The Memory Management Glossary
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Our aim is for these entries to be accurate, comprehensible, and useful, and also to have an entry for all common memory management terms. If you can't find the term you're looking for, if our definition doesn't help you, or if you'd like to suggest corrections or additions, please let us know via our feedback page.

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value object (also known as immutable object)

A value object or immutable object is an object whose identity depends solely upon its value or magnitude.

In a typed language, the compiler can often determine at compile time that certain types can be represented as value objects. Usually these types are a scalar data type with bounded magnitude.

Relevance to memory management: If value objects can be identified, the compiler and the memory manager can make certain optimizations: Value objects can be represented as immediate data to minimize storage overhead, they can be replicated to improve locality, and a vector data type of value objects can be represented as a leaf object.

Historical note: Some programming languages expose representational details such as the use of value objects. In Lisp, for example, numbers are often represented as value objects but not always as immediate data. The EQ predicate of Lisp tests if two objects are the identical representation, whereas the EQL predicate tests if two objects represent the same type and value (are computationally identical). Because the choice of representation is an optimization, exposing it at the language level could cause programs to behave differently under different compilers or optimization settings. Modern languages, such as DylanTM hide this representational distinction, permitting the compiler greater freedom in optimization.

Similar terms: immediate data.
See also: immutable.

Related publications:


value type

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vector data type

A vector data type is an aggregate type of more than one dimension whose objects have a value for each dimension, where each dimension is of the same type.

Examples of vector data types include: strings, arrays, and lists.

Relevance to memory management: Vector data types are seldom represented using value objects, but may be represented using leaf objects if they are an aggregate of a type that can be represented by value objects. Scanning information for vectors can be compactly encoded in terms of the aggregated type and the vector dimension.

See also: scalar data type; algebraic data type; value object; leaf object.

virtual address (also known as logical address)

In a virtual memory(1) system, the addresses that application programs deal with are known as virtual addresses.

The virtual addresses used by the application program are translated by the virtual memory system (often using TLBs and page-tables) to physical addresses. It is the physical address that is used to retrieve the contents from the memory(3).

Opposites: physical address.

virtual address space

The virtual address space is the space of virtual addresses.

On virtual memory(1) systems, user processes see the virtual address space, and commonly have a separate virtual address space each, so that they map the same addresses to different data. These systems often have shared memory as well.

Opposites: physical address space.

virtual memory(1) (also known as VM(1))

In a virtual memory (VM) system, the program code deals with virtual addresses. Upon use, the virtual address is translated by the MMU to obtain a physical address that is used to access physical memory(1).

Some operating systems can simulate having more memory(2) than is available as main memory, by storing part of the data in backing store, typically on disk. If the page referenced by the virtual address is not currently in main memory, a page fault occurs, triggering an operating system handler that swaps in the page. Some other page might be swapped out to make room.

Each process typically has its own separate virtual address space with its own mappings and protections.

Example of the relationship between the virtual address spaces of two processes, physical memory, and backing store
Diagram: Example of the relationship between the virtual address spaces of two processes, physical memory, and backing store

Virtual memory technology can be used in many useful memory management techniques, such as barriers(1), copy-on-write, and memory mapping.

"Virtual" means never knowing where your next byte is coming from.

Opposites: real memory(1).
See also: paging; paged in; paged out; swapping; swap space; mapped; reserved; unmapped; shared memory.

VM(1) (for full details, see virtual memory(1))

In a virtual memory (VM) system, the program code deals with virtual addresses. Upon use, the virtual address is translated by the MMU to obtain a physical address that is used to access physical memory(1).

VM(2)

In the PostScript® language, VM is the storage where the values of the composite objects reside.

VM is short for "virtual memory", but this has nothing to do with the usual sense of the phrase (see virtual memory(1)).

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