operations whilst the other is waiting on data to come from disks, more of the CPU power will be given to the first task. In a static system, each would get 50% an the unused portion would go to waste. Dynamic allocation actually opens up a communications channel between the two operating systems. Anywhere that two states can be indicated is sufficient to communicate in binary. Imagine both systems are extremely secure, and no information should be able to pass between one and the other, ever. Two people with access could collude to pass information between themselves by writing two programs that try to take large amounts of resources at the same time. When one takes a large amount of memory there is less available for the other. If both keep track of the maximum allocations, a bit of information can be transferred. Say they make a pact to check every second if they can allocate this large amount of memory. If the target can, that is considered binary 0, and if it can not (the other machine has all the memory), that is considered binary 1. A data rate of one bit per second is not astounding, but information is flowing. This is called a covert channel, and whilst admittedly far fetched there have been examples of security breaches from such mechanisms. It just goes to show that the life of a systems programmer is never simple! Userspace We call the theoretical place where programs run by the user userspace. Each program runs in userspace, talking to the kernel through system calls (discussed below). As previously discussed, userspace is unprivileged. User programs can only do a limited range of things, and should never be able to crash other programs, even if they crash themselves. Infact, the hypervisor shares much in common with a micro-kernel; both strive to be small layers to present the hardware in a safe fashion to layers above it. Page 10 Function of the Operating System What is the Purpose of an OS? An operating system acts as a communication bridge (interface) between the user and computer hardware. The purpose of an operating system is to provide a platform on which a user can execute programs in a convenient and efficient manner. An operating system is a piece of software that manages the allocation of computer hardware. The coordination of the hardware must be appropriate to ensure the correct working of the computer system and to prevent user programs from interfering with the proper working of the system. Example: Just like a boss gives order to his employee, in the similar way we request or pass our orders to the operating system. The main goal of the operating system is to thus make the computer environment more convenient to use and the secondary goal is to use the resources in the most efficient manner. What is operating system ? An operating system is a program on which application programs are executed and acts as an communication bridge (interface) between the user and the computer hardware. The main task an operating system carries out is the allocation of resources and services, such as allocation of: memory, devices, processors and information. The operating system also includes programs to manage these resources, such as a traffic controller, a scheduler, memory management module, I/O programs, and a file system. Important functions of an operating system: 1. Security The operating system uses password protection to protect user data and similar other techniques. it also prevents unauthorized access to programs and user data. 2. Control over system performance Monitors overall system health to help improve performance. records the response time between service requests and system response to have a complete view of the system health. This can help improve performance by providing important information needed to troubleshoot problems. 3. Job accounting Operating system keeps track of time and resources used by various tasks and users, this information can be used to track resource usage for a particular user or group of user. 4. Error detecting aids Operating system constantly monitors the system to detect errors and avoid the malfunctioning of computer system. 5. Coordination between other software and users Operating systems also coordinate and assign interpreters, compilers, assemblers and other software to the various users of the computer systems. 6. Memory Management The operating system manages the primary memory or main memory. Main memory is made up of a large array of bytes or words where each byte or word is assigned a certain address. Main memory is a fast storage and it can be accessed directly by the CPU. For a program to be executed, it should be first loaded in the main memory. An operating system performs the following activities for memory management: It keeps tracks of primary memory, i.e., which bytes of memory are used by which user program. The memory addresses that have already been allocated and the memory addresses of the memory that has not yet been used. In multi programming, the OS decides the order in which process are granted access to memory, and for how long. It Allocates the memory to a process when the process requests it and deallocates the memory when the process has terminated or is performing an I/O operation. 7. Processor Management In a multi programming environment, the OS decides the order in which processes have access to the processor, and how much processing time each process has. This function of OS is called process scheduling. An operating system performs the following activities for processor management. Keeps tracks of the status of processes. The program which perform this task is known as traffic controller. Allocates the CPU that is processor to a process. De-allocates processor when a process is no more required. Page 11  8. Device Management An OS manages device communication via their respective drivers. It performs the following activities for device management. Keeps tracks of all devices connected to system. designates a program responsible for every device known as the Input/Output controller. Decides which process gets access to a certain device and for how long. Allocates devices in an effective and efficient way. Deallocates devices when they are no longer required. 9. File Management A file system is organized into directories for efficient or easy navigation and usage. These directories may contain other directories and other files. An operating system carries out the following file management activities. It keeps track of where information is stored, user access settings and status of every file and more… These facilities are collectively known as the file system. Moreover, operating system also provides certain services to the computer system in one form or the other. The operating system provides certain services to the users which can be listed in the following manner: 1. Program Execution The operating system is responsible for execution of all types of programs whether it be user programs or system programs. The operating system utilizes various resources available for the efficient running of all types of functionalities. 2. Handling Input/Output Operations The operating system is responsible for handling all sort of inputs, i.e, from keyboard, mouse, desktop, etc. The operating system does all interfacing in the most appropriate manner regarding all kind of inputs and outputs. For example, there is difference in nature of all types of peripheral devices such as mouse or keyboard, then operating system is responsible for handling data between them. 3. Manipulation of File System The operating system is responsible for making of decisions regarding the storage of all types of data or files, i.e, floppy disk/hard disk/pen drive, etc. The operating system decides as how the data should be manipulated and stored. 4. Error Detection and Handling The operating system is responsible for detection of any types of error or bugs that can occur while any task. The well secured OS sometimes also acts as countermeasure for preventing any sort of breach to the computer system from any external source and probably handling them. 5. Resource Allocation The operating system ensures the proper use of all the resources available by deciding which resource to be used by whom for how much time. All the decisions are taken by the operating system. 6. Accounting The operating system tracks an account of all the functionalities taking place in the computer system at a time. All the details such as the types of errors occurred are recorded by the operating system. 7. Information and Resource Protection The operating system is responsible for using all the information and resources available on the machine in the most protected way. The operating system must foil an attempt from any external resource to hamper any sort of data or information. All these services are ensured by the operating system for the convenience of the users to make the programming task easier. All different kinds of operating system more or less provide the same services. Page 12 Types of Operating Systems What are the Types of Operating Systems An Operating System performs all the basic tasks like managing file,process, and memory. Thus operating system acts as manager of all the resources, i.e. resource manager. Thus operating system becomes an interface between user and machine. Types of Operating Systems: Some of the widely used operating systems are as follows- 1. Batch Operating System This type of operating system does not interact with the computer directly. There is an operator which takes similar jobs having same requirement and group them into batches. It is the responsibility of operator to sort the jobs with similar needs. Figure 3.6.1 : Depiction of a batch operating system. Advantages of Batch Operating System: It is very difficult to guess or know the time required by any job to complete. Processors of the batch systems know how long the job would be when it is in queue Multiple users can share the batch systems The processor idle time for batch system is very low It is easy to manage large tasks repeatedly in batch systems Disadvantages of Batch Operating System: The computer operators should have a good understanding of batch systems Batch systems are hard to debug It is sometime costly The other jobs will have to wait for an unknown time if any job fails Examples of Batch based Operating System: Payroll System, Bank Statements etc. 2. Time-Sharing Operating Systems Each task is given some time to execute, so that all the tasks work smoothly. Each user gets a time time slot on the CPU. These systems are also known as Multitasking Systems. The task can be from single user or from different users. The time that each task gets to execute is called quantum. After this time interval is over OS switches to next task. Page 13  3.6.1 Figure : Time sharing Operating System. Advantages of Time-Sharing OS: Each task gets an equal time on the processor Less chances of duplication of software CPU idle time can be reduced Disadvantages of Time-Sharing OS: The operating system must take care of security and integrity of user programs and data Data communication problems can arise with if the data storage or users are remotely located Examples of Time-Sharing OSs are: Linux, Unix etc. 3. Distributed Operating System Various autonomous interconnected computers communicate each other using a shared communication network. Independent systems possess their own memory unit and CPU. These are referred as loosely coupled systems or distributed systems. These system’s processors differ in size and function. The major benefit of working with these types of operating system is that it is always possible that one user can access the files or software which are not actually present on his system but on some other system connected within this network i.e., remote access is enabled within the devices connected in that network. Figure : Distributed Operating System. Page 14  3.6.1.1 Advantages of Distributed Operating System: Failure of one node will not affect the other network communication, since all systems are independent from each other Since resources are being shared, computation can be very fast Load on host computer is reduced These systems are easily scalable as many systems can be easily added to the network Disadvantages of Distributed Operating System: Failure of the main network will stop the entire communication With the increase of telecommunications capability and the Internet, some of the disadvantages have disappeared Examples of Distributed Operating System are- LOCUS. 4. Network Operating System Historically operating systems with networking capabilities were described as network operating system, because they allowed personal computers (PCs) to participate in computer networks and shared file and printer access within a local area network (LAN). This description of operating systems is now largely historical, as common operating systems include a network stack to support a client–server model. These limited client/server networks were gradually replaced by Peer-to-peer networks, which used networking capabilities to share resources and files located on a variety of computers of all sizes. A peer-to-peer network sets all connected computers equal; they all share the same abilities to use resources available on the network. The most popular peer-to-peer networks as of 2020 are Ethernet, Wi-Fi and the Internet protocol suite. Software that allowed users to interact with these networks, despite a lack of networking support in the underlying manufacturer's operating system, was sometimes called a network operating system. Page 15 https://eng.libretexts.org/@go/page/82844 Examples of such add-on software include Phil Karn's KA9Q NOS (adding Internet support to CP/M and MS-DOS), PC/TCP Packet Drivers (adding Ethernet and Internet support to MS-DOS), and LANtastic (for MS-DOS, Microsoft Windows and OS/2), and Windows for Workgroups (adding NetBIOS to Windows). Examples of early operating systems with peer-to-peer networking capabilities built-in include MacOS (using AppleTalk and LocalTalk), and the Berkeley Software Distribution. Today, distributed computing and groupware applications have become the norm. Computer operating systems include a networking stack as a matter of course. During the 1980s the need to integrate dissimilar computers with network capabilities grew and the number of networked devices grew rapidly. Partly because it allowed for multi-vendor interoperability, and could route packets globally rather than being restricted to a single building, the Internet protocol suite became almost universally adopted in network architectures. Thereafter, computer operating systems and the firmware of network devices tended to support Internet protocols. Figure 3.6.1. : Network Operating System. Advantages of Network Operating System: Highly stable centralized servers Security concerns are handled through servers New technologies and hardware up-gradation are easily integrated to the system Server access are possible remotely from different locations and types of systems Disadvantages of Network Operating System: Servers are costly User has to depend on central location for most operations Maintenance and updates are required regularly Examples of Network Operating System are: Microsoft Windows Server 2003, Microsoft Windows Server 2008, UNIX, Linux, Mac OS X, Novell NetWare, and BSD etc. Page 16 5. Real-Time Operating System These types of OSs are used in real-time systems. The time interval required to process and respond to inputs is very small. This time interval is called response time. Real-time systems are used when there are time requirements are very strict like missile systems, air traffic control systems, robots etc. Two types of Real-Time Operating System which are as follows: Hard Real-Time Systems: These OSs are meant for the applications where time constraints are very strict and even the shortest possible delay is not acceptable. These systems are built for saving life like automatic parachutes or air bags which are required to be readily available in case of any accident. Virtual memory is almost never found in these systems. Soft Real-Time Systems: These OSs are for applications where for time-constraint is less strict. 3.6.2.1 Figure : Real Time Operating System. Advantages of RTOS: Maximum Consumption: Maximum utilization of devices and system,thus more output from all the resources Task Shifting: Time assigned for shifting tasks in these systems are very less. For example in older systems it takes about 10 micro seconds in shifting one task to another and in latest systems it takes 3 micro seconds. Focus on Application: Focus on running applications and less importance to applications which are in queue. Real time operating system in embedded system: Since size of programs are small, RTOS can also be used in embedded systems like in transport and others. Error Free: These types of systems MUST be able to deal with any exceptions, so they are not really error free, but handle error conditions without halting the system. Memory Allocation: Memory allocation is best managed in these type of systems. Disadvantages of RTOS: Limited Tasks: Very few tasks run at the same time and their concentration is very less on few applications to avoid errors. Use heavy system resources: Sometimes the system resources are not so good and they are expensive as well. Complex Algorithms: The algorithms are very complex and difficult for the designer to write on. Device driver and interrupt signals: It needs specific device drivers and interrupt signals to response earliest to interrupts. Thread Priority: It is not good to set thread priority as these systems are very less prone to switching tasks. Examples of Real-Time Operating Systems are: Scientific experiments, medical imaging systems, industrial control systems, weapon systems, robots, air traffic control systems, etc. Page 17 Difference between multitasking, multithreading and multiprocessing Multi-programming In a modern computing system, there are usually several concurrent application processes which want to execute. It is the responsibility of the operating system to manage all the processes effectively and efficiently. One of the most important aspects of an operating system is to provide the capability to multi-program. In a computer system, there are multiple processes waiting to be executed, i.e. they are waiting while the CPU is allocated to other processes. The main memory is too small to accommodate all of these processes or jobs. Thus, these processes are initially kept in an area called job pool. This job pool consists of all those processes awaiting allocation of main memory and CPU. The scheduler selects a job out of the job pool, brings it into main memory and begins executing it. The processor executes one job until one of several factors interrupt its processing: 1) the process uses up its allotted time; 2) some other interrupt (we will talk more about interrupts) causes the processor to stop executing this process; 3) the process goes into a wait state waiting on an I/O request. Non-multi-programmed system concepts: In a non multi-programmed system, as soon as one job hits any type of interrupt or wait state, the CPU becomes idle. The CPU keeps waiting and waiting until this job (which was executing earlier) comes back and resumes its execution with the CPU. So CPU remains idle for a period of time. There are drawbacks when the CPU remains idle for a very long period of time. Other jobs which are waiting for the processor will not get a chance to execute because the CPU is still allocated to the job that is in a wait state. This poses a very serious problem - even though other jobs are ready to execute, the CPU is not available to them because it is still allocated to a job which is not even utilizing it. It is possible that one job is using the CPU for an extended period of time, while other jobs sit in the queue waiting for access to the CPU. In order to work around such scenarios like this the concept of multi-programming developed to increase the CPU utilization and thereby the overall efficiency of the system. The main idea of multi-programming is to maximize the CPU time. Multi-programmed system concepts: In a multi-programmed system, as soon as one job goes gets interrupted or goes into a wait state, the cpu selects the next job from the scheduler and starts its execution. Once the previous job resolves the reason for its interruption - perhaps the I/O completes - goes back into the job pool. If the second job goes into a wait state, the CPU chooses a third job and starts executing it. This makes for much more efficient use of the CPU. Therefore, the ultimate goal of multi-programming is to keep the CPU busy as long as there are processes ready to execute. This way, multiple programs can be executed on a single processor by executing a part of a program at one time, a part of another program after this, then a part of another program and so on, hence executing multiple programs In the image below, program A runs for some time and then goes to waiting state. In the mean time program B begins its execution. So the CPU does not waste its resources and gives program B an opportunity to run. There is still time slots where the processor is waiting - other programs could be run if necessary. Figure : Multiprogramming. Page 18 2. Multiprocessing In a uni-processor system, only one process executes at a time. Multiprocessing makes use of two or more CPUs (processors) within a single computer system. The term also refers to the ability of a system to support more than one processor within a single computer system. Since there are multiple processors available, multiple processes can be executed at a time. These multiprocessors share the computer bus, sometimes the clock, memory and peripheral devices also. Multiprocessing system’s working – With the help of multiprocessing, many processes can be executed simultaneously. Say processes P1, P2, P3 and P4 are waiting for execution. Now in a single processor system, firstly one process will execute, then the other, then the other and so on. But with multiprocessing, each process can be assigned to a different processor for its execution. If its a dual-core processor (2 processors), two processes can be executed simultaneously and thus will be two times faster, similarly a quad core processor will be four times as fast as a single processor. Why use multiprocessing The main advantage of multiprocessor system is to get more work done in a shorter period of time. These types of systems are used when very high speed is required to process a large volume of data. multiprocessing systems can save money in comparison to single processor systems because the processors can share peripherals and power supplies. It also provides increased reliability in that if one processor fails, the work does not halt, it only slows down. e.g. if we have 10 processors and 1 fails, then the work does not halt, rather the remaining 9 processors can share the work of the 10th processor. Thus the whole system runs only 10 percent slower, rather than failing altogether Multiprocessing refers to the hardware (i.e., the CPU units) rather than the software (i.e., running processes). If the underlying hardware provides more than one processor then that is multiprocessing. It is the ability of the system to leverage multiple processors’ computing power. Difference between multiprogramming and multiprocessing A system can be both multi programmed by having multiple programs running at the same time and multiprocessing by having more than one physical processor. The difference between multiprocessing and multi programming is that Multiprocessing is basically executing multiple processes at the same time on multiple processors, whereas multi programming is keeping several programs in main memory and executing them concurrently using a single CPU only. Multiprocessing occurs by means of parallel processing whereas Multi programming occurs by switching from one process to other (phenomenon called as context switching). 3. Multitasking As the name itself suggests, multitasking refers to execution of multiple tasks (say processes, programs, threads etc.) at a time. In the modern operating systems, we are able to play MP3 music, edit documents in Microsoft Word, surf the Google Chrome all simultaneously, this is accomplished by means of multitasking. Multitasking is a logical extension of multi programming. The major way in which multitasking differs from multi programming is that multi programming works solely on the concept of context switching whereas multitasking is based on time sharing alongside the concept of context switching. Multi tasking system’s concepts In a time sharing system, each process is assigned some specific quantum of time for which a process is meant to execute. Say there are 4 processes P1, P2, P3, P4 ready to execute. So each of them are assigned some time quantum for which they will execute e.g time quantum of 5 nanoseconds (5 ns). As one process begins execution (say P2), it executes for that quantum of time (5 ns). After 5 ns the CPU starts the execution of the other process (say P3) for the specified quantum of time. Thus the CPU makes the processes to share time slices between them and execute accordingly. As soon as time quantum of one process expires, another process begins its execution. Page 19  Here also basically a context switch is occurring but it is occurring so fast that the user is able to interact with each program separately while it is running. This way, the user is given the illusion that multiple processes/ tasks are executing simultaneously. But actually only one process/ task is executing at a particular instant of time. In multitasking, time sharing is best manifested because each running process takes only a fair quantum of the CPU time. In a more general sense, multitasking refers to having multiple programs, processes, tasks, threads running at the same time. This term is used in modern operating systems when multiple tasks share a common processing resource (e.g., CPU and Memory). Figure 3.7.1.1 : Depiction of Multitasking System. As depicted in the above image, At any time the CPU is executing only one task while other tasks are waiting for their turn. The illusion of parallelism is achieved when the CPU is reassigned to another task. i.e all the three tasks A, B and C are appearing to occur simultaneously because of time sharing. So for multitasking to take place, firstly there should be multiprogramming i.e. presence of multiple programs ready for execution. And secondly the concept of time sharing. Page 20 Multithreading A thread is a basic unit of CPU utilization. Multithreading is an execution model that allows a single process to have multiple code segments (i.e., threads) running concurrently within the “context” of that process. e.g. VLC media player, where one thread is used for opening the VLC media player, one thread for playing a particular song and another thread for adding new songs to the playlist. Multithreading is the ability of a process to manage its use by more than one user at a time and to manage multiple requests by the same user without having to have multiple copies of the program. Multithreading system examples Example 1 Say there is a web server which processes client requests. Now if it executes as a single threaded process, then it will not be able to process multiple requests at a time. First one client will make its request and finish its execution and only then, the server will be able to process another client request. This is quite inefficient, time consuming and tiring task. To avoid this, we can take advantage of multithreading. Now, whenever a new client request comes in, the web server simply creates a new thread for processing this request and resumes its execution to process more client requests. So the web server has the task of listening to new client requests and creating threads for each individual request. Each newly created thread processes one client request, thus reducing the burden on web server. Example 2 We can think of threads as child processes that share the parent process resources but execute independently. Take the case of a GUI. Say we are performing a calculation on the GUI (which is taking very long time to finish). Now we can not interact with the rest of the GUI until this command finishes its execution. To be able to interact with the rest of the GUI, this calculation should be assigned to a separate thread. So at this point of time, 2 threads will be executing i.e. one for calculation, and one for the rest of the GUI. Hence here in a single process, we used multiple threads for multiple functionality. The image helps to describe the VLC player example: Figure 3.7.2.1 : Example of Multithreading. Advantages of multithreading Benefits of multithreading include increased responsiveness. Since there are multiple threads in a program, so if one thread is taking too long to execute or if it gets blocked, the rest of the threads keep executing without any problem. Thus the whole program remains responsive to the user by means of remaining threads. Another advantage of multithreading is that it is less costly. Creating brand new processes and allocating resources is a time consuming task, but since threads share resources of the parent process, creating threads and switching between them is comparatively easy. Hence multithreading is the need of modern Operating Systems. Page 21