Operating Systems Notes PDF

UNIT-1 NOTES UNIT – I OPERATING SYSTEMS OVERVIEW: Introduction-operating system operations, process management,memory management, storage management, protection and security, System structures-Operating system services, systems calls, Types of system calls, system programs (T1: Ch-1, 2) (1.1-1.9, 2...

UNIT-1 NOTES UNIT – I OPERATING SYSTEMS OVERVIEW: Introduction-operating system operations, process management,memory management, storage management, protection and security, System structures-Operating system services, systems calls, Types of system calls, system programs (T1: Ch-1, 2) (1.1-1.9, 2.1-2.5) What is an Operating System? A program that acts as an intermediary between a user of a computer and the computer hardware Operating system goals: Execute user programs and make solving user problems easier Make the computer system convenient to use Use the computer hardware in an efficient manner 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 hardware among various applications and users Application programs – define the ways in which the system resources are used to solve the computing problems of the users Word processors, compilers, web browsers, database systems, video games Users People, machines, other computers Four Components of a Computer System Operating Systems Fundamentals Page 1  UNIT-1 NOTES Operating System Definition An operating system as a resource allocator. A computer system has many resources that may be required to solve a problem: CPU time, memory space, file-storage space, I/O devices, and so on. The operating system acts as the manager of these resources. Decides between conflicting requests for efficient and fair resource use OS is a control program A control program manages the execution of user programs to prevent errors and improper use of the computer. It is especially concerned with the operation and control of I/O devices. “OS is the one program running at all times on the computer” is the kernel. Everything else is either a system program (ships with the operating system) or an application program System programs which are associated with the operating system but are not part of the kernel, and application programs which include all programs not associated with the operation of the system.) Computer Startup bootstrap program is loaded at power-up or reboot Typically stored in ROM or EPROM, generally known as firmware Initializes all aspects of system Loads operating system kernel and starts execution Once the kernel is loaded and executing, it can start providing services to the system and its users. Some services are provided outside of the kernel, by system programs that are loaded into memory at boot time to become system processes, or system daemons that run the entire time the kernel is running. On UNIX, the first system process is “init,” and it starts many other daemons. Once this phase is complete, the system is fully booted, and the system waits for some event to occur. 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 Operating Systems Fundamentals Page 2  UNIT-1 NOTES Computer-System Operation I/O devices and the CPU can execute concurrently Each device controller has a local buffer CPU moves data from/to main memory to/from local buffers Device controller informs CPU that it has finished its operation by causing An interrupt The occurrence of an event is usually signaled by an Interrupt from either the hardware or the software. Hardware may trigger an interrupt at any time by sending a signal to the CPU, usually by way of the system bus. Software may trigger an interrupt executing a special operation called a System call (also called a monitor call) Common Functions of Interrupts Interrupt transfers control to the interrupt service routine generally, through the interruptvector, which contains the addresses of all the service routines Interrupt architecture must save the address of the interrupted instruction Incoming interrupts are disabled while another interrupt is being processed to prevent a lost interrupt. A trap is a software-generated interrupt caused either by an error or a user request An operating system is interrupt driven Interrupt Handling The operating system preserves the state of the CPU by storing registers and the program counter Determines which type of interrupt has occurred: Separate segments of code determine what action should be taken for each type of interrupt Operating Systems Fundamentals Page 3  UNIT-1 NOTES Interrupt Timeline I/O Structure A general-purpose computer system consists of CPUs and multiple device controllers that are connected through a common bus. Each device controller is in charge of a specific type of device. Depending on the controller, more than one device may be attached. For instance, seven or more devices can be attached to the small computer- systems interface (SCSI) controller. A device controller maintains some local buffer storage and a set of special-purpose registers. The device controller is responsible for moving the data between the peripheral devices that it controls and its local buffer storage. Typically, operating systems have a device driver for each device controller. This device driver understands the device controller and provides the rest of the operating system with a uniform interface to the device. To start an I/O operation, the device driver loads the appropriate registers within the device controller. The device controller, in turn, examines the contents of these registers to determine what action to take (such as “read a character from the keyboard”). The controller starts the transfer of data from the device to its local buffer. Once the transfer of data is complete, the device controller informs the device driver via an interrupt that it has finished its operation. The device driver then returns control to the operating system, possibly returning the data or a pointer to the data if the operation was a read. For other operations, the device driver returns status information. System call – request to the operating system to allow user to wait for I/O completion Device-status table contains entry for each I/O device indicating its type, address, and state Operating Systems Fundamentals Page 4  UNIT-1 NOTES Operating system indexes into I/O device table to determine device status and to modify table entry to include interrupt Storage Structure The CPU can load instructions only from memory, so any programs to run must be stored there. General-purpose computers run most of their programs from rewriteable memory, called main memory (also called or RAM). Main commonly is implemented in a semiconductor technology called DRAM. All forms of memory provide an array of words. Each word has its own address. Interaction is achieved through a sequence of load or store instructions to specific memory addresses. The load instruction moves a word from main memory to an internal register within the CPU, whereas the store instruction moves the content of a register to main memory. Ideally, we want the programs and data to reside in main memory permanently. This arrangement usually is not possible for the following two reasons: 1) Main memory is usually too small to store all needed programs and data permanently. 2) Main memory is a volatile storage device that loses its contents when power is turned off or otherwise lost. Thus, most computer systems provide secondary storage as an extension of main memory. The main requirement for secondary storage is that it be able to hold large quantities of data permanently. The most common secondary-storage device is a magnetic disk which provides storage for both programs and data. Main memory – only large storage media that the CPU can access directly Secondary storage – extension of main memory that provides large nonvolatile storage capacity Magnetic disks – rigid metal or glass platters covered with magnetic recording material Storage Hierarchy Storage systems organized in hierarchy Speed Operating Systems Fundamentals Page 5  UNIT-1 NOTES Cost Volatility Caching – copying information into faster storage system; main memory can be viewed as a last cache for secondary storage Computer-System Architecture Most systems use a single general-purpose processor (PDAs through mainframes) Most systems have special-purpose processors as well Multiprocessors systems growing in use and importance Also known as parallel systems, tightly-coupled systems Advantages include 1.Increased throughput 2.Economy of scale 3.Increased reliability – graceful degradation or fault tolerance Two types 1.Asymmetric Multiprocessing 2.Symmetric Multiprocessing Operating Systems Fundamentals Page 6  UNIT-1 NOTES How a Modern Computer Works Symmetric Multiprocessing Architecture Multiprocessing adds CPUs to increase computing power. If the CPU has an integrated memory controller, then adding CPUs can also increase the amount of memory addressable in the system. Either way, multiprocessing can cause a system to change its memory access model from uniform memory access to non-uniform memory access UMA is defined as the situation in which access to any RAM from any CPU takes the same amount of time. With NUMA, some parts of memory may take longer to access than other parts, creating a performance penalty. Operating systems can minimize the NUMA penalty through resource management. Operating Systems Fundamentals Page 7  UNIT-1 NOTES A Dual-Core Design We show a dual-core design with two cores on the samechip. In this design, each core has its own register set as well as its own localcache; other designs might use a shared cache or a combination of local andshared caches. Blade servers are a recent development in which multiple processor boards, I/0 boards, and networking boards are placed in the same chassis. The difference between these and traditional multiprocessor systems is that each blade-processor board boots independently and runs its own operating system. Clustered Systems Another type of multiprocessor system is a clustered system, which gathers together multiple CPUs. Clustered systems differ from the multiprocessor systems (loosely coupled). Clustering is usually used to provide high-availability service — that is, service will continue even if one or more systems in the cluster fail. Clustering can be structured asymmetrically or symmetrically. In asymmetric clustering, one machine is in hot-standby mode while the other is running the applications. The hot- standby host machine does nothing but monitor the active server. If that server fails, the hot- standby host becomes the active server. In symmetric mode, two or more hosts are running applications and are monitoring each other. This mode is obviously more efficient, as it uses all of the available hardware. It does require that more than one application be available to run. However, applications must be written to take advantage of the cluster by using a technique known as parallelization which consists of dividing a program into separate components that run in parallel on individual computers in the cluster. Operating Systems Fundamentals Page 8  UNIT-1 NOTES Because most operating systems lack support for simultaneous data access by multiple hosts, parallel clusters are usually accomplished by use of special versions of software and special releases of applications. For example, Oracle Real Application Cluster is a version of Oracle's database that has been designed to run on a parallel cluster. Each machine runs Oracle, and a layer of software tracks access to the shared disk. Each machine has full access to all data in the database. To provide this shared access to data, the system must also supply access control and locking to ensure that no conflicting operations occur. This function, commonly known as a is distributed lock manager (DLM). Operating System Structure Multiprogramming needed for efficiency Single user cannot keep CPU and I/O devices busy at all times Multiprogramming organizes jobs (code and data) so CPU always has one to Execute a subset of total jobs in system is kept in memory Main memory is too small to accommodate all jobs, the jobs are kept initially on the disk in the Job pool. This pool consists of all processes residing on disk awaiting allocation of main memory. The operating system picks and begins to execute one of the jobs in memory. One job selected and run via job scheduling When it has to wait (for I/O for example), OS switches to another job Timesharing (multitasking) is logical extension in which CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing Operating Systems Fundamentals Page 9  UNIT-1 NOTES Memory Layout for Multi programmed System Time sharing (or multitasking) is a logical extension of multiprogramming. In time-sharing systems, the CPU executes multiple jobs by switching among them, but the switches occur so frequently that the users can interact with each program while it is running. Time sharing requires an interactive computer system, which provides direct communication between the user and the system. The user gives instructions to the operating system or to a program directly, using a input device such as a keyboard, mouse, touch pad, or touch screen, and waits for immediate results on an output device. Accordingly, the response time should be short—typically less than one second. A time-shared operating system allows many users to share the computer simultaneously. Since each action or command in a time-shared system tends to be short, only a little CPU time is needed for each user.A program loaded into memory and executing is called a process. When a process executes, it typically executes for only a short time before it either finishes or needs to perform I/O. Time sharing and multiprogramming require that several jobs be kept simultaneously in memory. If several jobs are ready to be brought into memory,and if there is not enough room for all of them, then the system must chooseamong them. Making this decision involves job scheduling. If several jobs are ready to run at the same time, the system must choose which job will run first. Making this decision is CPU scheduling. In a time-sharing system, the operating system must ensure reasonable response time. This goal is sometimes accomplished through swapping, whereby processes are swapped in and out of main memory to the disk. Operating Systems Fundamentals Page 10  UNIT-1 NOTES A more common method for ensuring reasonable response time is virtual memory, a technique that allows the execution of a process that is not completely in memory. The main advantage of the virtual-memory scheme is that it enables users to run programs that are larger than actual physical memory Operating-System Operations Modern operating systems are interrupt driven. If there are no processes to execute, no I/O devices to service, and no users to whom to respond, an operating system will sit quietly, waiting for something to happen. Events are almost always signaled by the occurrence of an interrupt or a trap. A trap (or an exception) is a software-generated interrupt caused either by an error (for example, division by zero or invalid memory access) or by a specific request from a user program that an operating-system service be performed. For each type of interrupt, separate segments of code in the operating system determine what action should be taken. An interrupt service routine is provided to deal with the interrupt. Transition from User to Kernel Mode At the very least, we need two separate modes of operation: user mode and kernel mode (also called supervisor mode, system mode, or privileged mode). A bit, called the mode bit, is added to the hardware of the computer to indicate the current mode: kernel (0) or user (1). With the mode bit, we can distinguish between a task that is executed on behalf of the operating system and one that is executed on behalf of the user. When the computer system is executing on behalf of a user application, the system is in user mode. However, when a user application requests a service from the operating system (via a system call), the system must transition from user to kernel mode to fulfill the request. At system boot time, the hardware starts in kernel mode. The operating system is then loaded and starts user applications in user mode. Whenever a trap or interrupt occurs, the hardware switches from user mode to kernel mode (that is, changes the state of the mode bit to 0). Thus, whenever the operating system gains control of the computer, it is in kernel mode. The system always switches to user mode (by setting the mode bit to 1) before passing control to a user program. The hardware allows privileged instructions to be executed only in kernel mode. If an attempt is made to execute a privileged instruction in user mode, the hardware does not Operating Systems Fundamentals Page 11  UNIT-1 NOTES execute the instruction but rather treats it as illegal and traps it to the operating system. The instruction to switch to kernel mode is an example of a privileged instruction. Some other examples include I/O control, timer management, and interrupt management. System calls provide the means for a user program to ask the operating system to perform tasks reserved for the operating system on the user program’s behalf. A system call is invoked in a variety of ways, depending on the functionality provided by the underlying processor. In all forms, it is the method used by a process to request action by the operating system. A system call usually takes the form of a trap to a specific location in the interrupt vector. This trap can be executed by a generic trap instruction, although some systems (such as MIPS) have a specific syscall instruction to invoke a system call. When a system call is executed, it is typically treated by the hardware as a software interrupt. Control passes through the interrupt vector to a service routine in the operating system, and the mode bit is set to kernel mode. The system-call service routine is a part of the operating system. The kernel examines the interrupting instruction to determine what system call has occurred; a parameter indicates what type of service the user program is requesting. Timer We must ensure that the operating system maintains control over the CPU. We cannot allow a user program to get stuck in an infinite loop or to fail to call system services and never return control to the operating system. To accomplish this goal, we can use a timer. A timer can be set to interrupt the computer after a specified period. The period may be fixed (for example, 1/60 second) or variable (for example, from 1 millisecond to 1 second). A variable timer is generally implemented by a fixed-rate clock and a counter. The operating system sets the counter. Every time the clock ticks, the counter is decremented. When the counter reaches 0, Operating Systems Fundamentals Page 12  UNIT-1 NOTES an interrupt occurs. For instance, a 10-bit counter with a 1-millisecond clock allows interrupts at intervals from 1 millisecond to 1,024 milliseconds, in steps of 1 millisecond. We can use the timer to prevent a user program from running too long. Process Management A program does nothing unless its instructions are executed by a CPU. A program in execution, as mentioned, is a process. A time-shared user program such as a compiler is a process. A word-processing program being run by an individual user on a PC is a process. A system task, such as sending output to a printer, can also be a process (or at least part of one). A process needs certain resources—including CPU time, memory, files, and I/O devices—to accomplish its task. These resources are either given to the process when it is created or allocated to it while it is running. In addition to the various physical and logical resources that a process obtains when it is created, various initialization data (input) may be passed along. A single-threaded process has one program counter specifying the next instruction to execute. The execution of such a process must be sequential. The CPU executes one instruction of the process after another, until the process completes. A multithreaded process has multiple program counters, each pointing to the next instruction to execute for a given thread. A process is the unit of work in a system. A system consists of a collection of processes, some of which are operating-system processes (those that execute system code) and the rest of which are user processes (those that execute user code). The operating system is responsible for the following activities in connection with process management: Process Management Activities Scheduling processes and threads on the CPUs Creating and deleting both user and system processes Suspending and resuming processes Providing mechanisms for process synchronization Providing mechanisms for process communication Operating Systems Fundamentals Page 13  UNIT-1 NOTES A process is a program in execution. It is a unit of work within the system. Program is a passive entity, process is an active entity. Process needs resources to accomplish its task Process termination requires reclaim of any reusable resources Single-threaded process has one program counter specifying location of next instruction to execute Process executes instructions sequentially, one at a time, until completion Multi-threaded process has one program counter per thread Typically system has many processes, some user, some operating system running concurrently on one or more CPUs Memory Management The main memory is central to the operation of a modern computer system. Main memory is a large array of bytes, ranging in size from hundreds of thousands to billions. Each byte has its own address. Main memory is a repository of quickly accessible data shared by the CPU and I/O devices. The central processor reads instructions from main memory during the instruction-fetch cycle and both reads and writes data from main memory during the data- fetch cycle. For a program to be executed, it must be mapped to absolute addresses and loaded into memory. As the program executes, it accesses program instructions and data from memory by generating these absolute addresses. Eventually, the program terminates, its memory space is declared available, and the next program can be loaded and executed. To improve both the utilization of the CPU and the speed of the computer’s response to its users, general-purpose computers must keep several programs in memory, creating a need for memory management. Memory management activities Keeping track of which parts of memory are currently being used and by whom Deciding which processes (or parts thereof) and data to move into and out of memory Allocating and deallocating memory space as needed Operating Systems Fundamentals Page 14  UNIT-1 NOTES Storage Management To make the computer system convenient for users, the operating system provides a uniform, logical view of information storage. The operating system abstracts from the physical properties of its storage devices to define a logical storage unit, the file. The operating system maps files onto physical media and accesses these files via the storage devices. File-System Management File management is one of the most visible components of an operating system. Computers can store information on several different types of physical media. Magnetic disk, optical disk, and magnetic tape are the most common. Each of these media has its own characteristics and physical organization. Each medium is controlled by a device, such as a disk drive or tape drive, that also has its own unique characteristics. These properties include access speed, capacity, data-transfer rate, and access method (sequential or random). A file is a collection of related information defined by its creator. Commonly, files represent programs (both source and object forms) and data. Data files may be numeric, alphabetic, alphanumeric, or binary. OS File Management activities Creating and deleting files and directories Primitives to manipulate files and directories Mapping files onto secondary storage Backup files onto stable (non-volatile) storage media Mass-Storage Management The operating system is responsible for the following activities in connection with disk management: Free-space management Storage allocation Disk scheduling Magnetic tape drives and their tapes and CD and DVD drives and platters are typical tertiary storage devices. Operating Systems Fundamentals Page 15  UNIT-1 NOTES Caching When we need a particular piece of information, we first check whether it is in the cache. If it is, we use the information directly from the cache. If it is not, we use the information from the source, putting a copy in the cache under the assumption that we will need it again soon. In addition, internal programmable registers, such as index registers, provide a high- speed cache for main memory. The programmer (or compiler) implements the register- allocation and register-replacement algorithms to decide which information to keep in registers and which to keep in main memory. Other caches are implemented totally in hardware. For instance, most systems have an instruction cache to hold the instructions expected to be executed next. Without this cache, the CPU would have to wait several cycles while an instruction was fetched from main memory.Because caches have limited size, cache management is an important design problem. Careful selection of the cache size and of a replacement policy can result in greatly increased performance. Main memory can be viewed as a fast cache for secondary storage, since data in secondary storage must be copied into main memory for use and data must be in main memory before being moved to secondary storage for safekeeping. The file-system data, which resides permanently on secondary storage, may appear on several levels in the storage hierarchy. At the highest level, the operating system may maintain a cache of file-system data in main memory. Operating Systems Fundamentals Page 16  UNIT-1 NOTES I/O Subsystem The I/O subsystem consists of several components: A memory-management component that includes buffering, caching, and spooling A general device-driver interface Drivers for specific hardware devices Protection and Security Protection, then, is any mechanism for controlling the access of processes or users to the resources defined by a computer system. Protection and security require the system to be able to distinguish among all its users. Most operating systems maintain a list of user names and associated user identifiers (user IDs). In Windows parlance, this is a security ID (SID). These numerical IDs are unique, one per user. When a user logs in to the system, the authentication stage determines the appropriate user ID for the user. That user ID is associated with all of the user’s processes and threads. When an ID needs to be readable by a user, it is translated back to the user name via the user name list. In some circumstances, we wish to distinguish among sets of users rather than individual users. For example, the owner of a file on a UNIX system may be allowed to issue all operations on that file, whereas a selected set of users may be allowed only to read the file. To accomplish this, we need to define a group name and the set of users belonging to that group. Group functionality can be implemented as a system-wide list of group names and group identifiers. A user can be in one or more groups, depending on operating-system design decisions. The user’s group IDs are also included in every associated process and thread. Systems generally first distinguish among users, to determine who can do what User identities (user IDs, security IDs) include name and associated number, one per user User ID then associated with all files, processes of that user to determine access control Group identifier (group ID) allows set of users to be defined and controls managed, then also associated with each process, file Privilege escalation allows user to change to effective ID with more rights Operating Systems Fundamentals Page 17  UNIT-1 NOTES Operating-System Services An operating system provides an environment for the execution of programs. It provides certain services to programs and to the users of those programs. The specific services provided, of course, differ from one operating system to another, but we can identify common classes. User interface. Almost all operating systems have a user interface (UI). This interface can take several forms. One is a command-line interface (CLI), which uses text commands and a method for entering them (say, a keyboard for typing in commands in a specific format with specific options). Another is a batch interface, in which commands and directives to control those commands are entered into files, and those files are executed. Most commonly, a graphical user interface (GUI) is used. Here, the interface is a window system with a pointing device to direct I/O, choose from menus, and make selections and a keyboard to enter text. Program execution. The system must be able to load a program into memory and to run that program. The program must be able to end its 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. For specific devices, special functions may be desired (such as recording to a CD or DVD drive or blanking a display screen). For efficiency and protection, users usually cannot control I/O devices directly. Therefore, the operating system must provide a means to do I/O. Operating Systems Fundamentals Page 18  UNIT-1 NOTES File-system manipulation. The file system is of particular interest. Obviously, programs need to read and write files and directories. They also need to create and delete them by name, search for a given file, and list file information. Finally, some operating systems include permissions management to allow or deny access to files or directories based on file ownership. Communications. There are many circumstances in which one process needs to exchange information with another process. Such communication may occur between processes that are executing on the same computer or between processes that are executing on different computer systems tied together by a computer network. Communications may be implemented via shared memory, in which two or more processes read and write to a shared section of memory, or message passing, in which packets of information in predefined formats are moved between processes by the operating system. Error detection. The operating system needs to be detecting and correcting errors constantly. Errors may occur in the CPU and memory hardware (such as a memory error or a power failure), in I/O devices (such as a parity error on disk, a connection failure on a network, or lack of paper in the printer), and in the user program (such as an arithmetic overflow, an attempt to access an illegal memory location, or a too-great use of CPU time). For each type of error, the operating system should take the appropriate action to ensure correct and consistent computing. Resource allocation. When there are multiple users or multiple jobs running at the same time, resources must be allocated to each of them. The operating system manages many different types of resources. Some (such as CPU cycles, main memory, and file storage) may have special allocation code, whereas others (such as I/O devices) may have much more general request and release code. For instance, in determining how best to use the CPU, operating systems have CPU-scheduling routines that take into account the speed of the CPU, the jobs that must be executed, the number of registers available, and other factors. Accounting. We want to keep track of which users use how much and what kinds of computer resources. This record keeping may be used for accounting (so that users can be billed) or simply for accumulating usage statistics. Usage statistics may be a valuable tool for researchers who wish to reconfigure the system to improve computing services. Protection and security. The owners of information stored in a multiuser or networked computer system may want to control use of that information. When several separate processes execute concurrently, it should not be possible for one process to interfere with Operating Systems Fundamentals Page 19  UNIT-1 NOTES the others or with the operating system itself. Protection involves ensuring that all access to system resources is controlled. User and Operating-System Interface One provides a command-line interface, or command interpreter, that allows users to directly enter commands to be performed by the operating system. The other allows users to interface with the operating system via a graphical user interface, or GUI. interpreters are known as shells. For example, on UNIX and Linux systems, a user may choose among several different shells, including the Bourne shell, C shell, Bourne-Again shell, Korn shell, and others. The main function of the command interpreter is to get and execute the next user- specified command. Many of the commands given at this level manipulate files: create, delete, list, print, copy, execute, and so on. The MS-DOS and UNIX shells operate in this way. System Calls System calls provide an interface to the services made available by an operating system. These calls are generally available as routines written in C and C++, although certain low-level tasks (for example, tasks where hardware must be accessed directly) may have to be written using assembly-language instructions. An example to illustrate how system calls are used: writing a simple program to read data from one file and copy them to another file. Frequently, systems execute thousands of system calls per second. Most programmers never see this level of detail, however. Typically, application developers design programs according to an application programming interface (API). The API specifies a set of functions that are available to an application programmer, including the parameters that are passed to each function and the return values the programmer can expect. Three of the most common APIs available to application programmers are the Windows API for Windows systems, the POSIX API for POSIX-based systems (which include virtually all versions of UNIX, Linux, and Mac OSX), and the Java API for programs that run on the Java virtual machine. A programmer accesses an API via a library of code provided by the operating system. In the case of UNIX and Linux for programs written in the C language, the library is called libc. Behind the scenes, the functions that make up an API typically invoke the actual system calls on behalf of the application programmer. For example, the Windows function Operating Systems Fundamentals Page 20  UNIT-1 NOTES CreateProcess() (which unsurprisingly is used to create a new process) actually invokes the NTCreateProcess() system call in the Windows kernel. For most programming languages, the run-time support system (a set of functions built into libraries included with a compiler) provides a system call interface that serves as the link to Operating Systems Fundamentals Page 21  UNIT-1 NOTES system calls made available by the operating system. The system-call interface intercepts function calls in the API and invokes the necessary system calls within the operating system. Types of System Calls Process control File management Device management Information maintenance Communications Protection Process Control A running program needs to be able to halt its execution either normally (end()) or abnormally (abort()). If a system call is made to terminate the currently running program abnormally, or if the program runs into a problem and causes an error trap, a dump of memory is sometimes taken and an error message generated. The dump is written to disk and may be examined by a debugger—a system program designed to aid the programmer in finding and correcting errors, or bugs—to determine the cause of the problem. Under either normal or abnormal circumstances, the operating system must transfer control to the invoking command interpreter. The command interpreter then reads the next command. Operating Systems Fundamentals Page 22  UNIT-1 NOTES Operating Systems Fundamentals Page 23  UNIT-1 NOTES Operating Systems Fundamentals Page 24  UNIT-1 NOTES FreeBSD (derived from Berkeley UNIX) is an example of a multitasking system. When a user logs on to the system, the shell of the user’s choice is run. This shell is similar to the MS-DOS shell in that it accepts commands and executes programs that the user requests. However, since FreeBSD is a multitasking system, the command interpreter may continue running while another program is executed (Figure 2.10). To start a new process, the shell executes a fork() system call. Then, the selected program is loaded into memory via an exec() system call, and the program is executed. Depending on the way the command was issued, the shell then either waits for the process to finish or runs the process “in the background.” File Management We first need to be able to create() and delete() files. Either system call requires the name of the file and perhaps some of the file’s attributes. Once the file is created, we need to open() it and to use it. We may also read(), write(), or reposition() (rewind or skip to the end of the file, for example). Finally, we need to close() the file, indicating that we are no longer using it. Device Management Process may need several resources to execute—main memory, disk drives, access to files, and so on. If the resources are available, they can be granted, and control can be returned to the user process. Otherwise, the process will have to wait until sufficient resources are available. The various resources controlled by the operating system can be thought of as devices. Some of these devices are physical devices (for example, disk drives), while others Operating Systems Fundamentals Page 25  UNIT-1 NOTES can be thought of as abstract or virtual devices (for example, files).A system with multiple users may require us to first request() a device, to ensure exclusive use of it. After we are finished with the device, we release() it. Once the device has been requested (and allocated to us), we can read(), write(), and (possibly) reposition() the device, just as we can with files. Information Maintenance Many system calls exist simply for the purpose of transferring information between the user program and the operating system. For example, most systems have a system call to return the current time() and date(). Other system calls may return information about the system, such as the number of current users, the version number of the operating system, the amount of free memory or disk space, and so on. Another set of system calls is helpful in debugging a program. Many systems provide system calls to dump() memory. This provision is useful for debugging. Communication There are two common models of inter process communication: the message passing model and the shared-memory model. In the message-passing model, the communicating processes exchange messages with one another to transfer information. Messages can be exchanged between the processes either directly or indirectly through a common mailbox. Before communication can take place, a connection must be opened. Each computer in a network has a host name by which it is commonly known. A host also has a network identifier, such as an IP address. Similarly, each process has a process name, and this name is translated into an identifier by which the operating system can refer to the process. The get hostid() and get processid() system calls do this translation. The identifiers are then passed to the generalpurpose open() and close() calls provided by the file system or to specific open connection() and close connection() system calls, depending on the system’s model of communication. The recipient process usually must give its permission for communication to take place with an accept connection() call. In the shared-memory model, processes use shared memory create() and shared memory attach() system calls to create and gain access to regions of memory owned by other processes. Operating Systems Fundamentals Page 26  UNIT-1 NOTES Protection Protection provides a mechanism for controlling access to the resources provided by a computer system. Typically, system calls providing protection include set permission() and get permission(), which manipulate the permission settings of resources such as files and disks. The allow user() and deny user() system calls specify whether particular users can—or cannot—be allowed access to certain resources. System Programs System programs, also known as system utilities, provide a convenient environment for program development and execution. Some of them are simply user interfaces to system calls. Others are considerably more complex. They can be divided into these categories: File management: These programs create, delete, copy, rename, print, dump, list, and generally manipulate files and directories. Status information: Some programs simply ask the system for the date, time, amount of available memory or disk space, number of users, or similar status information. Others are more complex, providing detailed performance, logging, and debugging information. File modification: Several text editors may be available to create and modify the content of files stored on disk or other storage devices. There may also be special commands to search contents of files or perform transformations of the text. Programming-language support: Compilers, assemblers, debuggers, and interpreters for common programming languages (such as C, C++, Java, and PERL) are often provided with the operating system or available as a separate download. Program loading and execution: Once a program is assembled or compiled, it must be loaded into memory to be executed. The system may provide absolute loaders, relocatable loaders, linkage editors, and overlay loaders. Debugging systems for either higher-level languages or machine language are needed as well. Background services: All general-purpose systems have methods for launching certain system-program processes at boot time. Some of these processes terminate after completing their tasks, while others continue to run until the system is halted. Constantly running system-program processes are known as services, subsystems, or daemons. Operating Systems Fundamentals Page 27  UNIT-1 NOTES Along with system programs, most operating systems are supplied with programs that are useful in solving common problems or performing common operations. Such application programs include Web browsers, word processors and text formatters, spreadsheets, database systems, compilers, plotting and statistical-analysis packages, and games. Operating Systems Fundamentals Page 28  UNIT-2 NOTES UNIT – II PROCESS MANAGEMENT: Process concepts- Operations on processes, IPC, Process Scheduling (T1: Ch-3). PROCESS COORDINATION: Process synchronization- critical section problem, Peterson’s solution, synchronization hardware, semaphores, classic problems of synchronization, readers and writers problem, dining philosopher’s problem, monitors (T1: Ch-5). Introduction Process: A process, which is a program in execution. A process is the unit of work in a modern time-sharing system. A system therefore consists of a collection of processes: operating system processes executing system code and user processes executing user code. Potentially, all these processes can execute concurrently, with the CPU (or CPUs) multiplexed among them. By switching the CPU between processes, the operating system can make the computer more productive. A process is also known as job or task. A process is more than the program code, which is sometimes known as the text section. It also includes the current activity, as represented by the value of the program counter and the contents of the processor’s registers. A process generally also includes the process stack, which contains temporary data (such as function parameters, return addresses, and local variables), and a data section, which contains global variables. A process may also include a heap, which is memory that is dynamically allocated during process run time. The structure of a process in memory is shown in Figure 3.1. Operating Systems Page 1  UNIT-2 NOTES We emphasize that a program by itself is not a process. A program is a passive entity, such as a file containing a list of instructions stored on disk (often called an executable file). In contrast, a process is an active entity, with a program counter specifying the next instruction to execute and a set of associated resources. A program becomes a process when an executable file is loaded into memory. Process State As a process executes, it changes state. The state of a process is defined in part by the current activity of that process. A process may be in one of the following states: New. The process is being created. Running. Instructions are being executed. Waiting. The process is waiting for some event to occur (such as an I/O completion or reception of a signal). Ready. The process is waiting to be assigned to a processor. Terminated. The process has finished execution. It is important to realize that only one process can be running on any processor at any instant. Many processes may be ready and waiting, however. The state diagram corresponding to these states is presented in Figure 3.2. 2.3 Process Control Block Each process is represented in the operating system by a process control block (PCB)—also called a task control block. A PCB is shown in Figure 3.3. It contains many pieces of information associated with a specific process, including these: Operating Systems Page 2  UNIT-2 NOTES Process state. The state may be new, ready, running, and waiting, halted, and so on. Program counter. The counter indicates the address of the next instruction to be executed for this process. CPU registers. The registers vary in number and type, depending on the computer architecture. They include accumulators, index registers, stack pointers, and general-purpose registers, plus any condition-code information. Along with the program counter, this state information must be saved when an interrupt occurs, to allow the process to be continued correctly afterward (Figure 3.4). CPU-scheduling information. This information includes a process priority, pointers to scheduling queues, and any other scheduling parameters. Memory-management information. This information may include such items as the value of the base and limit registers and the page tables, or the segment tables, depending on the memory system used by the operating system. Accounting information. This information includes the amount of CPU and real time used, time limits, account numbers, job or process numbers, and so on. I/O status information. This information includes the list of I/O devices allocated to the process, a list of open files, and so on. In brief, the PCB simply serves as the repository for any information that may vary from process to process. Operating Systems Page 3  UNIT-2 NOTES Threads The process model discussed so far has implied that a process is a program that performs a single thread of execution. For example, when a process is running a word-processor program, a single thread of instructions is being executed. This single thread of control allows the process to perform only one task at a time. The user cannot simultaneously type in characters and run the spell checker within the same process, for example. Most modern operating systems have extended the process concept to allow a process to have multiple threads of execution and thus to perform more than one task at a time. This feature is especially beneficial on multicore systems, where multiple threads can run in parallel. Operating Systems Page 4  UNIT-2 NOTES 2.4 Operations on Processes The processes in most systems can execute concurrently, and they may be created and deleted dynamically. Process Creation During the course of execution, a process may create several new processes. As mentioned earlier, the creating process is called a parent process, and the new processes are called the children of that process. Each of these new processes may in turn create other processes, forming a tree of processes. Most operating systems (including UNIX, Linux, and Windows) identify processes according to a unique process identifier (or pid), which is typically an integer number. The pid provides a unique value for each process in the system, and it can be used as an index to access various attributes of a process within the kernel. When a process creates a new process, two possibilities for execution exist: 1. The parent continues to execute concurrently with its children. 2. The parent waits until some or all of its children have terminated. Process Termination A process terminates when it finishes executing its final statement and asks the operating system to delete it by using the exit() system call. At that point, the process may return a status value (typically an integer) to its parent process (via the wait() system call). All the Operating Systems Page 5  UNIT-2 NOTES resources of the process—including physical and virtual memory, open files, and I/O buffers—are deallocated by the operating system. A parent may terminate the execution of one of its children for a variety of reasons, such as these: The child has exceeded its usage of some of the resources that it has been allocated. (To determine whether this has occurred, the parent must have a mechanism to inspect the state of its children.) The task assigned to the child is no longer required. The parent is exiting, and the operating system does not allow a child to continue if its parent terminates. Some systems do not allow a child to exist if its parent has terminated. In such systems, if a process terminates (either normally or abnormally), then all its children must also be terminated. This phenomenon, referred to as cascading termination, is normally initiated by the operating system. A process that has terminated, but whose parent has not yet called wait(), is known as a zombie process. All processes transition to this state when they terminate, but generally they exist as zombies only briefly. 2.2 Inter process Communication Processes executing concurrently in the operating system may be either independent processes or cooperating processes. A process is independent if it cannot affect or be affected by the other processes executing in the system. Any process that does not share data with any other process is independent. A process is cooperating if it can affect or be affected by the other processes executing in the system. Clearly, any process that shares data with other processes is a cooperating process. There are several reasons for providing an environment that allows process cooperation: Information sharing. Since several users may be interested in the same piece of information (for instance, a shared file), we must provide an environment to allow concurrent access to such information. Computation speedup. If we want a particular task to run faster, we must break it into subtasks, each of which will be executing in parallel with the others. Notice that such a speedup can be achieved only if the computer has multiple processing cores. Modularity. We may want to construct the system in a modular fashion, dividing the system functions into separate processes or threads. Operating Systems Page 6  UNIT-2 NOTES Convenience. Even an individual user may work on many tasks at the same time. For instance, a user may be editing, listening to music, and compiling in parallel. Cooperating processes require an inter process communication (IPC) mechanism that will allow them to exchange data and information. There are two fundamental models of inter process communication: shared memory and message passing. In the shared-memory model, a region of memory that is shared by cooperating processes is established. Processes can then exchange information by reading and writing data to the shared region. In the message-passing model, communication takes place by means of messages exchanged between the cooperating processes. The two communications models are contrasted in Figure 3.12. Operating Systems Page 7  UNIT-2 NOTES To illustrate the concept of cooperating processes, let’s consider the producer–consumer problem, which is a common paradigm for cooperating processes. A producer process produces information that is consumed by a consumer process. For example, a compiler may produce assembly code that is consumed by an assembler. The assembler, in turn, may produce object modules that are consumed by the loader. One solution to the producer–consumer problem uses shared memory. To allow producer and consumer processes to run concurrently, we must have available a buffer of items that can be filled by the producer and emptied by the consumer. This buffer will reside in a region of memory that is shared by the producer and consumer processes. A producer can produce one item while the consumer is consuming another item. The producer and consumer must be synchronized, so that the consumer does not try to consume an item that has not yet been produced. Two types of buffers can be used. The unbounded buffer places no practical limit on the size of the buffer. The consumer may have to wait for new items, but the producer can always produce new items. The bounded buffer assumes a fixed buffer size. In this case, the consumer must wait if the buffer is empty, and the producer must wait if the buffer is full. Message passing provides a mechanism to allow processes to communicate and to synchronize their actions without sharing the same address space. It is particularly useful in a distributed environment, where the communicating processes may reside on different Operating Systems Page 8  UNIT-2 NOTES computers connected by a network. For example, an Internet chat program could be designed so that chat participants communicate with one another by exchanging messages. A message- passing facility provides at least two operations: send(message) receive(message) Messages sent by a process can be either fixed or variable in size. Message passing may be either blocking or nonblocking— also known as synchronous and asynchronous. Blocking send. The sending process is blocked until the message is received by the receiving process or by the mailbox. Nonblocking send. The sending process sends the message and resumes operation. Blocking receive. The receiver blocks until a message is available. Nonblocking receive. The receiver retrieves either a valid message or a null. 2.3 Process Scheduling The objective of multiprogramming is to have some process running at all times, to maximize CPU utilization. The objective of time sharing is to switch the CPU among processes so frequently that users can interact with each program while it is running. To meet these objectives, the process scheduler selects an available process (possibly from a set of several available processes) for program execution on the CPU. For a single-processor system, there will never be more than one running process. If there are more processes, the rest will have to wait until the CPU is free and can be rescheduled. Scheduling Queues As processes enter the system, they are put into a job queue, which consists of all processes in the system. The processes that are residing in main memory and are ready and waiting to execute are kept on a list called the ready queue. This queue is generally stored as a linked list. A ready-queue header contains pointers to the first and final PCBs in the list. Each PCB includes a pointer field that points to the next PCB in the ready queue. The list of processes waiting for a particular I/O device is called a device queue. A common representation of process scheduling is a queueing diagram, such as that in Figure 3.6. Each rectangular box represents a queue. Two types of queues are present: the ready queue and a set of device queues. The circles represent the resources that serve the queues, and the arrows indicate the flow of processes in the system. Operating Systems Page 9  UNIT-2 NOTES A new process is initially put in the ready queue. It waits there until it is selected for execution, or dispatched. Once the process is allocated the CPU and is executing, one of several events could occur: The process could issue an I/O request and then be placed in an I/O queue. The process could create a new child process and wait for the child’s termination. The process could be removed forcibly from the CPU, as a result of an interrupt, and be put back in the ready queue. Schedulers A process migrates among the various scheduling queues throughout its lifetime. The operating system must select, for scheduling purposes, processes from these queues in some fashion. The selection process is carried out by the appropriate scheduler. Often, in a batch system, more processes are submitted than can be executed immediately. These processes are spooled to a mass-storage device (typically a disk), where they are kept for later execution. The long-term scheduler, or job scheduler, selects processes from this pool and loads them into memory for execution. The short-term scheduler, or CPU scheduler, selects from among the processes that are ready to execute and allocates the CPU to one of them. The long-term scheduler executes much less frequently; minutes may separate the creation of one new process and the next. The long-term scheduler controls the degree of multiprogramming (the number of processes in memory). Operating Systems Page 10  UNIT-2 NOTES It is important that the long-term scheduler make a careful selection. In general, most processes can be described as either I/O bound or CPU bound. An I/O-bound process is one that spends more of its time doing I/O than it spends doing computations. A CPU-bound process, in contrast, generates I/O requests infrequently, using more of its time doing computations. It is important that the long-term scheduler select a good process mix of I/O- bound and CPU-bound processes. The system with the best performance will thus have a combination of CPU-bound and I/O-bound processes. Context Switch Interrupts cause the operating system to change a CPU from its current task and to run a kernel routine. Such operations happen frequently on general-purpose systems. When an interrupt occurs, the system needs to save the current context of the process running on the CPU so that it can restore that context when its processing is done, essentially suspending the process and then resuming it. Switching the CPU to another process requires performing a state save of the current process and a state restore of a different process. This task is known as a context switch. When a context switch occurs, the kernel saves the context of the old process in its PCB and loads the saved context of the new process scheduled to run. Context-switch times are highly dependent on hardware support. Operating Systems Page 11  UNIT-2 NOTES 2.4 Process synchronization A situation where several processes access and manipulate the same data concurrently and the outcome of the execution depends on the particular order in which the access takes place, is called a race condition. To guard against the race condition we need to ensure that only one process at a time can be manipulating the variable counter. To make such a guarantee, we require that the processes be synchronized in some way. 2.4.1 The Critical-Section Problem We begin our consideration of process synchronization by discussing the so called critical- section problem. Consider a system consisting of n processes {P0, P1,..., Pn−1}. Each process has a segment of code, called a critical section, in which the process may be changing common variables, updating a table, writing a file, and so on. The important feature of the system is that, when one process is executing in its critical section, no other process is allowed to execute in its critical section. That is, no two processes are executing in their critical sections at the same time. The critical-section problem is to design a protocol that the processes can use to cooperate. Each process must request permission to enter its critical section. The section of code implementing this request is the entry section. The critical section may be followed by an exit section. The remaining code is the remainder section. The general structure of a typical process Pi is shown in Figure 5.1. The entry section and exit section are enclosed in boxes to highlight these important segments of code. A solution to the critical-section problem must satisfy the following three requirements: Operating Systems Page 12  UNIT-2 NOTES 1. Mutual exclusion. If process Pi is executing in its critical section, then no other processes can be executing in their critical sections. 2. Progress. If no process is executing in its critical section and some processes wish to enter their critical sections, then only those processes that are not executing in their remainder sections can participate in deciding which will enter its critical section next, and this selection cannot be postponed indefinitely. 3. Bounded waiting. There exists a bound, or limit, on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted. Two general approaches are used to handle critical sections in operating systems: 1) Preemptive kernels and 2) non-Preemptive kernels. A preemptive (preemption is the act of temporarily interrupting a task being carried out by a computer system) kernel allows a process to be preempted while it is running in kernel mode. A nonpreemptive kernel does not allow a process running in kernel mode to preempted; a kernel-mode process will run until it exits kernel mode, blocks, or voluntarily yields control of the CPU. 2.4.2 Peterson’s Solution A classic software-based solution to the critical-section problem known as Peterson’s solution. Because of the way modern computer architectures perform basic machine- language instructions, such as load and store, there are no guarantees that Peterson’s solution will work correctly on such architectures. However, we present the solution because it provides a good algorithmic description of solving the critical-section problem and illustrates some of the complexities involved in designing software that addresses the requirements of mutual exclusion, progress, and bounded waiting. Operating Systems Page 13  UNIT-2 NOTES Peterson’s Solution is a classical software based solution to the critical section problem. In Peterson’s solution, we have two shared variables: boolean flag[i] :Initialized to FALSE, initially no one is interested in entering the critical section int turn : The process whose turn is to enter the critical section. // code for producer (j) // producer j is ready // to produce an item flag[j] = true; // but consumer (i) can consume an item turn = i; // if consumer is ready to consume an item // and if its consumer's turn while (flag[i] == true && turn == i) { // then producer will wait } // otherwise producer will produce // an item and put it into buffer (critical Section) Operating Systems Page 14  UNIT-2 NOTES // Now, producer is out of critical section flag[j] = false; // end of code for producer //-------------------------------------------------------- // code for consumer i // consumer i is ready // to consume an item flag[i] = true; // but producer (j) can produce an item turn = j; // if producer is ready to produce an item // and if its producer's turn while (flag[j] == true && turn == j) { // then consumer will wait } // otherwise consumer will consume // an item from buffer (critical Section) // Now, consumer is out of critical section flag[i] = false; // end of code for consumer Explanation of Peterson’s algorithm – Peterson’s Algorithm is used to synchronize two processes. It uses two variables, a bool array flag of size 2 and an int variable turn to accomplish it. In the solution i represents the Consumer and j represents the Producer. Initially the flags are false. When a process wants to execute it’s critical section, it sets it’s flag to true and turn as the index of the other process. This means that the process wants to execute but it will allow the other process to run first. The process performs busy waiting until the other process has finished it’s own critical section. After this the current process enters it’s critical section and adds or removes a random number from the shared buffer. After completing the critical section, it sets it’s own flag to false, indication it does not wish to execute anymore. Operating Systems Page 15  UNIT-2 NOTES Synchronization Hardware However, as mentioned, software-based solutions such as Peterson’s are not guaranteed to work on modern computer architectures. In the following discussions, we explore several more solutions to the critical-section problem using techniques ranging from hardware to software-based APIs available to both kernel developers and application programmers. All these solutions are based on the premise of locking —that is, protecting critical regions through the use of locks. Mutex Locks operating-systems designers build software tools to solve the critical-section problem. The simplest of these tools is the mutex lock. (In fact, the term mutex is short for mutual exclusion.) We use the mutex lock to protect critical regions and thus prevent race conditions. That is, a process must acquire the lock before entering a critical section; it releases the lock when it exits the critical section. The acquire()function acquires the lock, and the release() function releases the lock. A mutex lock has a boolean variable available whose value indicates if the lock is available or not. If the lock is available, a call to acquire() succeeds, and the lock is then considered unavailable. A process that attempts to acquire an unavailable lock is blocked until the lock is released. The definition of acquire() is as follows: acquire() { while (!available) ; available = false;; } Operating Systems Page 16  UNIT-2 NOTES The definition of release() is as follows: release() { available = true; } The main disadvantage of the implementation given here is that it requires busy waiting. While a process is in its critical section, any other process that tries to enter its critical section must loop continuously in the call to acquire(). In fact, this type of mutex lock is also called a spinlock because the process “spins” while waiting for the lock to become available. Semaphores Mutex locks, as we mentioned earlier, are generally considered the simplest of synchronization tools. In this section, we examine a more robust tool that can behave similarly to a mutex lock but can also provide more sophisticated ways for processes to synchronize their activities. A semaphore S is an integer variable that, apart from initialization, is accessed only through two standard atomic operations: wait() and signal(). The wait() operation was originally termed P; signal() was originally called V. The definition of wait() is as follows: wait(S) { while (S pairs, as shown below. Note that some domains may be disjoint while others overlap. System with three protection domains. The association between a process and a domain may be static or dynamic. o If the association is static, then the need-to-know principle requires a way of changing the contents of the domain dynamically. o If the association is dynamic, then there needs to be a mechanism for domain switching. Domains may be realized in different fashions - as users, or as processes, or as procedures. E.g. if each user corresponds to a domain, then that domain defines the access of that user, and changing domains involves changing user ID. Operating Systems Page 2  UNIT-5 NOTES A domain can be realized in a variety of ways: Each user may be a domain. In this case, the set of objects that can be accessed depends on the identity of the user. Domain switching occurs when the user is changed—generally when one user logs out and another user logs in. Each process may be a domain. In this case, the set of objects that can be accessed depends on the identity of the process. Domain switching occurs when one process sends a message to another process and then waits for a response. Each procedure may be a domain. In this case, the set of objects that can be accessed corresponds to the local variables defined within the procedure. Domain switching occurs when a procedure call is made. Example: UNIX UNIX associates domains with users. Certain programs operate with the SUID bit set, which effectively changes the user ID, and therefore the access domain, while the program is running. ( and similarly for the SGID bit. ) Unfortunately this has some potential for abuse. An alternative used on some systems is to place privileged programs in special directories, so that they attain the identity of the directory owner when they run. This prevents crackers from placing SUID programs in random directories around the system. Yet another alternative is to not allow the changing of ID at all. Instead, special privileged daemons are launched at boot time, and user processes send messages to these daemons when they need special tasks performed. Example: MULTICS The MULTICS system uses a complex system of rings, each corresponding to a different protection domain, as shown below: MULTICS ring structure Operating Systems Page 3  UNIT-5 NOTES Rings are numbered from 0 to 7, with outer rings having a subset of the privileges of the inner rings. Each file is a memory segment, and each segment description includes an entry that indicates the ring number associated with that segment, as well as read, write, and execute privileges. Each process runs in a ring, according to the current-ring-number, a counter associated with each process. A process operating in one ring can only access segments associated with higher ( farther out ) rings, and then only according to the access bits. Processes cannot access segments associated with lower rings. Domain switching is achieved by a process in one ring calling upon a process operating in a lower ring, which is controlled by several factors stored with each segment descriptor: o An access bracket, defined by integers b1 b2 o A list of gates, identifying the entry points at which the segments may be called. If a process operating in ring i calls a segment whose bracket is such that b1