Virtual and physical addresses

The benefit of using virtual addresses is that it allows management software, such as an Operating System (OS), to control the view of memory that is presented to software. The OS can control what memory is visible, the virtual address at which that memory is visible, and what accesses are permitted to that memory. This allows the OS to sandbox applications (hiding the resources of one application from another application) and to provide abstraction from the underlying hardware.

One benefit of using virtual addresses is that an OS can present multiple fragmented physical regions of memory as a single, contiguous virtual address space to an application.

Virtual addresses also benefit software developers, who will not know a system's exact memory addresses when writing their application. With virtual addresses, software developers do not need to concern themselves with the physical memory. The application knows that it is up to the OS and the hardware to work together to perform the address translation.

In practice, each application can use its own set of virtual addresses that will be mapped to different locations in the physical system. As the operating system switches between different applications it re-programs the map. This means that the virtual addresses for the current application will map to the correct physical location in memory.

Virtual addresses are translated to physical addresses through mappings. The mappings between virtual addresses and physical addresses are stored in translation tables (sometimes referred to as page tables) as this diagram shows:

 

A diagram showing virtual and physical addresses 

Translation tables are in memory and are managed by software, typically an OS or hypervisor. The translations tables are not static, and the tables can be updated as the needs of software change. This changes the mapping between virtual and physical addresses.

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