8.5: Process Description

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System Processes / System Resources

In a multiprocessing system, there are numerous processes competing to the system's resources. As each process takes its turn executing in the processor, the state of the other processes must be kept in the state that they were interrupted at so as to restore the next process to execute.

Processes run, make use of I/O resources, consume memory. At times processes block waiting for I/O, allowing other processes to run on the processors. Some processes are swapped out in order to make room in physical memory for other processes' needs. P₁is running, accessing I/O and memory. P₂is blocked waiting for P1 to complete I/O. P_nis swapped out waiting to return to main memory and further processing.

Multiple processes running on a system...they all need system resources. The state of each process must be saved when it is blocked or swapped out of main memory. — Figure \(\PageIndex{1}\): Processes and Resources. (Unknown source)

Process description and control

As the operating system manages processes and resources, it must maintain information about the current status of each process and the resources in use. The approach to maintaining this information is for the operating system to construct and maintain various tables of information about each entity to be managed.

Operating system maintains four internal components: 1) memory; 2) devices; 3) files; and 4) processes. Details differ from one operating system to another, but all operating systems maintain information in these four categories.

Memory tables: Memory is central to the operation of a modern computer system. Memory is a large array of words or bytes, each with its own address. Interaction is achieved through a sequence of reads or writes of specific memory address. The CPU fetches from and stores in memory.

In order to improve both the utilization of CPU and the speed of the computer's response to its users, several processes must be kept in memory. There are many different algorithms depends on the particular situation. Selection of a memory management scheme for a specific system depends upon many factor, but especially upon the hardware design of the system. Each algorithm requires its own hardware support.

The operating system is responsible for the following activities in connection with memory management.

Keep track of which parts of memory are currently being used and by whom.
Decide which processes are to be loaded into memory when memory space becomes available.
Allocate and deallocate memory space as needed.

We will cover memory management in a later section.

Device tables: One of the purposes of an operating system is to hide the peculiarities of specific hardware devices from the user. For example, in Linux, the peculiarities of I/O devices are hidden from the bulk of the operating system itself by the I/O system. The I/O system consists of:

A buffer caching system
A general device driver code
Drivers for specific hardware devices.

Only the device driver knows the peculiarities of a specific device. We will cover the details later in this course

File tables: File management is one of the most visible services of an operating system. Computers can store information in several different physical forms; disk - both magnetic disks and newer SSD devices, various USB devices, and to the cloud. Each of these devices has it own characteristics and physical organization.

For convenient use of the computer system, 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. Files are mapped, by the operating system, onto physical devices.

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 or alphanumeric. Files may be free-form, such as text files, or may be rigidly formatted. In general a files is a sequence of bits, bytes, lines or records whose meaning is defined by its creator and user. It is a very general concept.

The operating system implements the abstract concept of the file by managing mass storage device, such as types and disks. Also files are normally organized into directories to ease their use. Finally, when multiple users have access to files, it may be desirable to control by whom and in what ways files may be accessed.

The operating system is responsible for the following activities in connection with file management:

The creation and deletion of files
The creation and deletion of directory
The support of primitives for manipulating files and directories
The mapping of files onto disk storage.
Backup of files on stable (non volatile) storage.

This portion of the operating system will also be dealt with later.

Processes: The CPU executes a large number of programs. While its main concern is the execution of user programs, the CPU is also needed for other system activities. These activities are called processes. A process is a program in execution. Typically, a batch job is a process. A time-shared user program is a process. A system task, such as spooling, is also a process. For now, a process may be considered as a job or a time-shared program, but the concept is actually more general.

In general, a process will need certain resources such as CPU time, memory, files, I/O devices, etc., to accomplish its task. These resources are given to the process when it is created. In addition to the various physical and logical resources that a process obtains when its is created, some initialization data (input) may be passed along. For example, a process whose function is to display on the screen of a terminal the status of a file, say F1, will get as an input the name of the file F1 and execute the appropriate program to obtain the desired information.

We emphasize that a program by itself is not a process; a program is a passive entity, while a process is an active entity. It is known that two processes may be associated with the same program, they are nevertheless considered two separate execution sequences.

A process is the unit of work in a system. Such a system consists of a collection of processes, some of which are operating system processes, those that execute system code, and the rest being user processes, those that execute user code. All of those processes can potentially execute concurrently.

The operating system is responsible for the following activities in connection with processes managed.

The creation and deletion of both user and system processes
The suspension are resumption of processes.
The provision of mechanisms for process synchronization
The provision of mechanisms for deadlock handling