READ ONLY MEMORY (ROM)

       A read-only memory (ROM) consisits of an array of semiconductor devices that are interconnected to store an array of binary data. Once binary data is stored in the ROM, it can be read out whenever desired, but the data that is stored cannot be changed under normal operating conditions. The figure below shows a block diagram of a ROM which has three input lines and four output lines. And the truth table which relates the ROM inputs and outputs.

       For each combination of input values on the three input lines, the corresponding pattern of 0’s and 1’s appears on the ROM output lines. For example, if the combination ABC = 010 is applied to the input lines, the pattern F0F1F2F3 = 0111 appears on the output lines. Each output patterns that is stored in a ROM is called words. Since the ROM given above has three input lines, we have 2³ = eight different combinations of input values. Each combination serves as an address.

       The figure above a ROM which has n input lines and m output lines contains an array of 2n words and each words is m bits long. The input lines serve as an address to select one of the 2n words. When an input combination is applied to the ROM, the pattern of 0’s and 1’s which is stored in the corresponding word in the memory appears at the output lines. For example on the figure above, if 00 . . . 11 is applied to the input (address lines) of the ROM, the word 110 . . . 010 will be selected and transfered to the output  lines. A 2n X m ROM can realize m functions of n variables because it can store a truth table with 2n rows and m coloumns. Typical sizes for commercially available ROMs range from 32 words X 4 bits to 512K words X 8 bits, or longer.

By: Jonald Sarmiento