1. Independent process cannot affect or be affected by the execution of another process.
2. Cooperating process can affect or be affected by the execution of another process
3. Advantages of process cooperation

• Information sharing
• Computation speed-up
• Modularity
• Convenience

1. Message system – processes communicate with each other without resorting to shared variables.
2. IPC facility provides two operations:

• send(message) –message size fixed or variable
• receive(message)

3. If P and Q wish to communicate, they need to:

• establish a communication link between them
• exchange messages via send/receiven Implementation of communication link
• physical (e.g., shared memory, hardware bus) considered later
• logical (e.g., logical properties) now

• Batch system – jobs
  
• Time-shared systems – user programs or task

Textbook uses the terms job and process almost interchangeably.

Process – a program in execution; process execution must progress in sequential fashion.

A process includes:

• program counter
  
• stack


• data section
  
  

Process State

As a process executes, it changes state

• new: The process is being created.
  
• running: Instructions are being executed.


• waiting: The process is waiting for some event to occur


• ready: The process is waiting to be assigned to a processor


• terminated: The process has finished execution.

Process Control Block


Information associated with each process.


Process ID

Process staten Program counter

Threads


In computer science, a thread of execution results from a fork of a computer program into two or more concurrently running tasks. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process. Multiple threads can exist within the same process and share resources such as memory, while different processes do not share these resources.
On a single processor, multithreading generally occurs by time-division multiplexing (as in multitasking): the processor switches between different threads. This context switching generally happens frequently enough that the user perceives the threads or tasks as running at the same time. On a multiprocessor or multi-core system, the threads or tasks will generally run at the same time, with each processor or core running a particular thread or task. Support for threads in programming languages varies: a number of languages simply do not support having more than one execution context inside the same program executing at the same time.

1. Parent process creates children processes, which, in turn create other processes, forming a tree of processes.
2. Resource sharing

• Parent and children share all resources.
• Children share subset of parent’s resources.
• Parent and child share no resources.

3. Execution

• Parent and children execute concurrently.
• Parent waits until children terminate.

• Child duplicate of parent.
• Child has a program loaded into it.

• fork system call creates new process
• fork returns 0 to child , process id of child for parent
• exec system call used after a fork to replace the process’ memory space with a new program.

--Process Termination

Process executes last statement and asks the operating system to delete it (exit).

• Output data from child to parent (via wait).
• Process’ resources are deallocated by operating system.n Parent may terminate execution of children processes (abort).
• Child has exceeded allocated resources.
• Task assigned to child is no longer required.
• Parent is exiting.

----------- Operating system does not allow child to continue if its parent terminates.

----------- Cascading termination.

• In Unix, if parent exits children are assigned init as parent

• Process creation and deletion.
• Process suspension (process is in I/O wait queue, or “swapped” out to disk, …) and resumption (move to ready queue or execution) – manage the state of the process.

• Provision of mechanisms for:
• Process synchronization - concurrent processing is supported thus the need for synchronization of processes or threads.
• Process communication
• Deadlock handling

• File creation and deletion - system calls or commands.
• Directory creation and deletion - system calls or commands.
• Support of primitives for manipulating files and directories in an efficient manner - system calls or commands.
• Mapping files onto secondary storage.
• File backup on stable (nonvolatile) storage media.

EX: File Allocation Table (FAT) for Windows/PC systems

• Small piece of code – bootstrap loader, locates the kernel, loads it into memory, and starts it
  
• Sometimes two-step process where boot block at fixed location loads bootstrap loader
  
• When power initialized on system, execution starts at a fixed memory location
  -------Firmware used to hold initial boot code
  

• Implementation

Modes:
>>virtual user mode and virtual monitor mode,
>>Actual user mode and actual monitor mode
Time
>>Whereas the real I/O might have taken 100 milliseconds, the virtual I/O might take less time (because it is spooled) or more time (because it is interpreted.)
>>The CPU is being multi-programmed among many virtual machines, further slowing down the virtual machines in unpredictable ways.

• benefits
  

For testers it is important that they test software against the various supported operating systems that an application runs against. The traditional approach is to run multiple physical machines, each with a different operating system. This is bad for several reasons. Space, maintenance, power and feasibility come to mind. Deployment of the software to these various machines can also be an issue. Instead a tester can run multiple virtual machines on one physical machine. Each virtual machine could have a different operating system. The application can be deployed to the virtual machines and tested.

Another advantage of virtual machines is reproducibility. Build and test environments generally need to be well controlled. It would be undo work to have to wipe out a machine and rebuild it after each build or test run. A virtual machine allows the environment to be set up once. The environment is then captured. Any changes made after the capture can then be thrown away after the build or test run. Most emulation software packages offer this in some form or another

• Examples

A program written in Java receives services from the Java Runtime Environment (JRE) software by issuing commands to, and receiving the expected results from, the Java software. By providing these services to the program, the Java software is acting as a "virtual machine", taking the place of the operating system or hardware for which the program would ordinarily be tailored.

• Simple Structure

-any part of the system may use the functionality of the rest ofthe system

-MS-DOS (user programs can call low level I/O routines)View the OS as a series of levels

–Each level performs a related subset of functions

–Each level relies on the next lower level to perform more primitive functions

–This decomposes a problem into a number of more manageable subproblems

• Layered Approach
  
  The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.
  
  With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers
  
  

---Properties


Simplicity of construction

Simplicity of Debugging

Problems

Precise definition of layers

Example: Memory manager requires device driver of backing store (due to virtual memory)

The device driver requires CPU scheduler (since if the driver waits for IO, another task should be scheduled)

CPU scheduler may require virtual memory for large amount of information of some processes

Less efficiency: due to the number of layers a request should pass

• Process control

create/terminate a process (including self)

• File management
  Also referred to as simply a file system or filesystem. The system that an operating system or program uses to organize and keep track of files. For example, a hierarchical file system is one that uses directories to organize files into a tree structure.Although the operating system provides its own file management system, you can buy separate file management systems. These systems interact smoothly with the operating system but provide more features, such as improved backup procedures and stricter file protection.
• 
  

--->>Device management

Device Management is a set of technologies, protocols and standards used to allow the remote management of mobile devices, often involving updates of firmware over the air (FOTA). The network operator, handset OEM or in some cases even the end-user (usually via a web portal) can use Device Management, also known as Mobile Device Management, or MDM, to update the handset firmware/OS, install applications and fix bugs, all over the air. Thus, large numbers of devices can be managed with single commands and the end-user is freed from the requirement to take the phone to a shop or service center to refresh or update.

• Information maintenance –get time

–set system data (OS parameters)

– get process information (id, time used)

• Communications

– establish a connection

– send, receive messages

– terminate a connectionlProcess control

– create/terminate a process (including self)

1. Program execution – system capability to load a program into memory and to run it
2. I/O operations – since user programs cannot execute I/O operations directly, the operating system must provide some means to perform I/On
3. File-system manipulation – program capability to read, write, create, and delete files
4. Communications – exchange of information between processes executing either on the same computer or on different systems tied together by a network. Implemented via shared memory or message passing
5. Error detection – ensure correct computing by detecting errors in the CPU and memory hardware, in I/O devices, or in user programs

• Operating System Process Management

A process is a program in execution

A process needs certain resources, including CPU time, memory, files, and I/O devices, to accomplish its task.

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

1. Process creation and deletion
2. Process suspension and resumption
3. Provision of mechanisms for:

• process synchronization
• process communication

• Main memory Management

1.Memory is a large array of words or bytes, each with its own address

() It is a repository of quickly accessible data shared by the CPU and I/O devices

2. Main memory is a volatile storage device. It loses its contents in the case of system failure

3. The operating system is responsible for the following activities in connections with memory managementl

• Keep track of which parts of memory are currently being used and by whoml
• Decide which processes to load when memory space becomes availablel
• Allocate and deallocate memory space as needed

Memory management is the act of managing computer memory. In its simpler forms, this involves providing ways to allocate portions of memory to programs at their request, and freeing it for reuse when no longer needed. The management of main memory is critical to the computer system.

• File management

A file is a collection of related information defined by its creatorl Commonly, files represent programs (both source and object forms) and data

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

• File creation and deletion
• Directory creation and deletion
• Support of primitives for manipulating files and directories
• Mapping files onto secondary storage
• File backup on stable (nonvolatile) storage media

A file object provides a representation of a resource (either a physical device or a resource located on a physical device) that can be managed by the I/O system. Like other objects, they enable sharing of the resource, they have names, they are protected by object-based security, and they support synchronization. The I/O system also enables reading from or writing to the resource.

• I/O System management

The I/O system consists of:

>A buffer-caching system

>A general device-driver interface

>Drivers for specific hardware devices

• Secondary Storage Management

Since main memory (primary storage) is volatile and too small to accommodate all data and programs permanently, the computer system must provide secondary storage to back up main memory

Most modern computer systems use disks as the principle on-line storage medium, for both programs and datan

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

>> Free space managementl Storage allocationl

>>Disk scheduling

The I/O management subsystem controls all the input andoutput of the computer system. For the enforcement ofsecurity, the most important things that the I/O managementsubsystem does are· Managing the transfer of data.· Enforcing access controls (the DAC mechanisms) on datawhile it is being transferred. See “Discretionary access control(DAC)” for more information on DAC.During the transfer of blocks or streams of data, and duringcharacter I/O operation, each I/O transaction is completelyseparate from all others. It follows a well-known and well-defined path; therefore, the integrity of all data is maintainedduring data transactions.

• Protection System

Protection refers to a mechanism for controlling access by programs, processes, or users to both system and user resourcesn

The protection mechanism must:

1. distinguish between authorized and unauthorized usagel specify the controls to be imposedl
2. provide a means of enforcement

• Command-Interpreter system

Many commands are given to the operating system by control statements which deal with:

1. Process creation and management
2. I/O handling
3. Secondary-storage management
4. Main-memory management
5. File-system access
6. Protection
7. Networking

A command interpreter is the part of a computer operating system that understands and executes commands that are entered interactively by a human being or from a program. In some operating systems, the command interpreter is called the shell.

• Dual Mode Operation

• Sharing system resources requires operating system to ensurethat an incorrect program cannot cause other programs toexecute incorrect

• Provide hardware support to differentiate between at least twomodes of operations.

1. User mode – execution done on behalf of a user.

2. Monitor mode (also supervisor mode or system mode) –execution done on behalf of operating system.

• I/O Protection

• Memory Protection

• CPU Protection

-to prevent a user programs gets stuck in infinite loop and never returning back to the os

(2) Storage Heirarchy

• caching

--->>In computer science, a cache (pronounced /kæʃ/) is a collection of data duplicating original values stored elsewhere or computed earlier, where the original data is expensive to fetch (owing to longer access time) or to compute, compared to the cost of reading the cache. In other words, a cache is a temporary storage area where frequently accessed data can be stored for rapid access. Once the data is stored in the cache, it can be used in the future by accessing the cached copy rather than re-fetching or recomputing the original data.
A cache has proven to be extremely effective in many areas of computing because access patterns in typical computer applications have locality of reference. There are several kinds of locality, but this article primarily deals with data that are accessed close together in time (temporal locality). The data might or might not be located physically close to each other (spatial locality).

• coherency and consistency

In computing, cache coherence (also cache coherency) refers to the integrity of data stored in local caches of a shared resource. Cache coherence is a special case of memory coherence.
When clients in a system maintain caches of a common memory resource, problems may arise with inconsistent data. This is particularly true of CPUs in a multiprocessing system. Referring to the "Multiple Caches of Shared Resource" figure, if the top client has a copy of a memory block from a previous read and the bottom client changes that memory block, the top client could be left with an invalid cache of memory without any notification of the change. Cache coherence is intended to manage such conflicts and maintain consistency between cache and memory.

cache consistency

-->>The synchronisation of data in multiple caches such that reading a memory location via any cache will return the most recent data written to that location via any (other) cache.Some parallel processors do not cache accesses to shared memory to avoid the issue of cache coherency. If caches are used with shared memory then some system is required to detect when data in one processor's cache should be discarded or replaced because another processor has updated that memory location. Several such schemes have been devised.

Cache Consistency Mechanisms There are several ways to maintain cache consistency of files placed in a cache: time-to-live fields, active invalidation protocols, and client polling. Time-to-live fields are efficient to set up, but are inaccurate.

(1) Storage Structure

main memory

--->>>Primary storage, presently known as memory, is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them as required. Any data actively operated on is also stored there in uniform manner.Historically, early computers used delay lines, Williams tubes, or rotating magnetic drums as primary storage. By 1954, those unreliable methods were mostly replaced by magnetic core memory, which was still rather cumbersome. Undoubtedly, a revolution was started with the invention of a transistor, that soon enabled then-unbelievable miniaturization of electronic memory via solid-state silicon chip technology.This led to a modern random access memory (RAM). It is small-sized, light, but quite expensive at the same time. (The particular types of RAM used for primary storage are also volatile, i.e. they lose the information when not powered).

• magnetic Disk

--Moving Head Disk Mechanism

• Magnetic tapes

--->>Magnetic tape is a medium for magnetic recording generally consisting of a thin magnetizable coating on a long and narrow strip of plastic. Nearly all recording tape is of this type, whether used for recording audio or video or for computer data storage. It was originally developed in Germany, based on the concept of magnetic wire recording. Devices that record and playback audio and video using magnetic tape are generally called tape recorders and video tape recorders respectively. A device that stores computer data on magnetic tape can be called a tape drive, a tape unit, or a streamer.
Magnetic tape revolutionized the broadcast and recording industries. In an age when all radio (and later television) was live, it allowed programming to be prerecorded. In a time when gramophone records were recorded in one take, it allowed recordings to be created in multiple stages and easily mixed and edited with a minimal loss in quality between generations. It is also one of the key enabling technologies in the development of modern computers. Magnetic tape allowed massive amounts of data to be stored in computers for long periods of time and rapidly accessed when needed.