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This article is about computer memory addressing. For the meaning in networking, see MAC address.
Diagram of relationship between the virtual and physical address spaces.

In computing, a physical address (also real address, or binary address), is a 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, or a register of memory mapped I/O device.

Use by central processing unit[edit]

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 (MMU) to translate memory addresses, the virtual and physical addresses refer to an address before and after translation performed by the MMU, respectively.[1]

Unaligned addressing[edit]

Depending upon its underlying computer architecture, 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 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.[2][3]

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 (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's SIGBUS).[2]

Use by other devices[edit]

The direct memory access (DMA) feature allows other devices in the mother board but CPU to address the main memory. Such devices, therefore, also need to have a knowledge of physical addresses.

See also[edit]

References[edit]

  1. ^ Frank
 Uyeda (2009). "Lecture
 7: 
Memory 
Management" (PDF). CSE
120:
 Principles
 of 
Operating 
Systems. UC San Diego. Retrieved 2013-12-04. 
  2. ^ a b Daniel Drake (2007-12-04). "Memory access and alignment". LWN.net. Retrieved 2013-12-04. 
  3. ^ Daniel Drake; Johannes Berg. "Documentation/unaligned-memory-access.txt". kernel.org. Retrieved 2013-12-04. 



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Fri, 08 Aug 2014 11:15:02 -0700

About the decision, Facebook chimed in: “We're focused on ratings and reviews for places right now.” So, if you've lost your Page ratings yet aren't signed up as a 'Local Business' with a physical address don't fret. It's nothing you've done, just a ...

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Mon, 01 Sep 2014 06:41:15 -0700

Bill Cassidy in the GOP effort to unseat Landrieu and win control of the Senate, said he sued her because "I think it's important." Cassidy spokesman John Cummins said that Landrieu's physical address is in Washington, “but more importantly, she votes ...
 
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Tue, 02 Sep 2014 10:37:30 -0700

In fact we could find no physical address for Torres. He uses a PO Box in Lenexa. We did, however, reach him by phone. He told us he'd be happy to return 50 percent of the money Doss had paid him and that the only reason he hadn't is because Doss had ...
 
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Tue, 02 Sep 2014 06:30:00 -0700

A legitimate business or individual seller should give you a physical address and a working telephone number at which they can be contacted in case you have problems. -Use anti-virus and anti-spyware software, as well as a firewall, and update them all ...
 
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Tue, 02 Sep 2014 08:11:15 -0700

McRae's physical address is 1857 Woods Mountain Trail, Cullowhee. Efforts to reach school board Vice Chairman Ali Laird-Large for comment, via her home phone and through email, were unsuccessful; Chairman Ken Henke has been out of town. Murray ...

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Sun, 31 Aug 2014 10:59:08 -0700

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... Wellness Supervisor, by calling 610.273.9333, ext. 2154 or via email at jschweitzer@telhai.org. Directions to campus can be found on the website: www.telhai.org under the “Contact Us” tab on the home page. Using GPS please enter the physical ...
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