EX: File Allocation Table (FAT) for Windows/PC systems
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.
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
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.
-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
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. ---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
create/terminate a process (including self)
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.
–set system data (OS parameters)
– get process information (id, time used)
– establish a connection
– send, receive messages
– terminate a connectionlProcess control
– create/terminate a process (including self)
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.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
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.
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:
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.
The I/O system consists of:
>A buffer-caching system
>A general device-driver interface
>Drivers for specific hardware devices
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 refers to a mechanism for controlling access by programs, processes, or users to both system and user resourcesn
The protection mechanism must:
Many commands are given to the operating system by control statements which deal with:
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.
