Short note: ROM

ROM (Read-Only Memory) is a non-volatile semiconductor memory whose contents are written once (at manufacture or by programming) and then mainly read. It keeps data when power is removed, so it stores fixed programs such as BIOS/boot code, look-up tables and microcode.

Structure: a $2^n \times m$ ROM has $n$ address lines and $m$ data outputs. An $n$-to-$2^n$ decoder selects one word (row); a programmable OR array (links at row–column crossings) gives the $m$ output bits. So a ROM can implement any set of $m$ Boolean functions of $n$ variables in sum-of-minterms form.

 A(n-1)..A0 -->[ n:2^n decoder ]--> 2^n word lines
                     |
               [ OR array (links) ]
                     |
               D(m-1) ... D0  (outputs)

Types:

Example: a $32 \times 8$ ROM has 5 address lines and 8 outputs, storing 256 bits.