MyNOR Is a Single-Board Computer Built with a CPU Using a Single NOR Gate

Central processing units (CPUs) are excellent at performing fundamental tasks very quickly. As CPUs evolved, designers added more logic and complexity for a gain in performance. However, the MyNOR 8-bit single board computer from Dennis Kuschel takes CPU design in the opposite direction. At the core of this fully functional CPU are just three components: two transistors and a pull-up resistor.

Like other DIY 8-bit computer projects, MyNOR uses 7400 series logic chips to build the CPU. However, its CPU does not contain an arithmetic logic unit (ALU). An ALU is a combination digital circuit to perform arithmetic functions, like addition and subtraction. MyNOR does not have a combinational circuit. Instead, it has a single NOR gate that performs AND, OR, XOR, ADD, and SUB.

For example, to do a logical-OR on an 8-bit word, the data must travel through the NOR-gate 16 times. Amazingly, MyNOR's CPU only uses a look-up table for a few operations, like bit rotations, which are not possible with the single gate.

MyNOR's circuit board contains 15 CMOS logic chips, two transistors, 8 kilobytes RAM, 32 kilobytes ROM, and 64 kilobytes EEPROM. The ROM is a 27C256 one-time programmable (OTP) EPROM containing microcode and an operating system in machine program code. The OS's APIs are powerful enough to build a floating point calculator, which Kuschel provides example code and hardware design.

MyNOR will not break any speed records. Kuschel estimates it is comparable to a Microchip AVR running around 32kHz. However, we think it is an ingenious minimalist design that anyone can build or extend.

KiCad Schematics and PCB design are available on MyNOR's website. All of the components are available at the time of writing. The only "custom" parts are the PCB and the 27C256 OTP EPROM — both of which you can create. For detailed information on how it operates, check out Kuschel's well-written MyNOR operation guide.