The first commercially available MCU was Texas Instrument's TMS 1000, a general-purpose 4-bit, single-chip system. It was first made available for sale in 1974. The original model had 1 KB of ROM, 64 x 4 bits of RAM, and 23 I/O pins. They could be clocked at speeds from 100 to 400 KHz, with each instruction executing in six clock cycles.

Later models would increase the ROM and RAM sizes, though the basic design remained largely unchanged until production ceased in 1981:

The size of the MCU die was roughly 5 x 5 millimeters, small enough to fit in a DIP package. This type of MCU used mask-programmable ROM, meaning that you could not get a blank TMS 1000 chip and program it. Instead, you would have to send the debugged program to Texas Instruments to have it physically produced using a photolithography mask, resulting in a metallic bridge for each bit.

Being a fairly primitive design (relative to later MCUs), it lacked a stack and interrupts, had a set of 43 instructions and two general-purpose registers, making it quite similar to the Intel 4004 CPU. Some models had special peripherals for driving vacuum fluorescent displays (VFD) and for continuously reading inputs to handle user input via a keyboard without interrupting the main program. Its basic pinout looked as follows:

Obviously the pin functions predate the general purpose input/output (GPIO) pins we know today - the K pins can only be used for input, while output pins are denoted as O and control pins are marked with R. The OSC pins are to be connected to an external oscillator circuit. Much like with discrete logic ICs, the Init pin is used to initialize the chip on power-up and has to be kept high for at least six cycles, whereas recent MCUs have integrated Power-On Reset (POR) and a reset pin that needs at most a discrete resistor and capacitor.

According to the original Texas Instruments press release from 1974, these microcontroller could be had for as little as $3 or less if you bought them in large quantity. They would be used in popular toys such as the Speak and Spell, but also just about everywhere else, including household appliances, automobiles, and scientific equipment. By the time production ceased in the early 1980s, many millions had been sold.

It's also interesting to note that while one-time programmable low cost microcontrollers have gone down in price a lot, the class of products has persevered - as an example, the Padauk PMS150C can now be had for $0.03 and whilst offering an 8 bit architecture, its 1K words of ROM and 64 bytes of RAM sound oddly familiar.