The 4004 is part of the MCS-4 family of LSI chips that can be used to build digital computers with varying amounts of memory. The other members of the MCS-4 family are memories and input/output circuits, which are necessary to implement a complete computer. The 4001 is a ROM (read-only memory) with four lines of output; the 4002 is a RAM (random access memory) with four lines of input/output. The 4003 is a static shift register to be used for expanding the I/O lines; e.g., for keyboard scanning or controlling a printer.