Editing Intel 4040 (section)

=== 4004 ===
{{Main|Intel 4004}}
The 4004 was designed to be used in an [[electronic calculator]] and many of its design notes are related to this role. For instance, program code can only be read from an area dedicated to [[read only memory]], not RAM. Although one could implement the "ROM space" using RAM chips, there were no instructions able to write to that area of memory, and no instructions able to read program code from RAM space. The idea was that systems would supply the system program on ROM, and small amounts of RAM would be used only for data, if at all. In typical use, the internal "index registers" would be used for storage during calculations, with the 16 4-bit registers able to hold a single 8-digit [[binary-coded decimal]] value, the data format used by calculators. It also had four 12-bit registers for holding addresses, the top-most was the [[program counter]] and the next three operated as a [[Stack register|push-down stack]] for [[subroutine]] calls.

To reduce pin count to only 16, the original 4004 had only four data pins, [[multiplexed]] for both data and addresses. Additional pins indicated whether the address was in ROM or one of several "banks" of RAM. As addresses were 12 bits long, expressing an address took three cycles along with the subsequent read or write, and external circuitry had to [[flip-flop (electronics)|latch]] each 4 bit part of the address between cycles. Instructions were 8 bits, so loading the next instruction required many cycles, which led to the slow performance in spite of what was a relatively fast cycle time for the era.