PLA, 그리고 PAL

Programmable logic array

From Wikipedia, the free encyclopedia

A programmable logic array (PLA) is a programmable device used to implement

combinational logic circuits. The PLA has a set of programmable AND gate planes,

which link to a set of programmable OR gate planes, which can then be

conditionally complemented to produce an output. This layout allows for

a large number of logic functions to be synthesized in the sum of products

(and sometimes product of sums) canonical forms.

One application of a PLA is to implement the control over a datapath.

It defines various states in an instruction set, and produces the next state

(by conditional branching). [eg. if the machine is in state 2, and

will go to state 4 if the instruction contains an immediate field;

then the PLA should define the actions of the control in state 2,

will set the next state to be 4 if the instruction contains an immediate field,

and will define the actions of the control in state 4].

Programmable Logic Arrays should correspond to a state diagram

for the system.

Other commonly used programmable logic devices are PAL, CPLD and FPGA.

Note that the use of the word "programmable" does not indicate that all PLAs

are field-programmable; in fact many are mask-programmed during manufacture

in the same manner as a mask ROM. This is particularly true of PLAs that are embedded 

in more complex and numerous integrated circuits such as microprocessors.

PLAs that can be programmed after manufacture are called FPLA (Field-programmable PLA).

* PLA는 datapath 제어 구현하는데 쓰인다. 쉽게 말하면 CU 만드는데 쓰인다고 생각하면 된다.

* PLA는 PROM과 개념이 유사. 차이점이라면 PLA는 주어진 변수에 대한 완벽한 디코드 출력을

제공하지 않는다는 점과 모든 최소항을 발생시키지 않는다는 점.

* PAL은 프로그램 가능한 AND Array + 고정 OR Array

(PLA는 프로그램 가능한 AND Array + 프로그램 가능한 OR Array)

* PAL - 장: 상대적으로 PLA보다 프로그램 하기 쉽다.

          - 단: PLA보다 융통성이 떨어진다.