introduction to OS Lecture.ppt

Chapter 1: Introduction (OS Structure, Modes and Services) By: Navjot Kaur  What is an Operating System? It is a layer of system software that:  directly has privileged access to underlined hardware  hides hardware complexity  manages ha...

Chapter 1: Introduction (OS Structure, Modes and Services) By: Navjot Kaur  What is an Operating System? It is a layer of system software that:  directly has privileged access to underlined hardware  hides hardware complexity  manages hardware on behalf of one or more applications according to policies.  What is an Operating System? What is an Operating system?  A program that acts as an intermediate/ interface between a user of a computer and the computer hardware.  Resource allocator (Managing the resources efficiently)  Control Program Operating system goals:  Execute user programs and make problem solving easier.  Make the computer system convenient to use  Efficiently use available resources An operating system is the one program that is running at all the times on the computer- usually called the kernel. Kernel is a program that (allow) let the hardware to recognize and read the program/process.  Computer System Structure Computer system can be divided into four components:  Hardware – provides basic computing resources  CPU, memory, I/O devices  Operating system  Controls and coordinates use of resources among various applications and users  System/Application programs – define the ways in which the system resources are used to solving user problems  Word processors, compilers, web browsers, database systems, video games  Users  People, machines, other computers Four Components of a Computer System  Computer System Organization Computer-system operation  One or more CPUs, device controllers connect through common bus providing access to shared memory  Concurrent execution of CPUs and devices competing for memory cycles  TYPES OF OS Batch Systems “Batch operating system. The users of a batch operating system do not interact with the computer directly. Each user prepares his job on an off-line device like punch cards and submits it to the computer operator. To speed up processing, jobs with similar needs are batched together and run as a group”. *[https://www.tutorialspoint.com/operating_system/os_types.htm]  TYPES OF OS: Batch Systems There is no direct interaction between user and the computer. The user has to submit a job (written on cards or tape) to a computer operator. Then computer operator places a batch of several jobs on an input device. Jobs are batched together by type of languages and requirements. Then a special program, the monitor program, manages the execution of each program in the batch. The monitor program is always in the main memory and available for execution. Disadvantages: No interaction between user and computer. No mechanism to prioritize the processes  Multiprogrammed OS Multiprogramming: When 2 or more processes reside in memory at the same time  Single user processes cannot keep CPU and I/O devices busy at all times  Multiprogramming organizes jobs (code and data) so CPU always has one to execute  Multiprogramming assumes a single shared processor. One job selected and run via job scheduling  Multiprogramming increases CPU utilization.  It is mixture of I/O bound and CPU bound processes  Multiprogrammed OS In this the operating system picks up and begins to execute one of the jobs from memory. Once this job needs an I/O operation operating system switches to another job (CPU and OS always busy). Jobs in the memory are always less than the number of jobs on disk(Job Pool).  Multiprogrammed OS If several jobs are ready to run at the same time, then the system chooses which one to run through the process of CPU Scheduling. In Non-multiprogrammed system, there are moments when CPU sits idle and does not do any work. In Multiprogramming system, CPU will never be idle and keeps on processing.  Multitasking/Timesharing OS Timesharing (multitasking) when multiple jobs are executed by the CPU simultaneously by switching between them.  There is at least one program is executing in memory process  If several jobs ready to run at the same time  CPU scheduling  If processes don’t fit in memory, swapping moves them in and out to run  Only one CPU is involved, but it switches from one process to another so quickly that it gives the appearance of executing all of the processes at the same time.  Multitasking/ Time Sharing Types of Multitasking: 1. Preemptive: the operating system parcels CPU time slices to each program. 2. Cooperative: each program can control the CPU for as long as it needs it. If a program is not using the CPU, however, it can allow another program to use it temporarily. In time sharing systems the prime focus is on minimizing the response time, while in multiprogramming the prime focus is to maximize the CPU usage.  Multiprocessing OS A multiprocessor system consists of several processors that share a common physical memory. Multiprocessor system provides higher computing power and speed. In multiprocessor system all processors operate under single operating system.  Multiprocessing OS Multi-processor systems; that is, they have multiple CPU. Also known as parallel systems or tightly coupled systems Such systems have more than one processor in close communication, sharing the computer bus, the clock, and sometimes memory and peripheral devices.  Distributed Systems A network is a communication path between two or more systems. Each system over the network keeps copy of the data, and this leads to Reliability (Because if one system crashes , data is not lost).  Real Time Systems Time bound systems Real time systems are of 2 types: 1. Soft Real time Systems: Process should complete in specific time but May have some delay (Positive delay) and will not harm the system. Exp: Session expires but can be re-logged in. 2. Hard Real Time Systems: Each process is assigned a specific time instance, and Process must complete in that time otherwise system will crash. Real Time Embedded Systems is a computing environment that reacts to input within a specific time period. Time Driven Task specific Exp: Microwave, Washing Machine…  Operating System Views OS can be explored from 2 view points: 1. User view:  The goal of the Operating System is to maximize the work and minimize the effort of the user.  Operating System gives an effect to the user as if the processor is dealing only with the current task, but in background processor is dealing with several processes.  Operating System Views OS can be explored from 2 view points: 2. System View: Operating System is a program involved with the hardware. OS is a resource allocator  Allocates and Manages all resources and their sharing.  Decides between conflicting requests for efficient and fair resource use OS is a control program  Controls execution of programs to prevent errors and improper use of the computer  It prevents improper usage, error and handle deadlock conditions. Operating-System Operations OS’s are Interrupt driven. If no process, no I/o devices, No users then OS will sit quietly waiting for some event to occur. Program or software send, Generate Events by using system calls. Error or request by a software creates exception or trap  E.g. Division by zero To ensure proper execution of OS, we must distinguish between execution of OS code and user defined code. Operating-System Operations To protect OS, Dual-mode operations exist:  User mode (1) and kernel mode (0)  A Mode bit is added to hardware to indicate mode  Provides ability to distinguish when system is running user mode or kernel mode  System call changes mode to kernel, return from call resets it to user Transition from User to Kernel Mode Transition from User to Kernel Mode  Operating System Structure Various ways to structure a system  Simple Structure -- MS-DOS MS-DOS – written to provide the most functionality in the least space  Not divided into modules  Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated  Non Simple Structure -- UNIX The UNIX OS consists of two parts:  System programs  The kernel  Consists of everything below the system-call interface and above the physical hardware  Provides the file system, CPU scheduling, memory management, and other operating-system functions; a large number of functions for one level Traditional UNIX System Structure Traditional LINUX System Structure Kernel has many inbuilt modules  Layered Approach The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface. With modularity, layers are selected such that each uses functions (operations) and services of only lower- level layers  Operating System Services An operating system provides an environment for the programs to run. It provides certain services to programs  Operating System Services Operating-system services provides functions that are helpful to the user:  User interface - Almost all operating systems have a user interface (UI)  Varies between Command-Line (CLI), Graphics User Interface (GUI).  Program execution - The system must be able to load a program into memory and to run that program, end execution, either normally or abnormally (indicating error)  I/O operations - A running program may require I/O, which may involve a file or an I/O device.  File-system manipulation - read and write files and directories, create and delete them, search them, list file Information, permission management.  Operating System Services  Communications – Processes may exchange information, on the same computer or between computers over a network  Communications may be via shared memory or through message passing (packets moved by the OS)  Error detection – OS needs to be constantly aware of possible errors  May occur in the CPU and memory hardware, in I/O devices, in user program  For each type of error, OS should take the appropriate action to ensure correct and consistent computing  Debugging facilities can greatly enhance the user’s and programmer’s abilities to efficiently use the system.  Resource allocation – OS must ensure allocation of resources to all programs running.  Many types of resources - such as CPU cycle time, main memory, and file storage, I/O devices  Operating System Services  Accounting - To keep track of which users use how much and what kinds of computer resources.  Protection and security - The owners of information stored in a multiuser or networked computer system may want to control use of that information, concurrent processes should not interfere with each other  Protection involves ensuring that all access to system resources is controlled  Security of the system from outsiders requires user authentication, extends to defending external I/O devices from invalid access attempts  If a system is to be protected and secure, precautions must be instituted throughout it.  A chain is only as strong as its weakest link.  Kernel A kernel is a central component of an operating system. It acts as an interface between the user applications and the hardware. The sole aim of the kernel is to manage the communication between the software (user level applications) and the hardware (CPU, disk memory etc). The main tasks of the kernel are :  Process management  Device management  Memory management  Interrupt handling  I/O communication  File system...etc..  Kernel A kernel is the lowest level of software that interfaces with the hardware in your computer. It is responsible for interfacing all applications that are running in “user mode” down to the physical hardware, and allowing processes, to get information from each other using inter-process communication (IPC).  Kernel Types kernels fall into one of three types: Monolithic Microkernel Hybrid  Monolithic Kernel A monolithic kernel is an operating system architecture where the entire operating system (which includes the device drivers, file system, and the application IPC etc.) is working in kernel space, in supervisor mode. Monolithic kernels are able to dynamically load (and unload) executable modules at runtime. Examples of operating systems that use a monolithic kernel are - Linux, Solaris, OS-9, DOS, Microsoft Windows (95,98,Me) etc. Monolithic Kernel Monolithic Kernel  Monolithic Kernel Pros: More direct access to hardware for programs Easier for processes to communicate between each other If your device is supported, it should work with no additional installations Processes react faster because there isn’t a queue for processor time. Cons: Large install footprint Large memory is needed Less secure because everything runs in supervisor mode  MicroKernel In a Microkernel architecture, the core functionality is isolated from system services and device drivers. This architecture allows some basic services like device driver management, file system etc. to run in user space. This reduces the kernel code size and also increases the security and stability of OS as we have minimum code running in kernel. Examples of operating systems that use a microkernel are - QNX, Integrity, PikeOS, Symbian, L4Linux, Singularity, K42, Mac OS X, HURD, Minix, and Coyotos. MicroKernel Types of Kernel  MicroKernel Pros Portability Small install footprint Small memory Security Cons Hardware is more abstracted through drivers Hardware may react slower because drivers are in user mode Processes have to wait in a queue to get information Processes can’t get access to other processes without waiting  HybridKernel Hybrid kernels have the ability to pick and choose what they want to run in user mode and what they want to run in supervisor mode. Device drivers and file system I/O will be run in user mode while IPC and server calls will be kept in the supervisor mode. This require more work of the hardware manufacturer because all of the driver responsibility is up to them.  Hybrid Kernel Pros Developer can pick and choose what runs in user mode and what runs in supervisor mode Smaller install than monolithic kernel More flexible than other models Cons Processes have to wait in a queue to get information Processes can’t get access to other processes without waiting Device drivers need to be managed by user  Interrupts An interrupt is a signal from a device attached to a computer or from a program within the computer that causes the main program that operates the computer (the operating system) to stop and figure out what to do next. Interrupts can be of following type:  Generated by Hardware (Hardware Interrupt)  Generated by Software (Software Interrupt)  Hardware Interrupt 1. Hardware interrupts are used by devices to communicate that they require attention from the operating system. 2. Hardware interrupts by sending signal to CPU via system bus. 3. Hardware interrupts are referenced by an interrupt number. 4. These numbers are mapped with hardware that created the interrupt. This enables the system to monitor/understand which device created the interrupt and when it occurred.  Software Interrupt/ Trap Interrupt generated by executing a instruction. Software interrupts by a special operation called a System Call or Monitor Call. Exp: 1. cout in C++ is a kind of interrupt because it would make a system to print something. 2. division by zero  Basic steps when interrupt occurs: 1. Interrupt Occurred? 2. H/w Transfers control to OS 3. OS preserves current state of process by using Registers and Program counter 4. Determine which kind of interrupt has occurred and Provides resources 5. When interrupt is executed, address is loaded to program counter and interrupted services are resumed.  I/O Interrupt Once the I/O is started, 2 kind of interrupts may arise: Synchronous interrupt (I/O): The control is transferred to the user process when I/O completes. Asynchronous interrupt (I/O): Returns control to user process without waiting for I/O to complete. (a) Synchronous (b) Asynchronous Waiting of I/O may be accomplished in 2 ways: Wait: it let the CPU wait till the next interrupt. Loop: Loop continues until interrupt is active.  Device Status Table To keep the track of how many I/O requests are pending Device Status Table is maintained.  Device type  Address  State (idle,busy)  System Calls Allow user-level processes to request services of the operating system. It provides a way in which program talks to the operating system. Why system calls are required? It is a request to the operating system to perform some activity. It is a call to the kernel in order to execute a specific function that controls a device or executes a instruction. A system call looks like a procedure call  Accessing System Calls System calls are accessed by programs via a high-level Application Program Interface (API) (provides run time environment) System calls are implemented via API, API’s interact with kernel of OS.  Example of System Calls System call sequence to copy the contents of one file to another file  System Call Implementation A number is associated with each system call  System-call interface maintains a table indexed according to these numbers The system call interface invokes intended system call in OS kernel and returns status of the system call with a return value. The caller needs to know nothing about how the system call is implemented  Just needs to obey API and understand what OS will do as a result call  Most details of OS interface hidden from programmer by API API – System Call – OS Relationship Standard C Library Example C program invoking printf() library call, which calls write() system call  System Call Parameter Passing Passing Parameters to System Calls: Information required for a system call vary according to OS and call. Three general methods used to pass parameters to the OS 1. Pass the parameters in registers  When parameters are < 6. 2. Parameters stored in a block, or table, in memory, and address of block passed as a parameter in a register. (6 or more)  This approach taken by Linux and Solaris 3. Parameters placed, or pushed, onto the stack by the program and popped off the stack by the operating system. Parameter Passing via Table  Types of System Calls 5 Categories Process Control File Management  end, abort  create file, delete file  load, execute  open, close file  create process,  read, write, reposition terminate process  get and set file  get process attributes, attributes set process attributes  wait for time  wait event, signal event  allocate and free memory  Types of System Calls (Cont.) Device Management Communications  request device, release device  create, delete  read, write, reposition communication  get device attributes, set device connection attributes  send, receive  logically attach or detach devices messages  transfer status Information Maintenance information  get time or date, set time or date  attach and detach  get system data, set system data remote devices  get and set process, file, or device attributes Q1. Who controls the execution of programs to prevent errors and improper use of computer? a) Resource allocator b) Control Program c) Hardware d) None of the above Q2. The operating system switches from user mode to kernel mode so the mode bit will change from? a) 0 to 1 b) 1 to 0 c) Remain constant d) None Q3. In which type of operating system users do not interact directly with the computer system? a) Multiprogramming operating systems b) Multiprocessing operating systems c) Batch operating systems d) Distributed operating systems Ans 3) c