Some existing digital storage uses multiple levels beyond 0 and 1 for improved density. For instance, the 8087 floating-point coprocessor used a ROM with four l

Some existing digital storage uses multiple levels beyond 0 and 1 for improved density. For instance, the 8087 floating-point coprocessor used a ROM with four levels to store its microcode with two bits per transistor, as a regular ROM was too big for the die. Flash memory uses multi-level cells with up to 4 bits per cell.