The 4004 and the MCS-4 family

The 4004 and the MCS-4 family

The 4004 is a complete CPU (central processing unit) integrated in a single chip, making the 4004 the world's first microprocessor. Before the microprocessor, CPUs were built with many chips or with a few LSI (large scale integration) chips. The CPUs built with a few LSI chips were steps toward the microprocessor but were not microprocessors. The 4004 is part of a family of four LSI components - the MCS-4 family - that can be used to build digital computers with varying amounts of memory. The other components of the MCS-4 family are memories and input/output circuits, which are not considered part of a CPU in any computer classification, but are necessary to implement a complete computer. Specifically:

* the 4001 is a ROM (read-only memory) with 4 lines of I/O (input/output)
* the 4002 is a RAM (random access memory) with 4 lines of output
* the 4003 is a static shift register to be used for expanding the I/O lines, for example, for keyboard scanning or for controlling a printer

The functional elements integrated in the 4004 are:

* 16 general purpose registers
* program counter and three levels of return address stack
* ALU both binary and decimal
* instruction decoder and all the control logic for the internal functional blocks of the CPU
* generation of timing signal for the CPU and for the rest of the MCS-4 family
* control of the external bus for the memory and for the I/O functions.

The 4004 included also the control functions for the memory and the I/O which are not normally handled by the microprocessor. The 4004, therefore, is not only a complete CPU, but has also additional functionality that normally is not considered a part of a CPU (microcontroller). The first commercial product to use a microprocessor was the Busicom calculator 141-PF.