Why PLA Over Read-only Memory
A combinational circuit combinational logic may occasionally have don't-care conditions. When implemented with a read-only memory, a don't care condition becomes an address input that will never occur. The words at the don't care address need not be programmed and may be left in their original state (all 0's or all 1's). The result is that not all the bit patterns available in read-only memory are used which may be considered a waste of available component.
Read more about this topic: Programmable Logic Array
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“Our memory is like a shop in the window of which is exposed now one, now another photograph of the same person. And as a rule the most recent exhibit remains for some time the only one to be seen.”
—Marcel Proust (1871–1922)