Read only memory (rom):

The problem with RAM is that its memory is volatile, i.e. it loses all its data when the power supply is removed.  A non-volatile memory is a permanent memory that never forgets its data.  One type of non-volatile memory is the Read Only Memory (ROM).  A ROM has a pattern of 0s and 1s imprinted in its memory by the manufacturer.  It is not possible to write new data into a ROM, which is why it is called a Read-Only Memory.

The organisation of data in a ROM is similar to that of a RAM.  Thus a 256-bit ROM might be organised as a 256 X 4-bit memory, and so on.  The ROM may be regarded as the "Reference Library" of a computer.