In
computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, e ...
, a physical address (also real address, or binary address), is a
memory address
In computing, a memory address is a reference to a specific memory location used at various levels by software and hardware. Memory addresses are fixed-length sequences of digits conventionally displayed and manipulated as unsigned integers. Su ...
that is represented in the form of a
binary number
A binary number is a number expressed in the base-2 numeral system or binary numeral system, a method of mathematical expression which uses only two symbols: typically "0" (zero) and "1" ( one).
The base-2 numeral system is a positional notatio ...
on the
address bus
In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This ex ...
circuitry in order to enable the
data bus
In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This ex ...
to access a ''particular'' storage cell of
main memory
Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers.
The central processing unit (CPU) of a computer ...
, or a register of
memory-mapped I/O
Memory-mapped I/O (MMIO) and port-mapped I/O (PMIO) are two complementary methods of performing input/output (I/O) between the central processing unit (CPU) and peripheral devices in a computer. An alternative approach is using dedicated I/O pro ...
device.
Use by central processing unit
In a computer supporting
virtual memory
In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very l ...
, the term ''physical address'' is used mostly to differentiate from a ''
virtual address
In computing, a virtual address space (VAS) or address space is the set of ranges of virtual addresses that an operating system makes available to a process. The range of virtual addresses usually starts at a low address and can extend to the hig ...
''. In particular, in computers utilizing a
memory management unit
A memory management unit (MMU), sometimes called paged memory management unit (PMMU), is a computer hardware unit having all memory references passed through itself, primarily performing the translation of virtual memory addresses to physical ad ...
(MMU) to translate memory addresses, the virtual and physical addresses refer to an address before and after translation performed by the MMU, respectively.
Unaligned addressing
Depending upon its underlying
computer architecture
In computer engineering, computer architecture is a description of the structure of a computer system made from component parts. It can sometimes be a high-level description that ignores details of the implementation. At a more detailed level, t ...
, the performance of a computer may be hindered by ''unaligned'' access to memory. For example, a
16-bit
16-bit microcomputers are microcomputers that use 16-bit microprocessors.
A 16-bit register can store 216 different values. The range of integer values that can be stored in 16 bits depends on the integer representation used. With the two mos ...
computer with a 16-bit memory data bus, such as
Intel 8086
The 8086 (also called iAPX 86) is a 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, is a slightly modified chip with an external 8-bit data bus (allowi ...
, generally has less
overhead if the access is aligned to an even address. In that case fetching one 16-bit value requires a single memory read operation, a single transfer over a data bus.
If the 16-bit data value starts at an odd address, the processor may need to perform two memory read cycles to load the value into it, i.e. one for the low address (throwing away half of it) and then a second read cycle to load the high address (throwing away again half of the retrieved data). On some
processors
A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, ...
, such as the
Motorola 68000
The Motorola 68000 (sometimes shortened to Motorola 68k or m68k and usually pronounced "sixty-eight-thousand") is a 16/32-bit complex instruction set computer (CISC) microprocessor, introduced in 1979 by Motorola Semiconductor Products Sector ...
and
Motorola 68010
The Motorola MC68010 processor is a 16/32-bit microprocessor from Motorola, released in 1982 as the successor to the Motorola 68000. It fixes several small flaws in the 68000, and adds a few features.
The 68010 is pin-compatible with the 68000 ...
processors, and
SPARC
SPARC (Scalable Processor Architecture) is a reduced instruction set computer (RISC) instruction set architecture originally developed by Sun Microsystems. Its design was strongly influenced by the experimental Berkeley RISC system developed ...
processors, unaligned memory accesses will result in an exception being raised (usually resulting in a software exception, such as
POSIX
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming interf ...
's
SIGBUS, being raised).
Use by other devices
The
direct memory access
Direct memory access (DMA) is a feature of computer systems and allows certain hardware subsystems to access main system memory independently of the central processing unit (CPU).
Without DMA, when the CPU is using programmed input/output, it is t ...
(DMA) feature allows other devices in the
mother board
A motherboard (also called mainboard, main circuit board, mb, mboard, backplane board, base board, system board, logic board (only in Apple computers) or mobo) is the main printed circuit board (PCB) in general-purpose computers and other expan ...
besides the CPU to address the main memory. Such devices, therefore, also need to have a knowledge of physical addresses.
See also
*
Address constant {{short description, Assembly language data type
In IBM System/360 through present day z/Architecture, an address constant or "adcon" is an assembly language data type which contains the address of a location in computer memory. An address constant ...
*
Addressing mode
Addressing modes are an aspect of the instruction set architecture in most central processing unit (CPU) designs. The various addressing modes that are defined in a given instruction set architecture define how the machine language instructions in ...
*
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.
For software programs to save and retrieve st ...
*
Page address register
A page address register (PAR) contains the physical addresses of pages currently held in the main memory of a computer system. PARs are used in order to avoid excessive use of an address table in some operating systems. A PAR may check a page's n ...
*
Pointer (computer programming)
In computer science, a pointer is an object in many programming languages that stores a memory address. This can be that of another value located in computer memory, or in some cases, that of memory-mapped computer hardware. A pointer ''ref ...
*
Primary storage
Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers.
The central processing unit (CPU) of a computer ...
, also known as ''main memory''
*
Virtual memory
In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very l ...
**
Virtual address
In computing, a virtual address space (VAS) or address space is the set of ranges of virtual addresses that an operating system makes available to a process. The range of virtual addresses usually starts at a low address and can extend to the hig ...
, also known as
logical address
In computing, a logical address is the address at which an item ( memory cell, storage element, network host) appears to reside from the perspective of an executing application program.
A logical address may be different from the physical addr ...
**
Page 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 used by the program executed by the accessing process, ...
**
Memory management unit
A memory management unit (MMU), sometimes called paged memory management unit (PMMU), is a computer hardware unit having all memory references passed through itself, primarily performing the translation of virtual memory addresses to physical ad ...
(MMU)
*
Gray code addressing
References
{{Data types
Computer memory
Virtual memory
Data types