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A programmable logic device (PLD) is an electronic component used to construct reconfigurable digital circuits. Unlike a logic gate, which has a fixed function, a PLD has an indefinite function at the time of manufacture. Before you can use the PLD in a circuit, it must be programmed, that is, reconfigured.

Before inventing PLDs, read-only memory chips (ROMs) were used to create arbitrary combinatorial logic functions of several inputs. Consider a ROM with m inputs (the address lines) and n outputs (the data lines). When used as memory, the ROM contains 2m words of n bits each.

Now imagine that the inputs are driven not by an m-bit address, but by m independent logic signals. Theoretically, there are 22m possible boolean functions of these m input signals. By Boolean function in this context is meant a single function that assigns each of the 2m possible combinations of the m boolean inputs to a single Boolean output. There are 22 different ways of assigning each of the 2m inputs to a Boolean value, which explains why there are 22m of such boolean m inputs.

Now, consider that each of the output n pins acts, independently, as a logic device that is specially selected to sample only one of the 22m possible functions. At any given time, only one of the 2m possible input values ​​may be present in the ROM, but over time, since input values ​​span all of their possible domain, each output pin will map its particular function to the 2m values of possible inputs, among the possible 22m of such functions. Note that the structure of the ROM allows that only n of the 22m can produce such boolean functions in the output pins. Therefore, the ROM becomes equivalent to n separate logic circuits, each of which generates a chosen function of the m inputs.

The advantage of using a ROM in this manner is that any conceivable function of all possible combinations of the m inputs can be made to appear in any of the n outputs, making it the most general combinational logic device available for m input and output jacks.
PROMs (Programmable ROMs), EPROMs and EEPROMs (electrically erasable PROMs) are also available which can be programmed using a standard PROM programmer without the need for specialized hardware or software. However, there are several disadvantages:
• are often much slower than dedicated logic circuits,
• can not necessarily provide secure "covers" for asynchronous logic transitions, so PROM outputs may fail as the inputs switch,
• consume more energy,
• are often more expensive than programmable logic, especially if high speed is required.

Since most ROMs do not have input or output registers, they can not be used independently for sequential logic. An external TTL record was often used for sequential designs such as state machines. Common EPROMs, for example the 2716, are still sometimes used this way by amateur circuit designers, who often have something to lie about. This use is sometimes called "PAL of the poor man".