1.)What are the major activities of an Operating System with regards to PROCESS MANAGEMENT?
Ans.
-Process creation
-Process creation and deletion
-Process suspesion and resumption
-Provision of mechanism for:-Process synchronization
-Process communication
-Deadlock handling
2.)What are the major activities of an Operating System with regards to MEMORY MANAGEMENT?
Ans.
-To improve both CPU utilization and response speed,several programs are kept in memory.
-Keep track of which parts of memory are currently being used and by whom.
-Decide which process to load when memory space becomes available.
-Allocate and Deallocate memory space as needed.
3.)What are the major activities of an Operating System with regards to SECONDARY-STORAGE MANAGEMENT?
Ans.
-Free space management
-Storage allocation
-Disk scheduling
4.)What are the major activities of an Operating System with regards to FILE MANAGEMENT?
Ans.
-File creation and deletion
-Derictory creation and deletion
-Support of primitives for manipulating files and directories
-File backup on stable storage media
-Mapping files onto secondary storage
5.)What is the purpose of the COMMAND INTERPRETER?
Ans.
It reads commands from the user or from a file of commandsand executes them, usually by
turning them into one or more systemcalls. It is usually not part of the kernel since the
command interpreteris subject to changes.
Showing posts with label OS-3. Show all posts
Showing posts with label OS-3. Show all posts
Thursday, July 9, 2009
Tuesday, July 7, 2009
SYSTEM BOOT
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.
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 GENERATION
- System definition: The necessary application and z/TPF system knowledge required to select the hardware configuration and related values used by thez/TPF system software.
- System initialization: The process of creating the z/TPF system tables and configuration-dependent system software.
- System restart and switchover: The procedures used by the z/TPF system software to ready the configuration for online use.
- A program in the restart procedures, called the initializer program (thez/TPF system identification is CCCTIN), uses values assigned to system records used for main (core) storage management. The initializer program is not the same thing as the initialization process. The initializer program is a portion of the restart procedures.
- A system restart begins by pressing the initial program load (IPL) key. This invokes a standard S/390 IPL sequence, which in turn invokes the z/TPF system IPL program. The IPL program loads enough of the system to main storage to permit the initializer program to allocate system tables to main storage. In a loosely coupled complex, an IPL requires the coordination of each of the CPCs in the complex; if a CPC is a multiprocessor, the IPL is performed by only one of the CPUs.
An operational system is a combination of the z/TPF system, application programs, and people. People assign purpose to the system and use the system. The making of an operational system depends on three interrelated concepts:
- System definition: The necessary application and z/TPF system knowledge required to select the hardware configuration and related values used by thez/TPF system software.
- System initialization: The process of creating the z/TPF system tables and configuration-dependent system software.
- System restart and switchover: The procedures used by the z/TPF system software to ready the configuration for online use.
The first two items are sometimes collectively called system generation; also installing and implementing. System definition is sometimes called design. System restart is the component that uses the results of a system generation to place the system in a condition to process real-time input. The initial startup is a special case of restart and for this reason system restart is sometimes called initial program load, or IPL. System restart uses values found in tables set up during system generation and changed during the online execution of the system. A switchover implies shifting the processing load to a different central processing complex (CPC), and requires some additional procedures on the part of a system operator. A restart or switchover may be necessary either for a detected hardware failure, detected software failure, or operator option. In any event, system definition (design), initialization, restart, and switchover are related to error recovery. This provides the necessary background to use this information, which is the principal reference to be used to install the z/TPF system.
Performing a system generation requires a knowledge of the z/TPF system structure, system tables, and system conventions, a knowledge of the applications that will be programmed to run under the system, and a user's knowledge of z/OS. Knowledge of the z/TPF system, Linux, and the application are required to make intelligent decisions to accomplish the system definition of a unique z/TPF system environment. The use of z/OS and Linux is necessary because many programs used to perform system generation run under control of z/OS or Linux. Although this information does not rely on much z/OS or Linux knowledge, when the moment arrives to use the implementation information, the necessary z/OS and Linux knowledge must be acquired. You are assumed to have some knowledge of the S/370 assembly program as well as jargon associated with the z/OS and Linux operating systems. Some knowledge of C language is also helpful, because some of the programs that are used to generate the system are written in C.
System definition is a nontrivial endeavor because both z/TPF system and application knowledge is necessary. An understanding of basic data organization of the system is necessary to allocate the appropriate area of online file storage for all the programs. All z/TPF system programs and application programs that are used in the online environment are placed in file storage and reside in core memory.
An appreciation of defining a system is necessary to relate a unique z/TPF system to its environment. Clearly, no amount of information can anticipate all the variants of applications to be run under the system. However, the interrelationship of the applications, the z/TPF control program, and the corresponding data structures is of paramount importance. The initial system definition must be approached with the best available information. Usually, even this is not sufficient for an accurate definition. Through knowledge, analysis, and an understanding of system guidelines, a workable operational system can be defined and initialized. A system definition is not as insurmountable as it may first seem, because the z/TPF system includes components that allow the system programmer to change the system tables and programs, the application programs, and the physical components. In this sense, the operational system can be adapted to its environment.
The initial system generation, which requires analysis techniques and knowledge of the z/TPF system, represents the beginning of a continuous process required throughout the life of an operational system.
The terms system initialization and initialization process are used to emphasize the human activity and offline procedures required to produce programs and data to be placed on online system resident storage. The term offline procedures refers to the execution of programs, written to run under z/OS or Linux, that assist in the production of the online programs and data. (The term procedures is generally used to mean a program or collection of programs.) The goal of system initialization is to place the various levels of z/TPF system storage facilities into the condition where restart procedures may be invoked to allow online processing to begin. The restart procedures are involved with the programs that build system records used to control online processing. Many of these records are assigned values identified by the initialization process. Some important distinctions of terminology follow:
- A program in the restart procedures, called the initializer program (thez/TPF system identification is CCCTIN), uses values assigned to system records used for main (core) storage management. The initializer program is not the same thing as the initialization process. The initializer program is a portion of the restart procedures.
- A system restart begins by pressing the initial program load (IPL) key. This invokes a standard S/390 IPL sequence, which in turn invokes the z/TPF system IPL program. The IPL program loads enough of the system to main storage to permit the initializer program to allocate system tables to main storage. In a loosely coupled complex, an IPL requires the coordination of each of the CPCs in the complex; if a CPC is a multiprocessor, the IPL is performed by only one of the CPUs.
An initial restart differs from an online restart, which is shown in Figure 1. An initial restart uses input data and restart programs created in the offline environment. An online restart uses some of the same data, but the data and restart programs are accessed from online files. The basic restart procedure of loading the z/TPF core resident programs and executing the initializer program is identical for both the initial restart or the online restart. The location of the restart programs and data identifies the difference. During an initial restart, additional programs to load programs and data to system storage must be invoked. Normally, much of the data put in place during an initial restart does not need to be reloaded during an online restart. (If a reload is necessary, an initial restart is required.) The basic restart programs (such as the IPL and initializer programs) and the structure of the data that these restart programs process are identical. The location of the data is different, as well as some of the data content.
Figure 1. Initial vs. online restart
VIRTUAL MACHINE
Few books have been written on virtual machines, with only a few Java Virtual Machine titles available. Virtual Machine Design and Implementation by Bill Blunden is therefore a landmark book for anyone with an interest in virtual machines, or even system and processor architecture as a whole.
IMPLEMENTATION
"The concept of the virtual machine is one of the most important concepts in computer science today. Emulators use virtual machines, operating systems use virtual machines (Microsoft's .NET), and programming languages use virtual machines (Perl, Java)". Read on for his review of Virtual Machine Design and Implementation in C/C++, an attempt to examine and explain virtual machines and the concepts which allow them to exist.
Virtual machines are, in effect, a software model of a whole system architecture and processor. They take in bytecode (formed of opcodes, operands, and other data) and execute it, much in the same way a real system executes code. Running these operations in software, however, gives you more security, and total control over how the system works.
Virtual machines are popular for a number of reasons. The first is that they give programmers a third compiler option. You don't have to either go the dynamic interpreted route or the static compiled route, you can compile for a virtual machine instead. Another is that virtual machines aid portability. If you compile your code for a virtual machine, you can run that binary on any system to which the virtual machine has been ported.
Few books have been written on virtual machines, with only a few Java Virtual Machine titles available. Virtual Machine Design and Implementation by Bill Blunden is therefore a landmark book for anyone with an interest in virtual machines, or even system and processor architecture as a whole.
BENEFITS
Top Ten Benefits 2008
The latest version of System Center Virtual Machine Manager incorporates all the functionality of its predecessor and brings exciting new capabilities to the management of virtual machines. Here are 10 of the most valuable benefits that Virtual Machine Manager 2008 (VMM) can provide to your organization.
- Designed for virtual machines running on Windows Server 2008 and Microsoft Hyper-V Server
Hyper-V is the next-generation hypervisor-based virtualization platform from Microsoft, which is designed to offer high performance, enhanced security, high availability, scalability, and many other improvements. VMM is designed to take full advantage of these foundational benefits through a powerful yet easy-to-use console that streamlines many of the tasks necessary to manage virtualized infrastructure. Even better, administrators can manage their traditional physical servers right alongside their virtual resources through one unified console.
- Support for Microsoft Virtual Server and VMware ESX
With this release, VMM now manages VMware ESX virtualized infrastructure in conjunction with the Virtual Center product. Now administrators running multiple virtualization platforms can rely on one tool to manage virtually everything. With its compatibility with VMware VI3 (through Virtual Center), VMM now supports features such as VMotion and can also provide VMM-specific features like Intelligent Placement to VMware servers.
- Performance and Resource Optimization (PRO)
Performance and Resource Optimization (PRO) enables the dynamic management of virtual resources though Management Packs that are PRO enabled. Utilizing the deep monitoring capabilities of System Center Operations Manager 2007, PRO enables administrators to establish remedial actions for VMM to execute if poor performance or pending hardware failures are identified in hardware, operating systems, or applications. As an open and extensible platform, PRO encourages partners to design custom management packs that promote compatibility of their products and solutions with PRO’s powerful management capabilities.
- Maximize datacenter resources through consolidation
A typical physical server in the datacenter operates at only 5 to 15 percent CPU capacity. VMM can assess and then consolidate suitable server workloads onto virtual machine host infrastructure, thus freeing up physical resources for repurposing or hardware retirement. Through physical server consolidation, continued datacenter growth is less constrained by space, electrical, and cooling requirements.
- Machine conversions are a snap!
Converting a physical machine to a virtual one can be a daunting undertaking—slow, problematic, and typically requiring you to halt the physical server. But thanks to the enhanced P2V conversion in VMM, P2V conversions will become routine. Similarly, VMM also provides a straightforward wizard that can convert VMware virtual machines to VHDs through an easy and speedy Virtual-to-Virtual (V2V) transfer process.
- Quick provisioning of new machines
In response to new server requests, a truly agile IT department delivers new servers to its business clients anywhere in the network infrastructure with a very quick turnaround. VMM enables this agility by providing IT administrators with the ability to deploy virtual machines in a fraction of the time it would take to deploy a physical server. Through one console, VMM allows administrators to manage and monitor virtual machines and hosts to ensure they are meeting the needs of the corresponding business groups.
- Intelligent Placement minimizes virtual machine guesswork in deployment
VMM does extensive data analysis on a number of factors before recommending which physical server should host a given virtual workload. This is especially critical when administrators are determining how to place several virtual workloads on the same host machine. With access to historical data—provided by Operations Manager 2007—the Intelligent Placement process is able to factor in past performance characteristics to ensure the best possible match between the virtual machine and its host hardware.
- Delegated virtual machine management for Development and Test
Virtual infrastructures are commonly used in Test and Development environments, where there is constant provisioning and tear down of virtual machines for testing purposes. This latest version of VMM features a thoroughly reworked and improved self-service Web portal, through which administrators can delegate this provisioning role to authorized users while maintaining precise control over the management of virtual machines.
- The library helps keep virtual machine components organized
To keep a data center’s virtual house in order, VMM provides a centralized library to store various virtual machine “building blocks”—off-line machines and other virtualization components. With the library’s easy-to-use structured format, IT administrators can quickly find and reuse specific components, thus remaining highly productive and responsive to new server requests and modifications.
- Windows PowerShell provides rich management and scripting environment
The entire VMM application is built on the command-line and scripting environment, Windows PowerShell. This version of VMM adds additional PowerShell commandlets and “view script” controls, which allow administrators to exploit customizing or automating operations at an unprecedented level.
EXAMPLE
In figure 92 is an example of an explicitly authorized TSAF collection involving two z/VM systems sharing global resources. The entries within each box represent the CP directory entries for each CMS virtual machine.
Figure 92. TSAF Collection with Authorized Global Resource Managers and User Programs
In figure 92, users have the following authorization:
-USERa on VMSYS1 can connect only to RES2 on VMSYS2.
-USERb on VMSYS1 can connect only to RES1 on VMSYS1.
-USERc on VMSYS2 can connect to RES1 on VMSYS1 and to RES2 on VMSYS2.
-USERd on VMSYS2 can connect only to RES2 on VMSYS2.
-USERb on VMSYS1 can connect only to RES1 on VMSYS1.
-USERc on VMSYS2 can connect to RES1 on VMSYS1 and to RES2 on VMSYS2.
-USERd on VMSYS2 can connect only to RES2 on VMSYS2.
Thursday, July 2, 2009
SYSTEM STRUCTURE
SIMPLE STRUCTURE
Any part of the system may use the functionality of the rest of
the system.
MS-DOS (user programs can call low level I/O routines)
LAYERED APPROACH
– layer n can only see the functionality that layer n-1 exports
– provides good abstraction from the lower level details
• new hardware can be added if it provides the interface required of a particular layer
– system call interface is an example of layering
– can be slow if there are too many layers
SIMPLE STRUCTURE
Any part of the system may use the functionality of the rest of
the system.
MS-DOS (user programs can call low level I/O routines)
LAYERED APPROACH
– layer n can only see the functionality that layer n-1 exports
– provides good abstraction from the lower level details
• new hardware can be added if it provides the interface required of a particular layer
– system call interface is an example of layering
– can be slow if there are too many layers
SYSTEM CALLS
PROCESS CONTROL
– create/terminate a process (including self)
FILE MANAGEMENT
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.
For companies, a Device Management system means better control and safety as well as increased efficiency, decreasing the possibility for device downtime. As the number of smart devices increases in many companies today, there is a demand for managing, controlling and updating these devices in an effective way. As mobile devices have become true computers over the years, they also force organizations to manage them properly. Without proper management and security policies, mobile devices pose threat to security: they contain lots of information, while they may easily get into wrong hands. Normally an employee would need to visit the IT / Telecom department in order to do an update on the device. With a Device Management system, that is no longer the issue. Updates can easily be done "over the air". The content on a lost or stolen device can also easily be removed by "wipe" operations. In that way sensitive documents on a lost or a stolen device do not arrive in the hands of others.
INFORMATION MAINTENANCE
– get time
– set system data (OS parameters)
– get process information (id, time used)
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.
For companies, a Device Management system means better control and safety as well as increased efficiency, decreasing the possibility for device downtime. As the number of smart devices increases in many companies today, there is a demand for managing, controlling and updating these devices in an effective way. As mobile devices have become true computers over the years, they also force organizations to manage them properly. Without proper management and security policies, mobile devices pose threat to security: they contain lots of information, while they may easily get into wrong hands. Normally an employee would need to visit the IT / Telecom department in order to do an update on the device. With a Device Management system, that is no longer the issue. Updates can easily be done "over the air". The content on a lost or stolen device can also easily be removed by "wipe" operations. In that way sensitive documents on a lost or a stolen device do not arrive in the hands of others.
INFORMATION MAINTENANCE
– get time
– set system data (OS parameters)
– get process information (id, time used)
OPERATING SYSTEM STRUCTURES
System Components
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
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.
I/O SYSTEM MANAGEMENT
SECONDARY STORAGE MANAGEMENT
PROTECTION SYSTEM
Protection refers to a mechanism for controlling access by programs, processes, or users to both system and user resources.
The protection mechanism must:
COMMAND-INTERPRETER SYSTEM
OPERATING SYSTEMS PROCESS MANAGEMENT
In operating systems, process is defined as “A program in
execution” . Process can be considered as an entity that
consists of a number of elements, including: identifier,
state, priority, program counter, memory pointer, context
data, and I/O request. The above information about a
process is usually stored in a data structure, typically called
process block. Figure 1 shows a simplified process block
[10]. Because process management involves scheduling
(CPU scheduling, I/O scheduling, and so on), state
switching, and resource management, process block is one
of the most commonly accessed data type in operating
system. Its design directly affects the efficiency of the
operating system. As a result, in most operating systems,
there is a data object that contains information about all the
execution” . Process can be considered as an entity that
consists of a number of elements, including: identifier,
state, priority, program counter, memory pointer, context
data, and I/O request. The above information about a
process is usually stored in a data structure, typically called
process block. Figure 1 shows a simplified process block
[10]. Because process management involves scheduling
(CPU scheduling, I/O scheduling, and so on), state
switching, and resource management, process block is one
of the most commonly accessed data type in operating
system. Its design directly affects the efficiency of the
operating system. As a result, in most operating systems,
there is a data object that contains information about all the
current active processes. It is called process controller.
Figure 2 shows the structure of a process controller ,
which is implemented as a linked-list of process blocks.
Figure 2 shows the structure of a process controller ,
which is implemented as a linked-list of process blocks.
In order to achieve high efficiency, process controller is
usually implemented as a global variable that can be
accessed by both the kernel modules and nonkernel
modules. For example, any time a new process (task) is
created, the module that created this process should be able
to access the process controller to add this new process.
Therefore, process controller – the data object that controls
the current active process – is usually implemented as a
category-5 global variable. This means, both the kernel
modules and nonkernel modules can access process
controller to change its fields and these changes can affect
the uses of process controller in kernel modules.
usually implemented as a global variable that can be
accessed by both the kernel modules and nonkernel
modules. For example, any time a new process (task) is
created, the module that created this process should be able
to access the process controller to add this new process.
Therefore, process controller – the data object that controls
the current active process – is usually implemented as a
category-5 global variable. This means, both the kernel
modules and nonkernel modules can access process
controller to change its fields and these changes can affect
the uses of process controller in kernel modules.
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
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.
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
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.
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 refers to a mechanism for controlling access by programs, processes, or users to both system and user resources.
The protection mechanism must:
- distinguish between authorized and unauthorized usage.
- specify the controls to be imposed.
- provide a means of enforcement.
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.
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