Texas Instruments introduced the first commercial single-chip computer in 1974, combining the CPU, RAM, ROM, and I/O into one chip. This family of 4-bit processors was called the TMS1000. A 4-bit processor now seems very limited, but it was a good match for calculators, where each decimal digit fit into four bits. This microcontroller was also used in hand-held games and simple control applications such as microwave ovens. Since its software was in ROM, the TMS1000 needed to be custom-manufactured for each application, but it was inexpensive and sold for $2-$4 in quantity.
See the full post on Ken Shirriff’s blog.