What is at port 4009?

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).