Also, PROMs (programmable ROMs), EPROMs (ultraviolet-erasable PROMs) and EEPROMs (electrically erasable PROMs) are available that can be programmed using a standard PROM programmer without requiring specialised hardware or software. The advantage of using a ROM in this way is that any conceivable function of all possible combinations of the m inputs can be made to appear at any of the n outputs, making this the most general-purpose combinational logic device available for m input pins and n output pins. The ROM therefore becomes equivalent to n separate logic circuits, each of which generates a chosen function of the m inputs. Note that the structure of the ROM allows just n of the 22m possible such Boolean functions to be produced at the output pins. ![]() At any given time, only one of the 2m possible input values can be present on the ROM, but over time, as the input values span their full possible domain, each output pin will map out its particular function of the 2m possible input values, from among the 22m possible such functions.
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