Address Space - Address Mapping and Translation

Address Mapping and Translation

Another common feature of address spaces are mappings and translations, often forming numerous layers. This usually means that some higher-level address must be translated to lower-level ones in some way. For example, file system on a logical disk operates linear sector numbers, which have to be translated to absolute LBA sector addresses, in simple cases, via addition of the partition's first sector address. Then, for a disk drive connected via Parallel ATA, each of them must be converted to logical (means fake) cylinder-head-sector address due to the interface historical shortcomings. It is converted back to LBA by the disk controller and then, finally, to physical cylinder, head and sector numbers.

The Domain Name System maps its names to (and from) network-specific addresses (usually IP addresses), which in turn may be mapped to link layer network addresses via Address Resolution Protocol. Also, network address translation may occur on the edge of different IP spaces, such as a local area network and the Internet.

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. It is also possible that a single virtual address maps to zero, one, or more than one physical address.