virtual machines

implementation-A virtual machine was originally defined by Popek and Goldberg as "an efficient, isolated duplicate of a real machine". Current use includes virtual machines which have no direct correspondence to any real hardware.Virtual machines are separated into two major categories, based on their use and degree of correspondence to any real machine. A system virtual machine provides a complete system platform which supports the execution of a complete operating system (OS). In contrast, a process virtual machine is designed to run a single program, which means that it supports a single process. An essential characteristic of a virtual machine is that the software running inside is limited to the resources and abstractions provided by the virtual machine -- it cannot break out of its virtual world.

• benefits-So what does this mean to developers and testers. Let's look at a few scenarios that developers and testers find themselves in. 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.
  Another scenario, your application is currently released as version 1. Because of how the application is written you can only run a single version of your application on a machine. When you start development on version 2 you have to remove version 1. Part way through development an issue is found in the version 1 software that you need to replicate and fix. You can uninstall version 2 and install version 1, find and fix the issue and then revert back but that is a lot of work. A nicer approach is to have a virtual machine with version 1 installed. When you need to go back to version 1 you just start up the virtual machine. Even better is that you can easily compare the behavior of the two versions side by side rather than having to switch between two computers.
  IT departments have already found the benefits of running virtual servers over having multiple physical servers. Development and testing share many of the same benefits. Virtualization has become a buzzword in the industry. Windows is becoming more virtualized so even if you aren't using virtual machines today you may be in the future.

• Example: 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.

system generation

system generation (SYSGEN): The process of selecting optional parts of an operating system and of creating a particular operating system tailored to the requirements of a data processing installation.

system boot

The typical computer system boots over and over again with no problems, starting the computer's operating system (OS) and identifying its hardware and software components that all work together to provide the user with the complete computing experience. But what happens between the time that the user powers up the computer and when the GUI icons appear on the desktop?
In order for a computer to successfully boot, its BIOS, operating system and hardware components must all be working properly; failure of any one of these three elements will likely result in a failed boot sequence.
When the computer's power is first turned on, the CPU initializes itself, which is triggered by a series of clock ticks generated by the system clock. Part of the CPU's initialization is to look to the system's ROM BIOS for its first instruction in the startup program. The ROM BIOS stores the first instruction, which is the instruction to run the power-on self test (POST), in a predetermined memory address. POST begins by checking the BIOS chip and then tests CMOS RAM. If the POST does not detect a battery failure, it then continues to initialize the CPU, checking the inventoried hardware devices (such as the video card), secondary storage devices, such as hard drives and floppy drives, ports and other hardware devices, such as the keyboard and mouse, to ensure they are functioning properly.
Once the POST has determined that all components are functioning properly and the CPU has successfully initialized, the BIOS looks for an OS to load.
The BIOS typically looks to the CMOS chip to tell it where to find the OS, and in most PCs, the OS loads from the C drive on the hard drive even though the BIOS has the capability to load the OS from a floppy disk, CD or ZIP drive. The order of drives that the CMOS looks to in order to locate the OS is called the boot sequence, which can be changed by altering the CMOS setup. Looking to the appropriate boot drive, the BIOS will first encounter the boot record, which tells it where to find the beginning of the OS and the subsequent program file that will initialize the OS.
Once the OS initializes, the BIOS copies its files into memory and the OS basically takes over control of the boot process. Now in control, the OS performs another inventory of the system's memory and memory availability (which the BIOS already checked) and loads the device drivers that it needs to control the peripheral devices, such as a printer, scanner, optical drive, mouse and keyboard. This is the final stage in the boot process, after which the user can access the system’s applications to perform tasks.

system structure

Simple Structure



-->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.

system call

• Process control is a statistics and engineering discipline that deals with architectures, mechanisms, and algorithms for controlling the output of a specific process. See also control theory.
  For example, heating up the temperature in a room is a process that has the specific, desired outcome to reach and maintain a defined temperature (e.g. 20°C), kept constant over time. Here, the temperature is the controlled variable. At the same time, it is the input variable since it is measured by a thermometer and used to decide whether to heat or not to heat. The desired temperature (20°C) is the setpoint. The state of the heater (e.g. the setting of the valve allowing hot water to flow through it) is called the manipulated variable since it is subject to control actions.
  A commonly used control device called a programmable logic controller, or a PLC, is used to read a set of digital and analog inputs, apply a set of logic statements, and generate a set of analog and digital outputs. Using the example in the previous paragraph, the room temperature would be an input to the PLC. The logical statements would compare the setpoint to the input temperature and determine whether more or less heating was necessary to keep the temperature constant. A PLC output would then either open or close the hot water valve, an incremental amount, depending on whether more or less hot water was needed. Larger more complex systems can be controlled by a Distributed Control System (DCS) or SCADA system.
  In practice, process control systems can be characterized as one or more of the following forms:
  »Discrete – Found in many manufacturing, motion and packaging applications. Robotic assembly, such as that found in automotive production, can be characterized as discrete process control. Most discrete manufacturing involves the production of discrete pieces of product, such as metal stamping.
  »Batch – Some applications require that specific quantities of raw materials be combined in specific ways for particular durations to produce an intermediate or end result. One example is the production of adhesives and glues, which normally require the mixing of raw materials in a heated vessel for a period of time to form a quantity of end product. Other important examples are the production of food, beverages and medicine. Batch processes are generally used to produce a relatively low to intermediate quantity of product per year (a few pounds to millions of pounds).
  »Continuous – Often, a physical system is represented though variables that are smooth and uninterrupted in time. The control of the water temperature in a heating jacket, for example, is an example of continuous process control. Some important continuous processes are the production of fuels, chemicals and plastics. Continuous processes, in manufacturing, are used to produce very large quantities of product per year(millions to billions of pounds).

• File Management: These types of system calls are used to manage files. Some examples are Create file, delete file, open, close, read, write etc.

• Device Management: These types of system calls are used to manage devices. Some examples are Request device, release device, read, write, get device attributes etc.

• information maintenance

many system call exist simply for the purpose of transferring information between the user program ande the operating system. for example most system, most system have a system call to return thecurrent time and date. other system calls may return information about the system, sush as the number of current users, the version number of the operationg system, the ammount of free memoryor disk space, and so on

operating system services

Operating systems are responsible for providing essential services within a computer system:
»Initial loading of programs and transfer of programs between secondary storage and main »memory
»Supervision of the input/output devices
»File management
»Protection facilities

System Components

• operating system process management-Multiprogramming systems explicitly allow multiple processes to exist at any given time, where only one is using the CPU at any given moment, while the remaining processes are performing I/O or are waiting.
  The process manager is of the four major parts of the operating system. It implements the process abstraction. It does this by creating a model for the way the process uses CPU and any system resources. Much of the complexity of the operating system stems from the need for multiple processes to share the hardware at the same time. As a conseuence of this goal, the process manager implements CPU sharing ( called scheduling ), process synchronization mechanisms, and a deadlock strategy. In addition, the process manager implements part of the operating system's protection and security.

• Main memory management

Memory management is a tricky compromise between performance (access time) and quantity (available space). We always seek the maximum available memory space but we are rarely prepared to compromise on performance. Memory management must also perform the following functions:
»allow memory sharing (for a multi-threaded system);
»allocate blocks of memory space for different tasks;
»protect the memory spaces used (e.g. prevent a user from changing a task performed by »another user);
»optimise the quantity of available memory, specifically via memory expansion systems.

• file management

Specifically, one may create a new file or edit an existing file and save it; open or load a pre-existing file into memory; or close a file without saving it. Additionally, one may group related files in directories. These tasks are accomplished in different ways in different operating systems and depend on the user interface design and, to some extent, the storage medium being used.

• I/O system management

In a multi-computer system having a plurality of computers, an input/output device configuration definition table and an input/output device configuration reference table are adapted to be collectively managed. A configuration management program manages the configuration definition of all input/output devices of a plurality of computers by using the input/output device configuration definition table, and generates a changed data file when an input/output device configuration is changed. Dynamic system alteration is effected by changing the contents of the input/output device configuration reference table stored in a shared memory, in accordance with the changed data file. The input/output device configuration definition table and the input/output device configuration reference table each have an input/output device information part and an input/output device connection information part arranged in a matrix form to allow addition/deletion of an input/output device and a computer.

• secondary storage management

Secondary storage management is a classical feature of database management systems. It is usually supported through a set of mechanisms. These include index management, data clustering, data buffering, access path selection and query optimization.
None of these is visible to the user: they are simply performance features. However, they are so critical in terms of performance that their absence will keep the system from performing some tasks (simply because they take too much time). The important point is that they be invisible. The application programmer should not have to write code to maintain indices, to allocate disk storage, or to move data between disk and main memory. Thus, there should be a clear independence between the logical and the physical level of the system.

• protection system

protection agianst any harm from computer attacks.

• command -interpreter system

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.