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Programmable logic array

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 FPGA (Field-programmable gate array), or less frequently FPLA (Field-programmable logic array).

See also

Programmable logic device
Field-programmable gate array

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