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Process Description and Control Chapter 3 1 Requirements of an Operating System • Interleave the execution of multiple processes to maximize processor utilization while providing reasonable response time • Allocate resources to processes • Support interprocess communication and user creation of processes 2 Process • A program in execution • An instance of a program running on a computer • The entity that can be assigned to and executed on a processor • A unit of activity characterized by the execution of a sequence of instructions, a current state, and an associated set of system instructions 3 Process Elements • At any given time, while the program is executing, this process can be uniquely characterized by a number of elements, as below: • Identifier(a unique id to distinguish it from all other process). • State(process is executing it is in running state) • Priority(Priority level relative to other processes) • Program counter(address of the next instruction to be executed) • Memory pointers(pointers to the program code and data associated with this process and if nay memory blocks shared with other process) 4 Process Elements • Context data (processor’s register data when the process is executed) • I/O status information( Includes outstanding I/O requests,I/O devices(e.g. tape drives) assigned to this process, alist of files in use by the process etc) • Accounting information(includes amount of processor time and clock time used,time limits,account numbers etc) 5 Process Control Block • Contains the process elements • Created and manage by the operating system • Allows support for multiple processes 6 Process Control Block Identifier State Priority Program counter Memory pointers Context data I/O status information Accounting information Figure 3.1 Simplified Process Control Block 7 Trace of Process • Sequence of instruction that execute for a process • Dispatcher switches the processor from one process to another 8 Two-State Process Model • Process may be in one of two states – Running – Not-running 9 Not-Running Process in a Queue 10 Process Creation 11 Process Termination 12 13 Processes • Not-running – ready to execute • Blocked – waiting for I/O • some processes in the Not Running state are ready to execute, while others are blocked, waiting for an I/O operation to complete. • Thus, using a single queue, the dispatcher could not just select the process at the oldest end of the queue. • To handle this situation is to split the Not Running state into two states: • Ready • Blocked. • we have added two additional states that will prove useful. 14 A Five-State Model • Running (process that is currently executed) • Ready (prepared to execute when given the opportunity • Blocked (cannot be executed until some event occurs) • New (a process that is created but has not yet been admitted to the pool of executable process by the OS. A new process has not yet been loaded into Memory Management, although its PCB has been created) • Exit ( a process is released from the pool of executable process by the OS,either because it halted or aborted) 15 Five-State Process Model • The five states in this new diagram are as follows: 16 Using Two Queues 17 Multiple Blocked Queues Release Ready Queue Admit Dispatch Processor Timeout Event 1 Queue Event 1 Occurs Event 1 Wait Event 2 Queue Event 2 Wait Event 2 Occurs Event n Queue Event n Occurs Event n Wait (b) Multiple block ed queues 18 Suspended Processes • Processor is faster than I/O so all processes could be waiting for I/O • Swap these processes to disk to free up more memory • Blocked state becomes suspend state when swapped to disk • Two new states – Blocked/Suspend – Ready/Suspend 19 We have two new state with five state model as below: – Blocked /Suspend : Process is in secondary memory and awaiting an event. – Ready/Suspend : Process is in secondary memory but is available for execution as soon as it is loaded into memory. 20 One Suspend State 21 Two Suspend States 22 Characteristics of a suspended Process 1. Process is not immediately available for execution. 2. Process may or may not be waiting on an event. 3. Process was placed in a suspended state by an agent : either itself , a parent process, or the operating system,for the purpose of preventing its execution. 4. Process may not be removed form this state until the agent explicitly orders the removal. 23 Reasons for Process Suspension 24 Process Description 25 Processes and Resources 26 Operating System Control Structures • Information about the current status of each process and resource • Tables are constructed for each entity the operating system manages 27 Memory Tables Process 1 Memory Devices Process Image I/O Tables Files Processes File Tables Primary Process Table Process 1 Process 2 Process 3 Process Image Process n Process n Figure 3.11 General Structure of Operating System Control Tables 28 Memory Tables • Used to keep track of both main and secondary memory Memory tables must include the following information : • Allocation of main memory to processes • Allocation of secondary memory to processes • Protection attributes for access to shared memory regions • Information needed to manage virtual memory 29 I/O Tables • Used by the OS to manage the I/O devices and channels of the computer system. • I/O device is available or assigned • Status of I/O operation • Location in main memory being used as the source or destination of the I/O transfer 30 File Tables • • • • • Existence of files Location on secondary memory Current Status Attributes Sometimes this information is maintained by a file management system 31 Process Table • Where process is located • Attributes in the process control block – Program – Data – Stack To create tables the OS must have some knowledge about the capacity of the MM,what I/O devices are connected and their identifiers and so on. 32 Process Control Structures If the OS has to manage and control a process it must know • Where the process is located • Attributes of the process that are necessary for its management. (e.g process ID,process state,location in memory ). 33 Process Image • The collection of program,data,stack and attributes are referred to as the process image. 34 Process Image 35 3 categories of PCB(general) • 1. Process identification • 2. Processor State Information • 3. Process Control Information 36 Process Control Block • Process identification – Identifiers • Numeric identifiers that may be stored with the process control block include – Identifier of this process – Identifier of the process that created this process (parent process) – User identifier 37 Processor State Information • Contents of processor registers – User-visible registers – Control and status registers – Stack pointers • Program status word (PSW) – contains status information – Example: the EFLAGS register on Pentium machines 38 Process Control Block • Processor State Information – User-Visible Registers • A user-visible register is one that may be referenced by means of the machine language that the processor executes while in user mode. Typically, there are from 8 to 32 of these registers, although some RISC implementations have over 100. 39 Process Control Block • Processor State Information – Control and Status Registers These are a variety of processor registers that are employed to control the operation of the processor. These include • Program counter: Contains the address of the next instruction to be fetched • Condition codes: Result of the most recent arithmetic or logical operation (e.g., sign, zero, carry, equal, overflow) • Status information: Includes interrupt enabled/disabled flags, execution mode 40 Process Control Block • Processor State Information – Stack Pointers • Each process has one or more last-in-first-out (LIFO) system stacks associated with it. A stack is used to store parameters and calling addresses for procedure and system calls. The stack pointer points to the top of the stack. 41 Process Control Block • Process Control Information – Scheduling and State Information This is information that is needed by the operating system to perform its scheduling function. Typical items of information: •Process state: defines the readiness of the process to be scheduled for execution (e.g., running, ready, waiting, halted). •Priority: One or more fields may be used to describe the scheduling priority of the process. In some systems, several values are required (e.g., default, current, highest-allowable) •Scheduling-related information: This will depend on the scheduling algorithm used. Examples are the amount of time that the process has been waiting and the amount of time that the process executed the last time it was running. •Event: Identity of event the process is awaiting before it can be resumed 42 Process Control Block • Process Control Information – Data Structuring • A process may be linked to other process in a queue, ring, or some other structure. • For example, all processes in a waiting state for a particular priority level may be linked in a queue. • A process may exhibit a parent-child (creator-created) relationship with another process. • The process control block may contain pointers to other processes to support these structures. 43 Process Control Block • Process Control Information – Interprocess Communication • Various flags, signals, and messages may be associated with communication between two independent processes. Some or all of this information may be maintained in the process control block. – Process Privileges • Processes are granted privileges in terms of the memory that may be accessed and the types of instructions that may be executed. In addition, privileges may apply to the use of system utilities and services. 44 Process Control Block • Process Control Information – Memory Management • This section may include pointers to segment and/or page tables that describe the virtual memory assigned to this process. – Resource Ownership and Utilization • Resources controlled by the process may be indicated, such as opened files. A history of utilization of the processor or other resources may also be included; this information may be needed by the scheduler. 45 Modes of Execution • User mode – Less-privileged mode – User programs typically execute in this mode • System mode, control mode, or kernel mode – More-privileged mode – Kernel of the operating system 46 Process Creation • Assign a unique process identifier :a new entry is added to the primary process table,which contains one entry per process • Allocate space for the process : includes all elements of the process image . – OS must know how much space is needed for the private user address space(programs and data) and the user stack. – If a process is spawned by another process,the parent process can pass the needed values to the OS as a part of the processcreation request. – If any existing address space is to be shared by this new process,the appropriate linkages must be set up. – Space for a process control block must be allocated. 47 • Initialize process control block : Process identification contains the ID of this process plus other appropriate IDs( parent process). • The processor state information portion will typically be initialized with most entries zero,except for the program counter(set to the program entry point) and system stack pointers (set to define the process stack boundaries). • The process control information portion is initialized based on std default values plus attributed that have been requested for this process. 48 • Set up appropriate linkages – Ex: add new process to linked list used for scheduling queue • Create or expand other data structures – Ex: maintain an accounting file 49 When to Switch a Process A process switch may occur any time that the os has gained control from currently running process. • Clock interrupt – process has executed for the maximum allowable time slice • I/O interrupt • The OS determines what I/O action has occurred. • If the I/O action form an event for which one or more processes are waiting, then the OS moves all of the corresponding blocked process to the ready state. • Memory fault – memory address is in virtual memory so it must be brought into main memory 50 When to Switch a Process • Trap – error or exception occurred – may cause process to be moved to Exit state • Supervisor call – The OS may be activated by a supervisor call from the program being executed. – E.g. a user process is running and an instruction is executed that requests an I/O operation. 51 Change of Process State The steps involved in a full process switch are as follows: • Save context of processor including program counter and other registers • Update the process control block of the process that is currently in the Running state • Other relevant fields must also be updated, including the reasons foe leaving the running state and accounting information. • Move process control block to appropriate queue – ready; blocked; ready/suspend • Select another process for execution 52 Change of Process State • Update the process control block of the process selected • Update memory-management data structures • Restore context of the selected process to the which existed at the selected process was last switched out of the running state. 53 UNIX SVR4 Process Management • Most of the operating system executes within the environment of a user process P1 OS Functions P2 OS Functions Pn OS Functions Process Switching Functions (b) OS functions execute within user pr 54 Process creation in Unix Process creation in Unix is made by means of the Kernel system call, fork(). When a process issues a fork request ,the OS performs the following functions : 1. It allocates a slot in the process table for the new process. 2. It assigns a unique process ID to the child process. 3. It makes a copy of the process image of the parent. 4. It increments counters for any files owned by the parent ,to reflect that an additional process now also owns those files. 5. It assigns the child process to a ready to run state 6. It returns the ID number of the child to the parent process, and a 0 value to the child process. All this work is accomplished in Kernel mode in the parent process. 55 UNIX Process States 56 UNIX Process Image 57 58