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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, 15 Aug 2014 03:22:08 -0700

If you need extra details such as the network adapter's physical address (also known as the MAC address), then type 'ipconfig /all' (without the quotes) at the prompt and press Enter. The physical address of our laptop's Ethernet adaptor is F0-4D-A2-DE ...

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Marketing Land
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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Tue, 26 Aug 2014 19:09:55 -0700

Time is of the essence because the informational materials for volunteers and media need to include the location and physical address for the Project. For the 46th year, hundreds of big-hearted South Texans have donated their time and their resources ...
 
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Mon, 25 Aug 2014 14:22:30 -0700

Investigators acted on a tip that Griffin's email address had been used to transmit images containing child pornography. Agents traced the email address and physical address associated with those images and obtained a search warrant, prosecutors stated.
 
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Wed, 27 Aug 2014 08:11:15 -0700

Agents gave Naethe a physical address in Flathead County for the meeting, and he was confronted by officers when he arrived. He allegedly admitted that he was at the residence to engage in sexual acts with the girl, and had bought condoms for the event.
 
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Mon, 25 Aug 2014 11:28:03 -0700

... each firm application must contain the proposed, current and historical business of the applicant, jurisdictions where it conducts business, the type of business and products it proposes offering in New York, the physical address of any operation ...
 
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Newly proposed regulations would require digital currency shops operating in the state to record the identity of their customers, including their name, physical address and any transactions made. In an attempt to deter illegal activities, any Bitcoin ...
 
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In addition, the nonprofit Campaign For Vermont has put that information in an interactive map, that also includes the physical address of each polling location. "It is important that every Vermonter have a voice in how their government works," said ...
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