Q. What is Read-only-Memory?

Read only memory is an illustration of a Programmable Logic Device (PLD) it implies that binary information which is stored within a PLD is specified in some fashion and embedded within hardware. So information retains even when power goes.  

Figure: ROM Design

Above figure (a) presents block diagram of ROM.  It comprises 'k' input address lines and 'n' output data lines. An m ×n ROM is an array of binary cell arranged in m (2^k = m) words of 'n' bits each. ROM doesn't have any data input since write operation is not defined for ROM. ROM is categorized as a combinational circuit and constructed internally with decoder and a group of OR gates. In general a m × n ROM (where m= 2^k, k = no. of address lines) will have an internal k ×2^k decoder and 'n' OR gate. Every OR gates has 2^k inputs that are connected to each of the outputs of decoder.