Memory address
Encyclopedia
A digital computer's memory
, more specifically main memory, consists of many memory locations, each having a memory address, a number, analogous to a street address, at which computer program
s store (except when the address is in mask ROM
) and retrieve, machine code
or data
. Most application programs do not directly read and write to physical address
es. Rather, they address logical address
es, or virtual address
es, using operating system
APIs. Generally only system software
, i.e. the BIOS
, operating systems, and some specialized utility programs (e.g., memory tester
s), address physical memory using machine code operands or processor register
s, instructing the CPU to direct a hardware device, called the memory controller
, to use the memory bus
or system bus
, or separate control
, address
and data busses, to execute the program's commands. The memory controllers' bus consists of a number of parallel lines, each represented by a binary digit (bit). The width of the bus, and thus the number of addressable storage units, and the number of bits in each unit, varies among computers.
In a computer program, an absolute address, (sometimes called an explicit address or specific address), is a memory address that uniquely identifies a location in memory. This is different from a relative address, that is not unique and specifies a location only in relation to somewhere else (the base address
). Virtual memory also adds a level of indirection.
, with each address identifying a single eight bit byte
or character
of storage; data too large to be stored in a single byte may reside in multiple bytes occupying a sequence of consecutive addresses. Other computers are word-addressable
, with addressable storage units larger than a byte. For example, the Data General Nova
minicomputer
, and the Texas Instruments TMS9900
and National Semiconductor IMP-16
microcomputer
s used 16 bit words, and there were many 36-bit mainframe computer
s which used 18-bit word addressing. The efficiency of addressing of memory depends on the bit size of the bus used for addresses—the more bits used, the more addresses are available to the computer. For example, a byte-addressable machine with a 20-bit address bus (e.g. Intel 8086
) can address 220 (1,048,576) memory locations, or one megabyte
of memory, while a 32-bit bus (e.g. Intel 80386
) addresses 232 (4,294,967,296) locations, or a 4 gigabyte
address space
. In contrast, a 36-bit word-addressable machine with an 18-bit bus addresses only 218 (262,144) locations, but 1.125 megabytes—slightly more than the 8086.
Some older computers (decimal computer
s), were decimal
digit
-addressable. For example, each address in the IBM 1620
's magnetic-core memory identified a single six bit binary-coded decimal
digit, consisting of a parity bit
, flag bit and four numerical bits. The 1620 used 5-digit decimal addresses, so in theory the highest possible address was 99,999. In practice, the CPU supported 20,000 memory locations, and up to two optional external memory units could be added, each supporting 20,000 addresses, for a total of 60,000 (00000 - 59999).
holds a binary number of some sort. Its interpretation, as data of some data type
or as an instruction, and use are determined by the instructions which retrieve and manipulate it. A pointer is a data type whose value is the address of another memory location where data is stored.
Some early programmers combined instructions and data in words as a way to save memory, when it was expensive: The Manchester Mark 1
had space in its 40-bit words to store little bits of data— its processor ignored a small section in the middle of a word— and that was often exploited as extra data storage. Self-replicating programs such as viruses
treat themselves sometimes as data and sometimes as instructions. Self-modifying code
is generally deprecated nowadays, as it makes testing and maintenance disproportionally difficult to the saving of a few bytes, and can also give incorrect results because of the compiler or processor's assumptions about the machine's state
, but is still sometimes used deliberately, with great care.
. It denotes the number of bits that a CPU can process at one time. Historically it has been sized in multiples of four and eight bits (nibbles and bytes, respectively), so sizes of 4, 8, 12, 16, 24, 32, 48, 64, and larger came into vogue with technological advances.
Very often, when referring to the word size of a modern computer, one is also describing the size of address space on that computer. For instance, a computer said to be "32-bit
" also usually allows 32-bit memory addresses; a byte-addressable 32-bit computer can address
bytes of memory, or 4 gibibyte
s (GiB). This seems logical and useful, as it allows one memory address to be efficiently stored in one word.
But this does not always hold. Computers often have memory addresses larger or smaller than their word size. For instance, almost all 8-bit
processors, such as 6502
, supported 16-bit
addresses— if not they would have been limited to a mere 256 byte
memory. The 16-bit Intel 8088
had only an 8-bit external memory bus on early IBM PC
s, and the 16-bit Intel 8086
supported 20-bit addressing, allowing it to access 1 MiB
rather than 64 KiB
s of memory. Popular Intel Pentium processors since introduction of Physical Address Extension
s (PAE) support 36-bit physical addresses, while generally having only a 32-bit word.
A modern byte-addressable 64-bit
computer—with proper OS support can address
bytes (or 16 exbibyte
s) which is considered practically unlimited.
s. By doing so the operating system
can decide to allocate memory as it deems most efficient
without causing the program to halt from a long garbage collection
process. Some literature suggests that, on the whole, garbage collection is the most efficient of memory reclamation strategies , but it is not ideal if it cuts in to 'tidy up' memory just as the program is controlling the firing of a missile or a landing on the moon, because it is non-deterministic
.
The physical blocks of memory (typically 4 kb chunks) are mapped to virtual addresses by a virtual memory program in the kernel
, and supported by the processor hardware, though it was formerly done all in software. The purpose of virtual memory is to abstract memory allocation, allowing the physical space to be allocated as best for the hardware (that is, usually in good sized blocks), and to still be seen as contiguous from a program or compiler
's perspective. Virtual memory is supported by some operating systems (for example Linux and Windows but not MS-DOS). One may think of virtual memory as a filter, or an alternate set of memory addresses that allow programs to read from memory as quickly as possible without requiring it to be at a particular place. Programs use these contiguous virtual address
es, rather than real, and often fragmented, physical address
es, to store instructions and data. When the program is actually executed, the virtual addresses are translated by the processor into real memory addresses. logical address
is a synonym for virtual address.
Virtual memory also effectively allows the address space to be larger than - or rather, extend beyond - the amount of real (or physical) memory available; the computer can put rarely-accessed pages into secondary storage (similar, at a level, in behavior to CPU cache
) and use the real memory (RAM) for new or active tasks. So the virtual address space might contain, say, twice as many addresses as main memory with the extra addresses mapped to hard disk space in the form of a swap file (used in Linux) or page file (the default on Windows XP and later MS operating systems). When required they are copied back (called swapping
) into main memory as soon as they are needed. These movements are performed, typically, as a background process - and in that sense are transparent to programs. However, if due care is not taken, this can often lead to thrashing.
Computer memory
In computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...
, more specifically main memory, consists of many memory locations, each having a memory address, a number, analogous to a street address, at which computer program
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
s store (except when the address is in mask ROM
Mask ROM
Mask ROM is a type of read-only memory whose contents are programmed by the integrated circuit manufacturer...
) and retrieve, machine code
Machine code
Machine code or machine language is a system of impartible instructions executed directly by a computer's central processing unit. Each instruction performs a very specific task, typically either an operation on a unit of data Machine code or machine language is a system of impartible instructions...
or data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
. Most application programs do not directly read and write to physical address
Physical address
In computing, a physical address, also real address, or binary address, is the memory address that is represented in the form of a binary number on the address bus circuitry in order to enable the data bus to access a particular storage cell of main memory.In a computer with virtual memory, the...
es. Rather, they address logical address
Logical address
In computing, a logical address is the address at which an item appears to reside from the perspective of an executing application program....
es, or virtual address
Virtual address
In computer technology, a virtual address is an address identifying a virtual, i.e. non-physical, entity.-Description:The term virtual address is most commonly used for an address pointing to virtual memory or, in networking, when referring to a virtual network address...
es, using operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
APIs. Generally only system software
System software
System software is computer software designed to operate the computer hardware and to provide a platform for running application software.The most basic types of system software are:...
, i.e. the BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
, operating systems, and some specialized utility programs (e.g., memory tester
Memory tester
Memory testers are specialized test equipment used to test and verify memory modules typically in SIMM or DIMM configurations. It detects functional failures of memory modules.- Types :...
s), address physical memory using machine code operands or processor register
Processor register
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...
s, instructing the CPU to direct a hardware device, called the memory controller
Memory controller
The memory controller is a digital circuit which manages the flow of data going to and from the main memory. It can be a separate chip or integrated into another chip, such as on the die of a microprocessor...
, to use the memory bus
Memory bus
The memory bus is the computer bus which connects the main memory to the memory controller in computer systems. Originally, general-purpose buses like VMEbus and the S-100 bus were used, but to reduce latency, modern memory buses are designed to connect directly to DRAM chips, and thus are...
or system bus
System bus
A system bus is a single computer bus that connects the major components of a computer system. The technique was developed to reduce costs and improve modularity....
, or separate control
Control bus
A control bus is a computer bus, used by CPUs for communicating with other devices within the computer. While the address bus carries the information on which device the CPU is communicating with and the data bus carries the actual data being processed, the control bus carries commands from the...
, address
Address bus
An address bus is a computer bus that is used to specify a physical address. When a processor or DMA-enabled device needs to read or write to a memory location, it specifies that memory location on the address bus...
and data busses, to execute the program's commands. The memory controllers' bus consists of a number of parallel lines, each represented by a binary digit (bit). The width of the bus, and thus the number of addressable storage units, and the number of bits in each unit, varies among computers.
In a computer program, an absolute address, (sometimes called an explicit address or specific address), is a memory address that uniquely identifies a location in memory. This is different from a relative address, that is not unique and specifies a location only in relation to somewhere else (the base address
Base address
In computing, a base address is an address serving as a reference point for other addresses.In computers using relative addressing scheme, to obtain an absolute address, the relevant base address is taken and offset is added to it....
). Virtual memory also adds a level of indirection.
Unit of address resolution
Most modern computers are byte-addressableByte-addressable
Byte addressing refers to hardware architectures which support accessing individual bytes of data rather than only larger units called words, which would be word-addressable....
, with each address identifying a single eight bit byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
or character
Character (computing)
In computer and machine-based telecommunications terminology, a character is a unit of information that roughly corresponds to a grapheme, grapheme-like unit, or symbol, such as in an alphabet or syllabary in the written form of a natural language....
of storage; data too large to be stored in a single byte may reside in multiple bytes occupying a sequence of consecutive addresses. Other computers are word-addressable
Word-addressable
Word-addressable is a Computer Architecture term.In computer architecture, a word is an ordered set of bytes or bits that is the normal unit in which information may be stored, transmitted, or operated on within a given computer....
, with addressable storage units larger than a byte. For example, the Data General Nova
Data General Nova
The Data General Nova was a popular 16-bit minicomputer built by the American company Data General starting in 1969. The Nova was packaged into a single rack mount case and had enough power to do most simple computing tasks. The Nova became popular in science laboratories around the world, and...
minicomputer
Minicomputer
A minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...
, and the Texas Instruments TMS9900
Texas Instruments TMS9900
Introduced in June 1976 and based on the Texas Instruments 990 minicomputer CPU, the TMS9900 was one of the first true 16-bit microprocessors...
and National Semiconductor IMP-16
IMP-16
The IMP-16, by National Semiconductor, was the first multi-chip 16-bit microprocessor. It consisted of five PMOS integrated circuits: four four-bit RALU chips providing the data path, and one CROM providing control sequencing and microcode storage.The IMP-16 provided four 16-bit accumulators,...
microcomputer
Microcomputer
A microcomputer is a computer with a microprocessor as its central processing unit. They are physically small compared to mainframe and minicomputers...
s used 16 bit words, and there were many 36-bit mainframe computer
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...
s which used 18-bit word addressing. The efficiency of addressing of memory depends on the bit size of the bus used for addresses—the more bits used, the more addresses are available to the computer. For example, a byte-addressable machine with a 20-bit address bus (e.g. Intel 8086
Intel 8086
The 8086 is a 16-bit microprocessor chip designed by Intel between early 1976 and mid-1978, when it was released. The 8086 gave rise to the x86 architecture of Intel's future processors...
) can address 220 (1,048,576) memory locations, or one megabyte
Megabyte
The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: bytes generally for computer memory; and one million bytes generally for computer storage. The IEEE Standards Board has decided that "Mega will mean 1 000...
of memory, while a 32-bit bus (e.g. Intel 80386
Intel 80386
The Intel 80386, also known as the i386, or just 386, was a 32-bit microprocessor introduced by Intel in 1985. The first versions had 275,000 transistors and were used as the central processing unit of many workstations and high-end personal computers of the time...
) addresses 232 (4,294,967,296) locations, or a 4 gigabyte
Gigabyte
The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is...
address space
Address space
In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.- Overview :...
. In contrast, a 36-bit word-addressable machine with an 18-bit bus addresses only 218 (262,144) locations, but 1.125 megabytes—slightly more than the 8086.
Some older computers (decimal computer
Decimal computer
Decimal computers, computers which have a decimal architecture, represent numbers and/or addresses in decimal, and provide instructions to operate on those numbers and/or addresses directly; examples of encoding used are BCD, Excess-3, two-out-of-five code, ASCII, and EBCDIC.Many early computers,...
s), were decimal
Decimal
The decimal numeral system has ten as its base. It is the numerical base most widely used by modern civilizations....
digit
Numerical digit
A digit is a symbol used in combinations to represent numbers in positional numeral systems. The name "digit" comes from the fact that the 10 digits of the hands correspond to the 10 symbols of the common base 10 number system, i.e...
-addressable. For example, each address in the IBM 1620
IBM 1620
The IBM 1620 was announced by IBM on October 21, 1959, and marketed as an inexpensive "scientific computer". After a total production of about two thousand machines, it was withdrawn on November 19, 1970...
's magnetic-core memory identified a single six bit binary-coded decimal
Binary-coded decimal
In computing and electronic systems, binary-coded decimal is a digital encoding method for numbers using decimal notation, with each decimal digit represented by its own binary sequence. In BCD, a numeral is usually represented by four bits which, in general, represent the decimal range 0 through 9...
digit, consisting of a parity bit
Parity bit
A parity bit is a bit that is added to ensure that the number of bits with the value one in a set of bits is even or odd. Parity bits are used as the simplest form of error detecting code....
, flag bit and four numerical bits. The 1620 used 5-digit decimal addresses, so in theory the highest possible address was 99,999. In practice, the CPU supported 20,000 memory locations, and up to two optional external memory units could be added, each supporting 20,000 addresses, for a total of 60,000 (00000 - 59999).
Contents of each memory location
Each memory location in a stored-program computerStored-program computer
A stored-program computer is one which stores program instructions in electronic memory. Often the definition is extended with the requirement that the treatment of programs and data in memory be interchangeable or uniform....
holds a binary number of some sort. Its interpretation, as data of some data type
Data type
In computer programming, a data type is a classification identifying one of various types of data, such as floating-point, integer, or Boolean, that determines the possible values for that type; the operations that can be done on values of that type; the meaning of the data; and the way values of...
or as an instruction, and use are determined by the instructions which retrieve and manipulate it. A pointer is a data type whose value is the address of another memory location where data is stored.
Some early programmers combined instructions and data in words as a way to save memory, when it was expensive: The Manchester Mark 1
Manchester Mark 1
The Manchester Mark 1 was one of the earliest stored-program computers, developed at the Victoria University of Manchester from the Small-Scale Experimental Machine or "Baby" . It was also called the Manchester Automatic Digital Machine, or MADM...
had space in its 40-bit words to store little bits of data— its processor ignored a small section in the middle of a word— and that was often exploited as extra data storage. Self-replicating programs such as viruses
Computer virus
A computer virus is a computer program that can replicate itself and spread from one computer to another. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability...
treat themselves sometimes as data and sometimes as instructions. Self-modifying code
Self-modifying code
In computer science, self-modifying code is code that alters its own instructions while it is executing - usually to reduce the instruction path length and improve performance or simply to reduce otherwise repetitively similar code, thus simplifying maintenance...
is generally deprecated nowadays, as it makes testing and maintenance disproportionally difficult to the saving of a few bytes, and can also give incorrect results because of the compiler or processor's assumptions about the machine's state
State (computer science)
In computer science and automata theory, a state is a unique configuration of information in a program or machine. It is a concept that occasionally extends into some forms of systems programming such as lexers and parsers....
, but is still sometimes used deliberately, with great care.
Word size versus address size
A word size is characteristic to a given computer architectureComputer architecture
In computer science and engineering, computer architecture is the practical art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals and the formal modelling of those systems....
. It denotes the number of bits that a CPU can process at one time. Historically it has been sized in multiples of four and eight bits (nibbles and bytes, respectively), so sizes of 4, 8, 12, 16, 24, 32, 48, 64, and larger came into vogue with technological advances.
Very often, when referring to the word size of a modern computer, one is also describing the size of address space on that computer. For instance, a computer said to be "32-bit
32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....
" also usually allows 32-bit memory addresses; a byte-addressable 32-bit computer can address
bytes of memory, or 4 gibibyteGibibyte
The gibibyte is a standards-based binary multiple of the byte, a unit of digital information storage. The gibibyte unit symbol is GiB....
s (GiB). This seems logical and useful, as it allows one memory address to be efficiently stored in one word.
But this does not always hold. Computers often have memory addresses larger or smaller than their word size. For instance, almost all 8-bit
8-bit
The first widely adopted 8-bit microprocessor was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M operating system. The Zilog Z80 and the Motorola 6800 were also used in similar computers...
processors, such as 6502
MOS Technology 6502
The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of...
, supported 16-bit
16-bit
-16-bit architecture:The HP BPC, introduced in 1975, was the world's first 16-bit microprocessor. Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816. The Intel 8088 was program-compatible with the Intel 8086, and was 16-bit in that its registers were 16...
addresses— if not they would have been limited to a mere 256 byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
memory. The 16-bit Intel 8088
Intel 8088
The Intel 8088 microprocessor was a variant of the Intel 8086 and was introduced on July 1, 1979. It had an 8-bit external data bus instead of the 16-bit bus of the 8086. The 16-bit registers and the one megabyte address range were unchanged, however...
had only an 8-bit external memory bus on early IBM PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...
s, and the 16-bit Intel 8086
Intel 8086
The 8086 is a 16-bit microprocessor chip designed by Intel between early 1976 and mid-1978, when it was released. The 8086 gave rise to the x86 architecture of Intel's future processors...
supported 20-bit addressing, allowing it to access 1 MiB
Mebibyte
The mebibyte is a multiple of the unit byte for digital information. The binary prefix mebi means 220, therefore 1 mebibyte is . The unit symbol for the mebibyte is MiB. The unit was established by the International Electrotechnical Commission in 2000 and has been accepted for use by all major...
rather than 64 KiB
Kibibyte
The kibibyte is a multiple of the unit byte for quantities of digital information. The binary prefix kibi means 1024; therefore, 1 kibibyte is . The unit symbol for the kibibyte is KiB. The unit was established by the International Electrotechnical Commission in 1999 and has been accepted for use...
s of memory. Popular Intel Pentium processors since introduction of Physical Address Extension
Physical Address Extension
In computing, Physical Address Extension is a feature to allow x86 processors to access a physical address space larger than 4 gigabytes....
s (PAE) support 36-bit physical addresses, while generally having only a 32-bit word.
A modern byte-addressable 64-bit
64-bit
64-bit is a word size that defines certain classes of computer architecture, buses, memory and CPUs, and by extension the software that runs on them. 64-bit CPUs have existed in supercomputers since the 1970s and in RISC-based workstations and servers since the early 1990s...
computer—with proper OS support can address
bytes (or 16 exbibyteExbibyte
The exbibyte is a standards-based binary multiple of the byte, a unit of digital information storage. The exbibyte unit symbol is EiB....
s) which is considered practically unlimited.
Virtual memory versus physical memory
Virtual memory is now universally used on desktop machines. It maps physical memory to different addresses using page tablePage table
A 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...
s. By doing so the operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
can decide to allocate memory as it deems most efficient
Algorithmic efficiency
In computer science, efficiency is used to describe properties of an algorithm relating to how much of various types of resources it consumes. Algorithmic efficiency can be thought of as analogous to engineering productivity for a repeating or continuous process, where the goal is to reduce...
without causing the program to halt from a long garbage collection
Garbage collection (computer science)
In computer science, garbage collection is a form of automatic memory management. The garbage collector, or just collector, attempts to reclaim garbage, or memory occupied by objects that are no longer in use by the program...
process. Some literature suggests that, on the whole, garbage collection is the most efficient of memory reclamation strategies , but it is not ideal if it cuts in to 'tidy up' memory just as the program is controlling the firing of a missile or a landing on the moon, because it is non-deterministic
Nondeterministic algorithm
In computer science, a nondeterministic algorithm is an algorithm that can exhibit different behaviors on different runs, as opposed to a deterministic algorithm. There are several ways an algorithm may behave differently from run to run. A concurrent algorithm can perform differently on different...
.
The physical blocks of memory (typically 4 kb chunks) are mapped to virtual addresses by a virtual memory program in the kernel
Kernel (computing)
In computing, the kernel is the main component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources...
, and supported by the processor hardware, though it was formerly done all in software. The purpose of virtual memory is to abstract memory allocation, allowing the physical space to be allocated as best for the hardware (that is, usually in good sized blocks), and to still be seen as contiguous from a program or compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
's perspective. Virtual memory is supported by some operating systems (for example Linux and Windows but not MS-DOS). One may think of virtual memory as a filter, or an alternate set of memory addresses that allow programs to read from memory as quickly as possible without requiring it to be at a particular place. Programs use these contiguous virtual address
Virtual address
In computer technology, a virtual address is an address identifying a virtual, i.e. non-physical, entity.-Description:The term virtual address is most commonly used for an address pointing to virtual memory or, in networking, when referring to a virtual network address...
es, rather than real, and often fragmented, physical address
Physical address
In computing, a physical address, also real address, or binary address, is the memory address that is represented in the form of a binary number on the address bus circuitry in order to enable the data bus to access a particular storage cell of main memory.In a computer with virtual memory, the...
es, to store instructions and data. When the program is actually executed, the virtual addresses are translated by the processor into real memory addresses. logical address
Logical address
In computing, a logical address is the address at which an item appears to reside from the perspective of an executing application program....
is a synonym for virtual address.
Virtual memory also effectively allows the address space to be larger than - or rather, extend beyond - the amount of real (or physical) memory available; the computer can put rarely-accessed pages into secondary storage (similar, at a level, in behavior to CPU cache
CPU cache
A CPU cache is a cache used by the central processing unit of a computer to reduce the average time to access memory. The cache is a smaller, faster memory which stores copies of the data from the most frequently used main memory locations...
) and use the real memory (RAM) for new or active tasks. So the virtual address space might contain, say, twice as many addresses as main memory with the extra addresses mapped to hard disk space in the form of a swap file (used in Linux) or page file (the default on Windows XP and later MS operating systems). When required they are copied back (called swapping
Swapping
Swapping can mean:* In computer systems, an older form of memory management, similar to Paging* Swapping * Hot swapping* Book swapping* Wife swapping* Cumswapping...
) into main memory as soon as they are needed. These movements are performed, typically, as a background process - and in that sense are transparent to programs. However, if due care is not taken, this can often lead to thrashing.
See also
- Address spaceAddress spaceIn computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.- Overview :...
- Addressing modeAddressing modeAddressing modes are an aspect of the instruction set architecture in most central processing unit designs. The various addressing modes that are defined in a given instruction set architecture define how machine language instructions in that architecture identify the operand of each instruction...
- Base addressBase addressIn computing, a base address is an address serving as a reference point for other addresses.In computers using relative addressing scheme, to obtain an absolute address, the relevant base address is taken and offset is added to it....
- Offset (computer science), also known as a displacement
- Computer engineeringComputer engineeringComputer engineering, also called computer systems engineering, is a discipline that integrates several fields of electrical engineering and computer science required to develop computer systems. Computer engineers usually have training in electronic engineering, software design, and...
- Computer scienceComputer scienceComputer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...
- EndiannessEndiannessIn computing, the term endian or endianness refers to the ordering of individually addressable sub-components within the representation of a larger data item as stored in external memory . Each sub-component in the representation has a unique degree of significance, like the place value of digits...
- Physical addressPhysical addressIn computing, a physical address, also real address, or binary address, is the memory address that is represented in the form of a binary number on the address bus circuitry in order to enable the data bus to access a particular storage cell of main memory.In a computer with virtual memory, the...
- Virtual memoryVirtual memoryIn computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...
- Virtual addressVirtual addressIn computer technology, a virtual address is an address identifying a virtual, i.e. non-physical, entity.-Description:The term virtual address is most commonly used for an address pointing to virtual memory or, in networking, when referring to a virtual network address...
, also known as logical addressLogical addressIn computing, a logical address is the address at which an item appears to reside from the perspective of an executing application program.... - Page tablePage tableA 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...
- Memory management unitMemory management unitA memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...
(MMU)
- Virtual address