Intel 4004

The chip (limited by its 16 pin DIP packaging) had a single 4-bit bus.

Originally designed for Busicom for use in their calculators, Intel provided a family of custom chips to support it. These chips included all needed circuits. For example the 12-bit Program address, which could access 4K bytes of memory, was latched by the Program ROM chips as the 4004 transmitted it 4-bits at a time.

Features

• Separate Program and Data memories (Harvard architecture)
• Instruction set contains 46 instructions
• Register set contains 16 4-bit registers
• Internal Subroutine stack is 3 levels deep

Available support chips

• 4001 - 256 bytes ROM w/ 4-bit I/O port
• 4002 - 80 4-bit words RAM w/ 4-bit Output port
• 4003 - 10-bit Shift Register
• 4008 - Address Latch for access to standard memory chips
• 4009 - Program & I/O access to standard memory and I/O chips

For calculator and controller use this was a very effective design. There are even a few older traffic light control systems in use built with these chips!