Intel 4004

The Intel 4004, a 4-bit CPU, was the world's first microprocessor. It was released in 16-pin ceramic DIP packaging on November 15th, 1971. The 4004 was the first computer processor designed and manufactured by chip maker Intel, which previously made semiconductor memory chips.

The chief designers of the chip were Ted Hoff and Federico Faggin.

Originally designed for the Japanese company Busicom to be used in their line of calculators, the 4004 was also provided with a family of custom support chips (e.g., each "Program ROM" internally latched for its own use the 4004's 12-bit program address, which allowed 4 KB memory access from the 4-bit address bus if all 16 ROMs were installed). The 4004 circuit was built of 2,300 transistors, and was followed the next year by the first ever 8-bit microprocessor, the 3,300 transistor 8008 (and the 4040, a revised 4004).

As its fourth entry in the microprocessor market, Intel released the CPU that started the microcomputer revolution — the 8080.

Technical specifications

A clock speed of 740 kHz
Separate program and data storage (i.e., a Harvard architecture). Contrary to most Harvard architecture designs, however, which use separate buses, the 4004, with its need to keep pin count down, uses a single multiplexed 4-bit bus for transferring:
- 12-bit addresses
- 8-bit instructions, not to be placed in the same memory as
- 4-bit data words
Instruction set contains 46 instructions (41 - 8 bits wide and 5 - 16 bits wide)
Register set contains 16 registers of 4 bits
Internal subroutine stack is 3 levels deep

Custom support chips

4001: 256 bytes ROM (256 8-bit program instructions), and one built in 4-bit I/O port
4002: 40 bytes RAM ( 80 4-bit data words), and one built in 4-bit Output port; the RAM in each chip was organized into 4 "Registers" each composed of
- 16 4-bit data words (used for mantisa digits in the original calculator)
- 4 4-bit status words (used for exponent digits and signs in the original calculator)
4003: 10-bit parallel output shift register for scanning keyboards, displays, printers, etc.
4008: 8-bit address latch for access to standard memory chips, and one built in 4-bit chip select and I/O port
4009: program & I/O access converter to standard memory and I/O chips

Note: 4001s cannot be used in a system using a 4008/4009.