Read-only memory: Difference between revisions

The simplest type of [[Solid-state electronics|solid-state]] ROM is as old as the [[semiconductor]] technology itself. [[Combinational logic|Combinational]] [[logic gate]]s can be joined manually to map {{mvar|n}}-bit address input onto arbitrary values of {{mvar|m}}-bit data output (a [[look-up table]]). With the invention of the [[integrated circuit]] came [[mask ROM]]. Mask ROM consists of a grid of [[Word (data type)|word]] lines (the address input) and bit lines (the data output), selectively joined with [[transistor]] switches, and can represent an arbitrary look-up table with a regular physical layout and predictable [[propagation delay]]. Mask ROM is programmed with [[photomask]]s in [[photolithography]] during [[semiconductor manufacturing]]. The mask defines physical features or structures that will be removed, or added in the ROM chips, and the presence or absence of these features will represent either a 1 or a 0 bit, depending on the ROM design.<ref name="umich">{{cite web|url=https://web.eecs.umich.edu/~prabal/teaching/eecs373-f10/readings/rom-eprom-eeprom-technology.pdf|title=ROM, EPROM, and EEPROM Technology}}</ref> Thus by design, any attempts to electronically change the data will fail, since the data is defined by the presence or absence of physical features or structures that cannot be electronically changed. For every software program, even for revisions of the same program, the entire mask must be changed, which can be costly.