In computers, a physical address or binary address is the value of the finite monotonically ordered binary number presented on the computer's main memory control structure called a computer address bus which is used electronically by the circuitry to directly enable a particular memory storage cell (itself part of the circuitry) using other boolean logic circuitry, the Memory Management Unit (MMU). It is distinct from a memory address only in that the term "Memory address" in practice include both virtual and physical addresses.
In computer architectures, a logical address is the address at which a memory location appears to reside from the perspective of an executing application program. This may be different from the physical address due to the operation of a memory management unit (MMU) between the CPU and the memory bus. Physical memory may be mapped to different logical addresses for various purposes. For example, the same physical memory may appear at two logical addresses and if accessed by the program at one data will pass through the processor cache whereas if it is accesed at the other it will bypass the cache.
In a system supporting virtual memory, there may actually not be any physical memory mapped to a logical address until an access is attempted. The access triggers special functions of the operating system which reprogram the MMU to map the address to some physical memory, perhaps writing the old contents of that memory to disk and reading back from disk what the memory should contain at the new logical address. In this case, the logical address may be referred to as a virtual address.
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6:58 PM on August 23, 2008
