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, ...
, 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. ...
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 notati ...
on the
address bus circuitry in order to enable the
data bus to access a ''particular'' storage cell of
main memory, or a register of
memory-mapped I/O device.
Use by central processing unit
In a computer supporting
virtual memory, the term ''physical address'' is used mostly to differentiate from a ''
virtual address''. 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 ...
(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, the ...
, the performance of a computer may be hindered by ''unaligned'' access to memory. For example, a
16-bit 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 (allow ...
, 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, 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 Sect ...
and
Motorola 68010 processors, and
SPARC 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 inte ...
's
SIGBUS, being raised).
Use by other devices
The
direct memory access (DMA) feature allows other devices in the
mother board besides the CPU to address the main memory. Such devices, therefore, also need to have a knowledge of physical addresses.
See also
*
Address constant
*
Addressing mode
*
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 ...
*
Page address register
*
Pointer (computer programming)
*
Primary storage, also known as ''main memory''
*
Virtual memory
**
Virtual address, also known as
logical address
**
Page table
**
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 ...
(MMU)
*
Gray code addressing
The reflected binary code (RBC), also known as reflected binary (RB) or Gray code after Frank Gray, is an ordering of the binary numeral system such that two successive values differ in only one bit (binary digit).
For example, the representat ...
References
{{Data types
Computer memory
Virtual memory
Data types