Editing Intel 4004 (section)

=== Support chips ===
* 4001: 256-[[byte]] ROM (256 8-bit ''program'' instructions) and one built-in 4-bit [[input/output|I/O]] port. A 4001 ROM+I/O chip cannot be used in a system along with a 4008/4009 pair.<ref>IMPORTANT section at page 25: http://www.intel.com/Assets/PDF/Manual/msc4.pdf.</ref>
* 4002: 40-byte [[random-access memory|RAM]] (80 4-bit ''data'' words) and one built-in 4-bit output port; the RAM portion of the chip is organized into 4 "registers" of 20 4-bit words:
** 16 data words (used for [[significand|mantissa]] digits in the original calculator design), accessed in a relatively standard manner,
** 4 status words (used for [[exponent]] digits and signs in the original calculator design), accessed using I/O type commands in place of the ROM's input channel.
* 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.
* 4269: keyboard/display interface.
* 4289: memory interface (combined functions of 4008 and 4009).

The minimum system specification described by Intel consists of a 4004 with a single 256-byte 4001 program ROM; there is no explicit need for separate RAM in minimal-complexity applications thanks to the 4004's large number of onboard index registers, which represent the equivalent of 16&nbsp;× 4-bit or 8&nbsp;× 8-bit characters (or a mixture) of working RAM, nor for simple interface chips thanks to the ROM's built-in I/O lines. However, as project complexity increases, the various other support chips start to become useful.