A little history: a processor in your pocket

Intel 4004: a processor for a calculator

In 1969, the Japanese company Busicom orders a set of chips for a desktop calculator from a young company called Intel. Intel engineers Ted Hoff and Federico Faggin, together with Masatoshi Shima from Busicom, propose replacing a dozen special-purpose chips with a single general-purpose one: it executes instructions from ROM, and the program decides what exactly to do. And so in 1971 the Intel 4004 appeared — the first microprocessor you could buy.

  • 4 bits: one decimal digit, 0–9, fits exactly into 4 bits — just what a calculator needs.
  • About 2,300 transistors on a die the size of a fingernail.
  • The program lives in a separate ROM chip, the data in a separate RAM chip.

Everything on one chip

The next step was to put the processor, the program ROM, the RAM and the pins for buttons and a screen all on one die. Such a chip is called a microcontroller. One of the first was Texas Instruments' TMS1000 (1974): 4 bits, with the program built in at the factory. It ran calculators, toys and the electronic game Simon (1978). A microcontroller cost next to nothing, and processors moved into things nobody called computers.

Sharp SM5: the brain of the Game & Watch

Inside a Game & Watch is a 4-bit microcontroller from the Sharp SM5 series (SM5A, SM510 and their relatives). It was designed specifically for handheld devices:

  • the ROM with the game program and the RAM with 4-bit cells are on the same die;
  • the LCD driver is right on the chip: some of the RAM cells are screen segments — write a bit and a segment lights up;
  • a 32,768 Hz watch crystal and a seconds divider keep the clock running even when the game is off;
  • the power draw is so low that a pair of button cells lasts for months.

Every game in the series is the same chip with a different program in ROM. That explains how fast Nintendo could release new games: the hardware stayed the same, only the program changed.

In the USSR, the "Elektronika IM-02" — "Nu, Pogodi!" — ran on the KB1013VK1-2 chip, a Soviet counterpart of the Sharp SM5A.

Why our processor is the way it is

Our processor is not a copy of the SM5: the SM5 instruction set is tangled, hard to build by hand and hard to understand in an evening. We took the main ideas of those chips and made them as simple as possible:

  • 4 bits, like the 4004 and the SM5: a digit of the clock or the score is one memory cell and one instruction.
  • 16 instructions, one instruction per clock cycle: the opcode is 4 bits, and the decoder is a single DEC16. The whole processor fits into 6 lessons.
  • Program and data kept separate, like in the SM5: a ROM of 512 instructions and a memory of 256 cells of 4 bits.
  • The screen is memory, like in the SM5: write a number into a screen cell and a digit or a crystal lights up.
  • Hardware around the processor — a 60 Hz divider, the step timer, randomness, the beep — like in the SM5, where the clock and the sound are separate blocks on the same die.

The result is the same design as a real Game & Watch: a small processor, a program in ROM and ready-made hardware around it. Only every part of it is built with your own hands.

Game & Watch is a trademark of Nintendo; Intel, Texas Instruments and Sharp are trademarks of their respective owners. This kit is not affiliated with or endorsed by them; the names are mentioned to tell the story. The processor, the game "Star Harvest" and all the kit pictures are our own.