Q. Working of Read-Only Memory?

A ROM is fundamentally a memory or storage device in which a fixed set of binary information is stored. A block diagram of ROM is as displayed in Figure. It comprises n input lines and m output lines. Every bit combination of input variables is known as an address. Every bit combination which comes out of output lines is known as a word. Total number of bits per word is equal to number of output lines m. Number of distinct addresses possible with n input variables is 2ⁿ.

Figure: ROM

A ROM is characterised by number of words (2ⁿ ) and number of bits (m) per word. For illustration a 32 × 8 ROM that can be written as 2⁵ × 8 comprises 32 words of 8 bit each that means there are 8 output lines and 32 distinct words stored in unit. There are only 5 input lines since 32 = 2⁵and with 5 binary variables we can specify 32 addresses.  

A ROM is fundamentally a combinational circuit and can be constructed as displayed in Figure below. On applying an Input I₁ = 0, I₂ = 0 the 00 line of decoder is selected and we will get O₁ = 0 and O₂ = 1 on applying I₁ = 0 and I₂ = 1 we will obtain O₁ = 1 AND O₂ = 0. This same logic can be used for creating larger ROMs.