Read only memory (ROM)

ROM plays a major role in digital electronics and computers. It is a general purpose code conversion unit where large scale integration is carried out. The input addressing in a ROM are generally termed as linear addressing. A ROM is generally composed of a decoder (AND logic) and an encoder (OR logic). Encoder block hardware is comprised of a functional relationship between the output and input. This is a permanent relationship which cannot be erased or modified. Commonly used ROMs are having sizes in the range 256 to 64K bits with 4 or 8 output lines. 256 kb ROMs are used in VLSI. For the construction of a ROM, diodes, BJTs or MOSFETs are used. BJTs and MOSFETs are more preferred than diodes. Commonly used ROM consists of an array of MOSFETs where gate oxide thicknesses are varied suitably for different devices to facilitate an efficient layout for the ROM encoder. This helps to achieve high storage capacity in a limited volume. ROMs are widely used nowadays in code conversion, waveform generation, look-up tables, sequence generation, display drivers, combinational logic and character generation. There are three types pg programmable ROMs. They are ultraviolet erasable PROM (UVPROM), electrically erasable PROM (EEPROM) and fusible PROM (FPROM).