OPERATING SYSTEMS
What is an Operating System
The 1960’s definition of an operating system is “the software that controls the hardware”. However, today, due to microcode we need a better definition. We see an operating system as the programs that make the hardware useable. In brief, an operating system is the set of programs that controls a computer. Some examples of operating systems are UNIX, Mach, MS-DOS, MS-Windows, Windows/NT, Chicago, OS/2, MacOS, VMS, MVS, and VM.
Controlling the computer involves software at several levels. We will differentiate kernel services, library services, and application-level services, all of which are part of the operating system. Processes run Applications, which are linked together with libraries that perform standard services. The kernel supports the processes by providing a path to the peripheral devices. The kernel responds to service calls from the processes and interrupts from the devices. The core of the operating system is the kernel, a control program that functions in privileged state (an execution context that allows all hardware instructions to be executed), reacting to interrupts from external devices and to service requests and traps from processes. Generally, the kernel is a permanent resident of the computer. It creates and terminates processes and responds to their request for service.
Operating Systems are resource managers. The main resource is computer hardware in the form of processors, storage, input/output devices, communication devices, and data. Some of the operating system functions are: implementing the user interface, sharing hardware among users, allowing users to share data among themselves, preventing users from interfering with one another, scheduling resources among users, facilitating input/output, recovering from errors, accounting for resource usage, facilitating parallel operations, organizing data for secure and rapid access, and handling network communications.
Objectives of Operating Systems
Modern Operating systems generally have following three major goals. Operating systems generally accomplish these goals by running processes in low privilege and providing service calls that invoke the operating system kernel in high-privilege state.
• To hide details of hardware by creating abstraction
An abstraction is software that hides lower level details and provides a set of higher-level functions. An operating system transforms the physical world of devices, instructions, memory, and time into virtual world that is the result of abstractions built by the operating system. There are several reasons for abstraction.
First, the code needed to control peripheral devices is not standardized. Operating systems provide subroutines called device drivers that perform operations on behalf of programs for example, input/output operations.
Second, the operating system introduces new functions as it abstracts the hardware. For instance, operating system introduces the file abstraction so that programs do not have to deal with disks.
Third, the operating system transforms the computer hardware into multiple virtual computers, each belonging to a different program. Each program that is running is called a process. Each process views the hardware through the lens of abstraction.
Fourth, the operating system can enforce security through abstraction.
• To allocate resources to processes (Manage resources)
An operating system controls how processes (the active agents) may access resources (passive entities).
• Provide a pleasant and effective user interface
The user interacts with the operating systems through the user interface and usually interested in the “look and feel” of the operating system. The most important components of the user interface are the command interpreter, the file system, on-line help, and application integration. The recent trend has been toward increasingly integrated graphical user interfaces that encompass the activities of multiple processes on networks of computers.
One can view Operating Systems from two points of views: Resource manager and Extended machines. Form Resource manager point of view Operating Systems manage the different parts of the system efficiently and from extended machines point of view Operating Systems provide a virtual machine to users that is more convenient to use. The structurally Operating Systems can be design as a monolithic system, a hierarchy of layers, a virtual machine system, an exokernel, or using the client-server model. The basic concepts of Operating Systems are processes, memory management, I/O management, the file systems, and security.
Definition of Process
The notion of process is central to the understanding of operating systems. There are quite a few definitions presented in the literature, but no "perfect" definition has yet appeared.
Definition
The term "process" was first used by the designers of the MULTICS in 1960's. Since then, the term process, used somewhat interchangeably with 'task' or 'job'. The process has been given many definitions for instance
• A program in Execution.
• An asynchronous activity.
• The 'animated sprit' of a procedure in execution.
• The entity to which processors are assigned.
• The 'dispatchable' unit.
and many more definitions have given. As we can see from above that there is no universally agreed upon definition, but the definition "Program in Execution" seem to be most frequently used. And this is a concept are will use in the present study of operating systems.
Now that we agreed upon the definition of process, the question is what is the relation between process and program. It is same beast with different name or when this beast is sleeping (not executing) it is called program and when it is executing becomes process. Well, to be very precise. Process is not the same as program. In the following discussion we point out some of the difference between process and program. As we have mentioned earlier.
Process is not the same as program. A process is more than a program code. A process is an 'active' entity as oppose to program which consider to be a 'passive' entity. As we all know that a program is an algorithm expressed in some suitable notation, (e.g., programming language). Being a passive, a program is only a part of process. Process, on the other hand, includes:
• Current value of Program Counter (PC)
• Contents of the processors registers
• Value of the variables
• The process stack (SP) which typically contains temporary data such as subroutine parameter, return address, and temporary variables.
• A data section that contains global variables.
A process is the unit of work in a system.
In Process model, all software on the computer is organized into a number of sequential processes. A process includes PC, registers, and variables. Conceptually, each process has its own virtual CPU. In reality, the CPU switches back and forth among processes. (The rapid switching back and forth is called multiprogramming).
Process State
The process state consist of everything necessary to resume the process execution if it is somehow put aside temporarily. The process state consists of at least following:
• Code for the program.
• Program's static data.
• Program's dynamic data.
• Program's procedure call stack.
• Contents of general purpose registers.
• Contents of program counter (PC)
• Contents of program status word (PSW).
• Operating Systems resource in use.
Process Control Block
A process in an operating system is represented by a data structure known as a process control block (PCB) or process descriptor. The PCB contains important information about the specific process including
• The current state of the process i.e., whether it is ready, running, waiting, or whatever.
• Unique identification of the process in order to track "which is which" information.
• A pointer to parent process.
• Similarly, a pointer to child process (if it exists).
• The priority of process (a part of CPU scheduling information).
• Pointers to locate memory of processes.
• A register save area.
• The processor it is running on.
The PCB is a certain store that allows the operating systems to locate key information about a process. Thus, the PCB is the data structure that defines a process to the operating systems.
Threads
• Threads
• Processes Vs Threads
• Why Threads?
• User-Level Threads
• Kernel-Level Threads
• Advantages of Threads over Multiple Processes
• Disadvantages of Threads over Multiprocesses
• Application that Benefits from Threads
• Application that cannot benefit from Threads
• Resources used in Thread creation and Process Creation
• Context Switch
• Major Steps of Context Switching
• Action of Kernel to Context switch among threads
• Action of kernel to Context switch among processes
Threads
Despite of the fact that a thread must execute in process, the process and its associated threads are different concept. Processes are used to group resources together and threads are the entities scheduled for execution on the CPU.
A thread is a single sequence stream within in a process. Because threads have some of the properties of processes, they are sometimes called lightweight processes. In a process, threads allow multiple executions of streams. In many respect, threads are popular way to improve application through parallelism. The CPU switches rapidly back and forth among the threads giving illusion that the threads are running in parallel. Like a traditional process i.e., process with one thread, a thread can be in any of several states (Running, Blocked, Ready or Terminated). Each thread has its own stack. Since thread will generally call different procedures and thus a different execution history. This is why thread needs its own stack. An operating system that has thread facility, the basic unit of CPU utilization is a thread. A thread has or consists of a program counter (PC), a register set, and a stack space. Threads are not independent of one other like processes as a result threads shares with other threads their code section, data section, OS resources also known as task, such as open files and signals.
Processes Vs Threads
As we mentioned earlier that in many respect threads operate in the same way as that of processes. Some of the similarities and differences are: