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Information about 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.

Features

Memory management systems on multi-tasking operating systems usually deal with the following issues.

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

Main article: 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

Main article: Shared memory
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.

See also

External links

Computer data storage, computer memory, and often casually storage or memory refer to computer components, devices and recording media that retain digital data used for computing for some interval of time.
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This article is about the computer term. For the TBN game show, see Virtual Memory (game show).
Virtual memory is an abstraction implemented in a computer that gives an application program the impression it has contiguous working memory, while in fact it is
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This article is about the computer term. For the TBN game show, see Virtual Memory (game show).
Virtual memory is an abstraction implemented in a computer that gives an application program the impression it has contiguous working memory, while in fact it is
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In computer science, garbage collection (GC) is a form of automatic memory management. The garbage collector, or just collector, attempts to reclaim garbage, or memory used by objects that will never be accessed or mutated again by the application.
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Manual memory management, in computer science, refers to the usage of manual instructions by the programmer in order to identify and deallocate unused objects, or garbage. Up until the mid 1990s, the majority of programming languages used in industry supported manual memory
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In computing, multitasking is a method by which multiple tasks, also known as processes, share common processing resources such as a CPU. In the case of a computer with a single CPU, only one task is said to be running
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This article is about the computer term. For the TBN game show, see Virtual Memory (game show).
Virtual memory is an abstraction implemented in a computer that gives an application program the impression it has contiguous working memory, while in fact it is
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Memory protection is a way for controlling memory usage on a computer, and is core to virtually every OS. The main purpose of memory protection is to prevent processes on an operating system from accessing the memory of other processes.
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Memory protection is a way for controlling memory usage on a computer, and is core to virtually every OS. The main purpose of memory protection is to prevent processes on an operating system from accessing the memory of other processes.
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shared memory is a memory that may be simultaneously accessed by multiple programs with an intent to provide communication among them. Depending on context, programs may run on the same physical processor or on separate ones.
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Segmentation is one of the most common ways to achieve memory protection; another common one is paging. Segmentation means that a part or parts of the memory will be sealed off from the currently running process, through the use of hardware registers.
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MS-DOS (short for Microsoft Disk Operating System) is an operating system commercialized by Microsoft. It was the most commonly used member of the DOS family of operating systems and was the dominant operating system for the PC compatible
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A memory manager is a part of a computer program which accepts requests from the program to allocate and deallocate chunks of memory.

The objective of the memory manager is generally to allow dynamic memory allocation.
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Conventional memory is the first 640 kilobytes of an IBM PC's memory.

640 kB barrier

The 640 kB barrier is an aspect of the IBM PC and compatibles. The architectural limitation of the Intel 8088 CPU, used in the original IBM PC was 1024 kB (1 MB or 220
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The name EMM386 was used for the expanded memory managers of both Microsoft's MS-DOS and Digital Research's DR-DOS, which created expanded memory using extended memory on Intel 80386 CPUs.
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QEMM (sometimes pronounced IPA: [kwɛm], though not by those who developed it), the Quarterdeck Expanded Memory Manager by Quarterdeck, was the most popular third party memory manager for the DOS operating system.
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In computer science, dynamic memory allocation is the allocation of memory storage for use in a computer program during the runtime of that program. It is a way of distributing ownership of limited memory resources among many pieces of data and code.
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In computer science, garbage collection (GC) is a form of automatic memory management. The garbage collector, or just collector, attempts to reclaim garbage, or memory used by objects that will never be accessed or mutated again by the application.
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reference is an object containing information which refers to data stored elsewhere, as opposed to containing the data itself. Accessing the value referred to by a reference is called dereferencing it.
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reference is an object containing information which refers to data stored elsewhere, as opposed to containing the data itself. Accessing the value referred to by a reference is called dereferencing it.
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page table is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses. Virtual addresses are those unique to the accessing process. Physical addresses are those unique to the CPU, i.e.
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paging, sometimes called swapping, is a transfer of pages between main memory and an auxiliary store, such as hard disk drive.[1] Paging is an important part of virtual memory implemention in most contemporary general-purpose operating systems, allowing to easily
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In computer operating systems, demand paging is an application of virtual memory. In a system that uses demand paging, the operating system copies a disk page into physical memory only if an attempt is made to access it (i.e., if a page fault occurs).
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pointer is a programming language data type whose value refers directly to (or “points to”) another value stored elsewhere in the computer memory using its address. Obtaining the value to which a pointer refers is called dereferencing the pointer.
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This article is about the computer term. For the TBN game show, see Virtual Memory (game show).
Virtual memory is an abstraction implemented in a computer that gives an application program the impression it has contiguous working memory, while in fact it is
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A memory management unit (MMU), sometimes called paged memory management unit (PMMU), is a computer hardware component responsible for handling accesses to memory requested by the central processing unit (CPU).
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