Intel 4004

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The Intel 4004, a 4-bit central processing unit (CPU) released by Intel Corp. in 1971, is widely considered to be the world's first commercial single-chip microprocessor.In 1970, over a year prior to the introduction of the 4004, the single-chip military F14 CADC microprocessor was deployed, though its existence remained classified until 1998. David A. Patterson, however, argues that the F14 CADC should not be considered a microprocessor in the modern sense and instead a "microprogrammed special purpose computer, using a variable number of custom chips."

Although originally designed to be a component in an Intel customer's calculator products, the 4004 soon found many uses as a flexible replacement for collections of simple logic chips in a variety of applications, thus indicating that there existed an untapped market for microprocessors as such. This prompted Intel and some other integrated circuit manufacturers to embark on a path of developing steadily more capable microprocessors—a trend that eventually created the multibillion-dollar microprocessor and microcomputer industries of today.

History and description

The 4004 was released in 16-pin CERDIP packaging on November 15th, 1971. The 4004 is 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 of Intel and Masatoshi Shima of Busicom (later of ZiLOG).

Originally designed for the Japanese company Busicom to be used in their line of calculators (instead of the complex special purpose calculator chipset that Busicom had designed themselves and brought to Intel to have made, which Intel determined was too complex to make with the technology they had at the time), 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

• Maximum clock speed is 740 kHzAll of Intel's 4004 data sheets, including the very first data sheet from November 1971, clearly indicate that the minimum clock period is 1350 nanoseconds, which results in a maximum clock speed of 740 kHz. Unfortunately, many apparently reputable web pages and other sources list an incorrect clock speed of 108 kHz; even Intel's own pages on the 4004's history say this. The 4004's minimum instruction cycle time is 10.8 microseconds (8 clock cycles), and it seems most likely that someone in the past confused this with a clock speed. This error has now propagated very widely.
• 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
• * 4-bit data words
• Instruction set contains 46 instructions (of which 41 are 8 bits wide and 5 are 16 bits wide)
• Register set contains 16 registers of 4 bits each
• Internal subroutine stack is 3 levels deep

Microarchitecture and pinout

Intel 4004 architectural block diagram.

Custom support chips

• 4001: 256-byte ROM (256 8-bit program instructions), and one built-in 4-bit I/O porta 4001 ROM+I/O chip cannot be used in a system along with a 4008/4009 pair.
• 4002: 40-byte RAM (80 4-bit data words), and one built-in 4-bit output port; the RAM portion of the chip is organized into four "registers" of twenty 4-bit words:
• *16 data words (used for mantissa digits in the original calculator design)
• *4 status words (used for exponent digits and signs in the original calculator design)
• 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 and I/O access converter to standard memory and I/O chips

Collectability

The Intel 4004, naturally, is one of world's most sought-after collectable/antique chips. Of highest value are 4004's that are gold and white, with visible so called 'grey traces' on the white portion (the original package type). As of 2006, such chips reached around US$1000 each on eBay. The slightly less valuable white and gold chips without grey traces typically reach $300 to $500. Those chips without a 'date code' underneath are earlier versions, and therefore worth slightly more. Other valuable chips include the Intel 4040.

Notes