Hardware used for virtual memory allocation. A base-bound register is associated with each segment of data or code and defines the position in physical memory of word zero for that segment, the so-called base, and the number of words available to that segment, the so-called bound or limit(or alternatively the physical memory address of the next word after the end of the segment, in which case it is a bounds register). Whenever a process attempts to access the memory segment, the hardware of the system checks that the address of the word lies within the range

and then adds the address to the value contained in the base register to give the physical address. A restriction on this system is that the storage for the segment must be allocated in a contiguous area of memory (see best fit, first fit).

The base register, used in the construction of relative addresses, should not be confused with the base of a base-bound system; the result of modifying an address by a base register’s contents is still an address within virtual memory space of the process, and is not necessarily a physical address.