Operating Systems - Operating-System Structures (Night Class)

Compiled By: Aurelie A. Peralta

Operating-System Structures

An operating system provides the environment within which programs are executed. Internally, operating systems vary greatly in their makeup, being organized, along many different lines. An operating system may be viewed from several vantage points. One is by examining the services that it provides. Another is by looking at the interface that it makes available to users and programmers. A third is by disassembling the system into its components and their interconnections.

System Components

We can create a system as large and complex as an operating system only by partitioning it into smaller pieces. Each piece should be a well-delineated portion of the system, with carefully defined inputs, outputs, and functions.

Process Management

A program does nothing unless its instructions are executed by a CPU. A process can be thought of as a program in execution, but its definition will broaden as we explore it further. A time-shared user program such as a compiler is a process. A word-processing program being run by an individual user on a PC is a process. A system task, such as sending output to a printer, is also a process.

A process needs certain resources - including CPU time, memory, files, and I/O devices - to accomplish its task. When the process terminates, the operating system will reclaim any reusable resources.

We emphasize that a program by itself is not a process; a program is a passive entity, such as the contents of a file stored on disk, whereas a process is an active entity, with a program counter specifying the next instruction to execute. The execution of a process must be sequential. The CPU executes one instruction of the process after another, until the process completes.

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 and the rest of which are user processes. All these processes can potentially execute concurrently.

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

1. Creating and deleting both user and system processes

2. Suspending and resuming processes

3. Providing mechanisms for process synchronization

4. Providing mechanisms for process communication

5. Providing mechanisms for deadlock handling

Main-Memory Management

The main memory is central to the operation of a modern computer system. Main memory is a large array of words or bytes, ranging in size from hundreds of thousands to billions. Each word or byte has its own address. Main memory is a repository of quickly accessible data shared by the CPU and I/O devices.

For a program to be executed, it must be mapped to absolute addresses and loaded into memory. As the program executes, it accesses program instructions and data from memory by generating these absolute addresses. Eventually, the program terminates, its memory space is declared available, and the next program can be loaded and executed.

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

1. Keeping track of which parts of memory are currently being used and by whom

2. Deciding which processes are to be loaded into memory when memory space becomes available

3. Allocating and deallocating memory space as needed

File Management

File management is one of the most visible components of an operating system. Computers can store information on several different types of physical media. Magnetic tape, magnetic disk, and optical disk are the most common media.

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. The operating system maps files onto physical media, and accesses these files via the storage devices.

A file is a collection of related information defined by its creator.

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

1. Creating and deleting files

2. Creating and deleting directories

3. Supporting primitives for manipulating files and directories

4. Mapping files onto secondary storage

5. Backing up files on stable storage media

I/O-System Management

One of the purpose of an operating system is to hide the peculiarities of specific hardware devices from the user. This is usually done through the use of an I/O subsystem which is consists of the following:

1. A memory-management component that includes buffering, caching, and spooling

2. A general device-driver interface

3. Drivers for specific hardware devices

Only the device driver knows the peculiarities of the specific device to which it is assigned.

Secondary-Storage Management

Because main memory is too small to accommodate all the data and programs, and because data that it holds are lost when power is lost, the computer system must provide secondary storage to back up main memory.

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

1. Free-space management

2. Storage allocation

3. Disk Scheduling

Because secondary storage is used frequently, it must be used efficiently. The entire speed of operation of a computer may hinge on the speeds of the disk subsystem and of the algorithms that manipulate that subsystem.

Networking

A distributed system is a collection of processors that do not share memory, peripheral devices, or a clock. The processors in the system are connected through a communication network, which can be configured in a number of different ways. The communication-network design must consider message routing and connection strategies, and the problems of contention and security.

A distributed system collects physically separate, possibly heterogeneous systems into a single coherent system, providing the user with access to the various resources that the system maintains. Access to shared resource allows computation speed up, increased functionality, increased data availability, and enhanced reliability.

Protection System

If a computer system has multiple users and allows the concurrent execution of multiple processes, then the various processes must be protected from one another’s activities. For that purpose, mechanisms ensure that the files, memory segments, CPU, and other resources can be operated on by only those processes that have gained proper authorization from the operating system.

Protection is any mechanism for controlling the access of programs, processes, or users to the resources defined by a computer system. This mechanism must provide means for specification of the controls to be imposed and means for enforcement.

Protection can improve reliability by detecting latent errors at the interfaces between component subsystems.

Command-Interpreter System

One of the most important systems programs for an operating system is the command interpreter, which is the interface between the user and the operating system. Some operating systems include the command interpreter in the kernel.

Many commands are given to the operating system by control statements. A program sometimes called control-card interpreter or command-line interpreter reads and interprets control statements. It is often known as the shell.

The command statements themselves deal with process creation and management, I/O handling, secondary-storage management, main-memory management, file-system access, protection, and networking.

Operating-System Services

An operating system provides an environment for the execution of programs. It provides certain services to programs and to the users of those programs. The specific services provided, of course, differ from one operating system to another, but we can identify common classes. These operating-system services are provided for the convenience of the programmer, to make programming task easier:

1. Program execution: The system must be able to load a program into memory and to run that program. The program must be able to end its execution, either normally or abnormally (indicating error).

2. I/O Operations: The operating system must provide a means to do I/O.

3. File-System Manipulation: Programs need to read, write, create, and delete files by name.

4. Communications: In many circumstances, one process needs to exchange information with another process. Communications may be implemented via shared memory, or by the technique of message passing, in which packets of information are moved between processes by the operating system.

5. Error Detection: The operating system constantly needs to be aware of possible errors. For each type of error, the operating system should take the appropriate action to ensure correct and consistent computing.

Another set of operating-system functions exists not for helping the user, but for ensuring the efficient operation of the system itself. Systems with multiple users can gain efficiency by sharing the computer resources among the users.

1. Resource Allocation: When multiple users are logged on the system or multiple jobs are running at the same time, resources must be allocated to each of them.

2. Accounting: We want to keep track of which users uses how many and which kinds of computer resources. This record keeping may be used for accounting or simply or accumulating usage statistics.

3. Protection: Protection involves ensuring that all access to system resources is controlled. Security of the system from outsiders is also important.

System Calls

System calls provide the interface between a process and the operating system. These calls are generally available as assembly-language instructions, and they are usually listed in the various manuals used by the assembly-language programmers.

Certain system allow system calls to be made directly from a higher-level language program, in which case the calls normally resemble predefined function or subroutine calls.

As an example of how system calls are used, consider writing a simple program to read data from one file and to copy them to another file. The first input that the program will need is the names of the two files: the input file and the output file. These names can be specified in many ways, depending on the operating-system design. One approach is for the program to ask the user for the names of the two files. In an interactive system, this approach will require a sequence of system calls, first to write a prompting message on the screen, and then to read from the keyboard the characters that define the two files. On mouse-based window-and-menu systems, a menu bar of file names is usually displayed in a window. The user can then use the mouse to select the source name, and a similar window can be opened for the destination name to be specified.

Most users never see this level of detail, however. The run-time support system (the set of functions built into libraries included with a compiler) for most programming languages provides a much simpler interface. Thus, most of the details of the operating system interface are hidden from the programmer by the compiler and by the run-time support package.

System calls occur in different ways, depending on the computer in use. Often, more information is required than simply the identity of the desired system call. The exact type and amount of information vary according to the particular operating system and call.

System calls can be grouped roughly into five major categories: process control, file management, device management, information maintenance, and communications.

Process Control

1. end, abort

2. load, execute

3. create process, terminate process

4. get process attributes, set process attributes

5. wait for time

6. wait event, signal event

7. allocate and free memory

File Management

1. create file, delete file

2. open, close

3. read, write, reposition

4. get file attributes, set file attributes

Device Management

1. request device, release device

2. read, write, reposition

3. get device attributes, set device attributes

4. logically attach or detach devices

Information Maintenance

1. get time or date, set time or date

2. get system data, set system data

3. get process, file, or device attributes

4. set process, file, or device attributes

Communications

1. create, delete communication connection

2. send, receive messages

3. transfer status information

4. attach or detach remote devices

System Programs

Another aspect of a modern system is the collection of system programs. System programs provide a convenient environment for program development and execution. Some of them are simply user interfaces to system calls; others are considerably more complex. They can be divided into these categories:

1. File Management

2. Status Information

3. File Modification

4. Programming-language support

5. Program loading and execution

6. Communication

System Structure

System structure refers to the way the different components of an operating system are interconnected and melded into a kernel.

1. Simple structure - many commercial systems do not have a well-defined structures. Frequently, such operating systems started as small, simple, and limited systems, and then grew beyond their original scope. MS-DOS is an example of such a system.

2. Layered approach - the modularization of a system can be done in many ways. One method is the layered approach, in which operating system is broken up into a number of layers or levels, each built on top of lower layers. The bottom layer is the hardware, the highest layer is the user interface. The main advantage of the layered approach is modularity.

3. Microkernels - modularization of the kernel, this method structures the operating system by removing all nonessential components from the kernel, and implementing them as system- and user-level programs. The result is a smaller kernel. The main function of the microkernel is to provide a communication facility between the client program and the various services that are also running in user space.

Virtual Machines

Conceptually, a computer system is made up of layers. The hardware is the lowest level in all such systems. The kernel running at the next level uses the hardware instructions to create a set of system calls for use by outer layers. The system programs above the kernel are therefore able to use either system calls or hardware instructions, and in some ways these programs do not differentiate between these two. Thus, although they are accessed differently, they both provide functionality that the program can use to create even more advanced functions. System programs, in turn, treat the hardware and the system calls as though they were both at the same level.

Some systems carry this scheme a step further by allowing the system programs to be called easily by the application programs. As before, although the system programs are at a level higher than that of the other routines, the application programs may view everything under them in the hierarchy as though the latter were part of the machine itself. This layered approach is taken to its logical conclusion in the concept of a virtual machine. The VM operating system of IBM systems is the best example of the virtual-machine concept, because IBM pioneered the work in this area.

Java is a very popular object-oriented language introduced by Sun Microsystems in late 1995. In addition to a language specification and a large API Library, Java also provides a specification for a Java virtual machine (JVM). The JVM is a specification for an abstract computer. The JVM consists of a class loader, a class verifier, and a Java interpreter that executes the architectural-neutral bytecodes.

System Design and Implementation

No complete solutions to the design problems exist, but some approaches have been successful.

Design Goals

At the highest level, the design of the system will be affected by the choice of hardware and type of system: batch, time shared, single user, multiuser, distributed, real time, or general purpose.

Mechanisms and Policies

One important principle is the separation of policy from mechanism. Mechanisms determine how to do something while policies determine what will be done.

Implementation

Once an operating system is designed, it must be implemented. Traditional operating systems have been written in assembly language. Now, however, they are often written in higher level languages such as C or C++.

Assignment: Explain System Generation in Operating Systems.

Reference: Operating System Concepts by Silberschatz, Galvin, and Gagne, 2003