Research Topics 2

Virtual memory in UNIX

Virtual memory is an internal “trick” that relies on the fact that not every executing task is always referencing it’s RAM memory region. Since all RAM regions are not constantly in-use, UNIX has developed a paging algorithm that move RAM memory pages to the swap disk when it appears that they will not be needed in the immediate future.

RAM demand paging in UNIX;

As memory regions are created, UNIX will not refuse a new task whose RAM requests exceeds the amount of RAM. Rather, UNIX will page out the least recently referenced RAM memory page to the swap disk to make room for the incoming request. When the physical limit of the RAM is exceeded UNIX can wipe-out RAM regions because they have already been written to the swap disk. When the RAM region is been removed to swap, any subsequent references by the originating program require UNIX copy page in the RAM region to make the memory accessible. UNIX page in operations involve disk I/O and are a source of slow performance. Hence, avoiding UNIX page in operations is an important concern for the Oracle DBA.

Memory management

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.

Virtual memory systems separate the memory addresses used by a process from actual physical addresses, allowing separation of processes and increasing the effectively available amount of RAM using disk swapping. The quality of the virtual memory manager can have a big impact on overall system performance.

Garbage collection is the automated allocation, and deallocation of computer memory resources for a program. This is generally implemented at the programming language level and is in opposition to manual memory management, the explicit allocation and deallocation of computer memory resources.

Relocation

In systems with virtual memory, programs in memory must be able to reside in different parts of the memory at different times. This is because when the program is swapped back into memory after being swapped out for a while it can not always be placed in the same location. Memory management in the operating system should therefore be able to relocate programs in memory and handle memory references in the code of the program so that they always point to the right location in memory.

Protection

Processes should not be able to reference the memory for another process without permission. This is called memory protection, and prevents malicious or malfunctioning code in one program from interfering with the operation of other running programs.

Sharing

Even though the memory for different processes is protected from each other different processes should be able to share information and therefore access the same part of memory.

Logical organization

Programs are often organized in modules. Some of these modules could be shared between different programs, some are read only and some contain data that can be modified. The memory management is responsible for handling this logical organization that is different from the physical linear address space. One way to arrange this organization is segmentation.

Physical organization

Memory is usually divided into fast primary storage and slow secondary storage. Memory management in the operating system handles moving information between these two levels of memory.

DOS memory managers

In addition to standard memory management, the 640 KB barrier of MS-DOS and compatible systems led to the development of programs known as memory managers when PC main memories started to be routinely larger than 640 KB in the late 1980s (see conventional memory). These move portions of the operating system outside their normal locations in order to increase the amount of conventional or quasi-conventional memory available to other applications. Examples are EMM386, which was part of the standard installation in DOS's later versions, and QEMM. These allowed use of memory above the 640 KB barrier, where memory was normally reserved for RAMs, and high and upper memory.

Research Topics

1. Review article about Operating System

An operating system (OS) is the software that manages the sharing of the resources of a computer and provides programmers with an interface used to access those resources. An operating system processes system data and user input, and responds by allocating and managing tasks and internal system resources as a service to users and programs of the system. At the foundation of all system software, an operating system performs basic tasks such as controlling and allocating memory, prioritizing system requests, controlling input and output devices, facilitating networking and managing file systems. Most operating systems come with an application that provides a user interface for managing the operating system, such as a command line interpreter or graphical user interface. The operating system forms a platform for other system software and for application software.

The most important program that runs on a computer. Every general-purpose computer must have an operating system to run other programs. Operating systems perform basic tasks, such as recognizing input from the keyboard, sending output to the display screen, keeping track of files and directories on the disk, and controlling peripheral devices such as disk drives and printers.
For large systems, the operating system has even greater responsibilities and powers. It is like a traffic cop -- it makes sure that different programs and users running at the same time do not interfere with each other. The operating system is also responsible for security, ensuring that unauthorized users do not access the system.

Operating systems can be classified as follows:

> multi-user : Allows two or more users to run programs at the same time. Some operating systems permit hundreds or even thousands of concurrent users.

> multiprocessing : Supports running a program on more than one CPU.

> multitasking : Allows more than one program to run concurrently.
multithreading : Allows different parts of a single program to run concurrently.

> real time: Responds to input instantly. General-purpose operating systems, such as DOS, UNIX, are not real-time.

Operating systems provide a software platform on top of which other programs, called application programs, can run. The application programs must be written to run on top of a particular operating system. Your choice of operating system, therefore, determines to a great extent the applications you can run. For PCs, the most popular operating systems are DOS, OS/2, and Windows, but others are available, such as Linux.
As a user, you normally interact with the operating system through a set of commands. For example, the DOS operating system contains commands such as COPY and RENAME for copying files and changing the names of files, respectively. The commands are accepted and executed by a part of the operating system called the command processor or command line interpreter. Graphical user interfaces allow you to enter commands by pointing and clicking at objects that appear on the screen.

2. 2 reasons why a regional bank might decide to buy 6 server computers instead of one supercomputer:

a. for their files to have a backup in case of errors or troubles that their computer may adapt or acquire.

b. for them to continue their transactions when other computer units are having their maintenance and repairs.