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A logical address may be different from the physical address due to the operation of an address translator or mapping function. Such mapping functions may be, in the case of a computer memory architecture, a memory management unit (MMU) between the CPU and the memory bus. There may be more than one level of mapping.
An iconic example of virtual-to-physical address translation is virtual memory, where different pages of virtual address space map either to page file or to main memory physical address space. It is possible that several numerically different virtual addresses all refer to one physical address and hence to the same physical byte of RAM .
Virtualization of storage helps achieve location independence by abstracting the physical location of the data. The virtualization system presents to the user a logical space for data storage and handles the process of mapping it to the actual physical location. It is possible to have multiple layers of virtualization or mapping.
However, with the introduction of virtual memory most application programs do not deal directly with physical addresses. Instead, they use logical or virtual addresses, which are translated to physical addresses by the computer's memory management unit (MMU) and the operating system's memory mapping mechanisms.
A page table is a data structure used by a virtual memory system in a computer to store mappings between virtual addresses and physical addresses. Virtual addresses are used by the program executed by the accessing process, while physical addresses are used by the hardware, or more specifically, by the random-access memory (RAM) subsystem. The ...
Drive mapping is how MS-DOS and Microsoft Windows associate a local drive letter (A-Z) with a shared storage area to another computer (often referred as a File Server) over a network. After a drive has been mapped , a software application on a client 's computer can read and write files from the shared storage area by accessing that drive, just ...
This makes providing direct access to the computer hardware difficult, because if the guest OS tried to instruct the hardware to perform a direct memory access (DMA) using guest-physical addresses, it would likely corrupt the memory, as the hardware does not know about the mapping between the guest-physical and host-physical addresses for the ...
In real mode, in order to calculate the physical address of a byte of memory, the hardware shifts the contents of the appropriate segment register 4 bits left (effectively multiplying by 16), and then adds the offset. For example, the logical address 7522:F139 yields the 20-bit physical address: