Programmable logic devices.

  Programmed Logic Array (PLA) 
    or 
  Programmable Array Logic (PAL)

    Combination of large array of one or two types of gates connected with 
    fuses.

    By blowing fuses, specific circuits can be created economically.
    Newer versions UV erasable or flash programmable.

  Used to create 
    Decoders, custom adders, or other unusual logic circuits.
      Such as a condition in which inputs 1,2,7,9 or 2,3,4,6 are true
      and all others are false.
     
      Or a sine look-up (more efficient than straight ROM).
 
  Cost effective in quantity.

  PLA vs ROM - different inputs may generate same output.
    ROM requires multiple recorded instances of the output.
    PLA - multiple input instances can select a single output. 

Two major types, 
  Programmable Logic Arrays (PLA) 
    and 
  Programmable Array Logic (PAL) 

  PLAs are simpler and faster.

  PALs more complex (may contain additional ROM) but supports more complex
    input conditions.
  
  Wikipedia topic : Programmable logic device

  A full adder implemented with type of PLA
   In this design, the connections are blow-able fuses.









A  B  C   S  C
1  1  1   1  1



1  1  0   0  1



1  0  1   0  1




1  0  0   1  0



0  1  1   0  1



0  1  0   1  0



0  0  1   1  0







 
The gate makeup of a designed adder