02_Handout_1(18).pdf

IT2105 Processes I/O status information – T...

IT2105 Processes I/O status information – This includes different information associated Process State Models with the input/output status of a process, such as the outstanding I/O In studying operating systems, the term process can be defined in different requests and I/O devices assigned to the process. ways, depending on its actual composition or utilization. Below are some of its Accounting information – This includes different tracking information basic definitions (Stallings, 2018). associated with the process, such as the amount of processor time, the A process can be a program in execution. clock time used, and time limits. A process can be an instance of a program running on a computer. A process can be considered as an entity that can be assigned to and The elements of a process are stored in a data structure called process executed on a processor. control block. The core concept of the process control block is that it contains A process can be treated as a unit of activity that executes a sequence of sufficient information so it is possible to interrupt a running process and later instructions, a current state, and an associated set of system resources. resume the operation as if no interruption occurred. Thus, making the process A process can also be defined as an environment that consists of a control block the key tool that enables the operating system (OS) to support number of elements for executing a user-level program. multiple processes and perform multiprocessing (Stallings, 2018). There are two (2) essential elements of a process, the program code – which Two-State Process Model (Stallings, 2018) can be shared with other processes that are executing the same program, and An operating system controls the course of execution that involves the process the set of data associated with the program code. Note that a computer's of determining an interleaving pattern for execution and allocating resources kernel can execute multiple processes at a time, thus supporting thousands of to processes. In a two-state process model, a process can either be in a processes on a single system (Gregg, 2021). At any given point in time, while running state or in a not running state. a program is executing, the process itself can be uniquely characterized by the Not running state – When the OS creates a new process, it creates a following elements: process control block for the new process and adds the new process into Identifier – It is a unique identifier for each process. the not running state of the system. Then, the process waits for an Process State – It indicates the current activity of a process. A process opportunity to execute. state can be any of the following (Silberschatz, Galvin & Gagne, 2018) Running state – The dispatch portion of the OS selects a particular o New – The process is being created. process to run. Then, it enters the running state. Intermittently, the o Ready – The process is waiting to be assigned to a processor. currently running process will be interrupted and the dispatch portion has o Running – The instructions are being executed. to select other processes to run. Subsequently, the interrupted process o Waiting – The process is waiting for a particular event to occur. returns to the queue in the not running state, while a completed process o Terminated – The process has finished the execution. exits the system. Priority – It refers to the priority level relative to other processes. Program counter – It indicates the address of the next instruction to be executed for a particular process. Memory pointer – It contains pointers to program codes and data associated with the process, including any memory blocks shared with other processes. Context data – These are the data present in a processor's registers during process execution. Figure 1. The state transition in a two-state process model. Source: Operating Systems: Internal and Design Principles (9th ed.), 2018 p. 136 02 Handout 1 *Property of STI  [email protected] Page 1 of 4  IT2105 This process model is effective only if all processes are always ready to Note that the not running state in a two-state process model was technically execute. The queue in the not running state would be in a first-in-first-out divided into two (2): The Ready state and the Block state. This is to improve (FIFO) arrangement, wherein each process in the queue is given a certain the overall efficiency of the operating system. In addition, the New and the Exit amount of time, alternately, to execute and then return to the queue, unless state are useful constructs for process management. The following are the completed. possible state transitions for a five-state process model: However, some processes in the not running state are ready to execute but Null → New Running → Exit Blocked → Ready are blocked, waiting for an I/O operation to complete. The dispatcher could not New → Ready Running → Ready Ready → Exit just select any process since there is only one (1) queue. Rather, the Ready → Running Running → Blocked Blocked → Exit dispatcher would have to scan the queue looking for the process that is not blocked and that has been in the queue the longest. This makes the Suspended Processes (Stallings, 2018) implementation of the two-state process model inadequate. The three (3) principal states which are Ready, Running, and Blocked, provide a systematic way of modeling the behavior of a process and guide the Five-State Process Model (Stallings, 2018) implementation of an operating system. Some OS are constructed using only This model naturally handles processes through the implementation of five these three (3) states. However, there are good justifications for adding other unique states which are the following: states to a model. New state: A process that has just been created and has not yet been admitted to the pool of executable processes by the OS. In a suspended process model, processes that are not immediately available Ready state: A process that is prepared to execute when given the for execution can occur. These processes may or may not be waiting for a opportunity. specific event. If a process is waiting for a particular event, this means that the Running state: The process that is currently being executed. blocked state is independent of the suspended state, and the occurrence of Blocked state/Waiting state: A process that cannot execute until some the blocking event does not enable the process to be executed immediately. event occurs, such as the completion of an I/O operation. Exit state: A process that has been released from the pool of executable A process can be placed in a suspended state by any of the following: processes by the OS, either because it halted or it was terminated. a. The process itself b. A parent process c. The operating system Figure 2. The state transition in a five-state process model. Source: Operating Systems: Internal and Design Principles (9th ed.), 2018 p. 140 Figure 3. A five-state process model with a suspended state. Source: Operating Systems: Internal and Design Principles (9th ed.), 2018 p. 145 02 Handout 1 *Property of STI  [email protected] Page 2 of 4  IT2105 Process Description (Stallings, 2018) global variables and any defined constants. Thus, a process will always An operating system (OS) controls all the events within a computer system. It encompass sufficient memory to hold a program and its corresponding data. schedules and dispatches processes for execution by the processor, allocates resources to different processes, and responds to requests by user processes Process control blocks are also referred to as process images. The location for basic services. of a process image will depend on the memory management scheme being used. In the simplest case, a process image is maintained as a continuous The OS can generally be considered as an entity that manages the utilization block of memory maintained in the secondary memory. To execute a process, of system resources by processes. During the course of a process execution, the entire process image must be loaded into the main memory, or at least to each process requires access to certain system resources such as the the virtual memory. Thus, the OS needs to know the location of each process processor, input and output devices, and main memory. In a multiprogramming on the disk. environment, there are numerous processes that can be created and exist in a virtual memory. Process Attributes: A multiprogramming system requires a great deal of information about each process that resides in a process control block. There An OS must have information about the current status of each process and are different types of information that might be of use to an OS without resource in order to effectively manage all operations. Thus, the operating considering any details as to how that information is organized. The system constructs and maintains tables of information about each process that information in a process control block can be grouped into three (3) categories: it is managing. The following are the four (4) different types of tables 1. Process identification – This involves unique numeric identifiers that maintained by an operating system: are assigned to specific processes, which can simply be an index into 1. Memory tables – These are used to keep track of both the main and a primary process table, or identifiers for cross-referencing process the secondary memory. A part of the main memory is reserved for OS tables. utilization, while the remaining parts are available for the processes' 2. Processor state information – This consists of processor registers' utilization. Note that processes are maintained on the secondary information. During process execution, information are in the registers. memory using some sort of virtual memory or a simple swapping When a process is interrupted, all information in the registers must be mechanism. The memory tables must encompass the following saved so it can be restored when the process resumes execution. information: 3. Process control information – This contains the additional o The allocation of the main memory processes information needed by the OS to control and coordinate various active o The allocation of the secondary memory processes processes, such as the data structure, resource ownership and o Any protection attribute for the main or the virtual memory utilization information, and process privileges. o Any information needed to manage the virtual memory 2. I/O tables – These are used by the OS to manage the input and output The process control block is the most important data structure in operating devices and channels of a computer system at any given point in time. systems, since it contains all the information about a process that are needed 3. File tables – These tables hold information regarding the existence of by the OS. The process control blocks are read and/or modified by every files and its corresponding attributes through a file management module in an OS, including those involved in resource allocation, scheduling, system. interrupt processing, analysis, and performance monitoring. 4. Process tables – These tables are maintained by the OS to manage processes. Process Control (Stallings, 2018) Modes of Execution: Specific processes require certain privileges in order to Process Location: At a minimum, a process must include a program to be protect the hardware and the OS from impending malfunction. Most executed. Associated to this program is a set of data locations for local and processors support at least two (2) modes of execution which are the following: 02 Handout 1 *Property of STI  [email protected] Page 3 of 4  IT2105 User mode – This is referred to as the less-privileged mode, since user In some operating systems, a process can be terminated by the process that programs execute in this mode. created it, or when the parent process itself is terminated. Kernel mode – This is referred to as the more-privileged mode, where the software has complete control of the processor and its instructions, References: Gregg, B. (2021). System performance: Enterprise and Cloud (2nd ed.). Pearson Education, Inc. registers, and memory. Silberschatz, A., Galvin, P. & Gagne, G. (2018). Operating systems concepts (10th ed.). John Wiley & Sons, Inc. Stallings, W. (2018). Operating systems: Internal and design principles (9th ed.). Pearson Education Limited Certain instructions and memory regions can only be executed and accessed in the more-privileged mode. This is necessary to protect the OS and the key operating system tables, such as process control blocks, from interferences by user programs. Process Creation: When a new process is to be added to those currently being managed, the operating system builds the data structure that will manage the process and allocates address space in the main memory for the process. Contemporary operating systems commonly create processes in a way that is transparent to the user or application program. With the evolution of operating systems, programmers realized that it can be useful and efficient, especially in structuring applications, to allow one (1) process to cause the creation of another. Process creation can proceed as follows: 1. Assign a unique process identifier to the new process 2. Allocate space for the process 3. Initialize the process control block 4. Set the appropriate linkages 5. Create or expand other data structures Process Switching: When the operating system controls the currently running processes, a process switch may occur at any time. Thus, the OS needs to provide a regaining control feature if a process is blocked, suspended, or paused. Process switching usually involves interrupts or traps. Interrupts are external events that are independent of the currently running process, while traps are related to errors or exception conditions generated within the currently running process. On the other hand, a supervisor call due to an explicit request can still occur to interrupt a process execution. Process Termination: Any computer system must provide means for a process to indicate its completion. A batch job may include a halt instruction or an explicit OS service call for termination. Generally, the halt instruction generates an interrupt to alert the OS that a process has been completed. For interactive applications, specific action of user can trigger process completion. 02 Handout 1 *Property of STI  [email protected] Page 4 of 4