In computing

Computing

Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...

, a physical address, also real address, or binary address, is the memory address

Memory address

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 programs store and retrieve, machine code or data. Most application programs do not directly read and write to...

 that is represented in the form of a binary number on the address bus

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...

 circuitry in order to enable the data bus to access a particular storage cell of main memory.

In a computer with virtual memory

Virtual memory

In 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...

, the term physical address is used mostly to differentiate from a 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...

. In particular, in computers utilizing memory management unit

Memory management unit

A 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) to translate memory addresses, the virtual and physical addresses refer to an address before and after MMU translation respectively.

In computer networking, physical address is sometimes a synonym of MAC address

MAC address

A Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...

. The address is actually used on the network's data link layer

Data link layer

The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....

, not on the physical layer

Physical layer

The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....

, as the name would suggest.

Unaligned addressing

Depending upon its underlying computer architecture

Computer 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....

, the performance of a computer may be hindered by unaligned access to memory. As an example, a 16 bit computer with a 16 bit memory data bus such as an 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...

 generally works most efficiently

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...

 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. Obviously, if the 16 bit data value starts at an odd address, the processor may actually need to perform two memory read cycles to load the value into it, i.e. one for the low address (throwing half of it away) and then a second to load the high address (again throwing half of the retrieved data away). In fact, on some architectures

Computer 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....

 (such as the Motorola 68000 family), unaligned memory accesses will result in an exception being raised (usually resulting in a software exception being raised like POSIX

POSIX

POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...

's SIGBUS

SIGBUS

On POSIX-compliant platforms, SIGBUS is the signal sent to a process when it causes a bus error. The symbolic constant for SIGBUS is defined in the header file signal.h. Symbolic signal names are used because signal numbers can vary across platforms....

).

See also

• Address constant
  Address constant
  In IBM/360 , an address constant or "adcon" is an Assembly language data type whose value refers directly to another value stored elsewhere in the computer memory using its address. An address constant can be one, two, three or four bytes long...
  
  
• Addressing mode
  Addressing mode
  Addressing 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...
  
  
• 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 :...
  
  
• Page address register
  Page address register
  A page address register 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 number against all entries in the PAR simultaneously,...
  
  
• Pointer (computer science)
• Primary storage, also known as main memory
• Virtual memory
  Virtual memory
  In 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 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...
    
    , also known as 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....
    
    
  • Page table
    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 those unique to the accessing process...
    
    
  • Memory management unit
    Memory management unit
    A 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)