ROM is essentially a memory (or storage) device in which permanent binary information is stored. The binary information must be specified by the designer and is then embedded in the unit to form the required interconnection pattern. ROMs come with special internal electronic fuses that can be "programmed" for a specific configuration. Once the pattern is established, it stays within the unit even when power is turned off and on again. It consists of n input lines and m output lines. Each bit combination of the input variables is called an address. Each bit combination that comes out of the output lines is called a word. The number of bits per word is equal to the number of output lines, m. An address is essentially a binary number that denotes one of the min terms of n variables. The number of distinct addresses possible with n input variables is 2n. An output word can be selected by a unique address, and since there are 2n distinct addresses in a ROM, there are 2n distinct words that are said to be stored in the unit.