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Building the TD4 4-Bit CPU

The TD4 is a famous 4-bit CPU featured in the book How to Build a CPU by Kaoru Tonami. The book focuses on constructing a functional processor entirely from basic 74-series TTL logic ICs. While the book is unfortunately only available in Japanese, a friend from Japan sent me a copy along with a TD4 PCB. I believe the PCB is based on the open-source design files available on BG5DIW's GitHub repository.

"How to Build CPU" book and the PCB.

Recently, I finally found the time to build and experiment with it. The project took a few months, as I had to translate the book myself to grasp the core concepts. The overall design is simple and elegant, offering a set of 12 instructions and a 16-byte ROM (implemented via DIP switches) for programming. The board operates on 5V and can be powered via USB. Most components were sourced from local shops, though I had to order a few 74HC-series ICs online. Later, I tested the circuit by replacing some 74HC components with 74LS series chips, and it worked without any issues.

The PCB is straightforward to solder. The only SMD components are four LEDs and a Micro-USB socket. If you haven't done SMD soldering before, the USB socket might look intimidating, but you only need to solder pins 1 and 5 (the remaining pins are not used in this design). 

Assembled TD4 CPU.

The most time-consuming part of the assembly was soldering the 1N4148 diodes. The book suggests using a 1S1588 or a D8-1A diode network, but these are difficult to source locally or online. Therefore, I opted for individual 1N4148 diodes. In the TD4 architecture, these diodes function as a Diode Matrix ROM. Since the "program" is stored via physical DIP switches, the diodes act as one-way gates. They prevent current from back-feeding into other parts of the circuit when multiple switches are flipped, effectively creating the logic gates necessary to decode the 16-byte memory addresses into specific 4-bit instructions for the CPU to execute.

Once assembly is complete, the TD4 is ready for immediate use; it requires no firmware or calibration. The PCB offers two power options: the easiest is via the Micro-USB port, while the second is a 2.54mm pin header. I used the pin header to attach the power leads for my logic probe, which was essential while I studied the circuit's operation alongside the book.

Functional TD4 CPU.

The TD4 is a hard-wired logic CPU, meaning it doesn't use a microprogram; every instruction is executed by direct electrical paths through the logic gates. The chipset typically consists of:

  • 74HC161: Used for the A and B registers and the Output port.
  • 74HC161: Another 4-bit counter used for the Program Counter (PC).
  • 74HC153: Multiplexers that act as the selector for the ALU inputs.
  • 74HC283: Configured as 4-bit ALU.
  • 74HC10 and 74HC32: NAND/AND gates used for the instruction decoder.

The clock is generated by a simple 74HC14 oscillator circuit, allowing you to step through instructions manually or run them at a very low frequency to watch the LEDs toggle as data moves between registers.

System clock can monitor from pin 2 of any 74HC161 counter.

The most interesting part of this project is programming the system. Due to the limited address space, the tasks you can perform are modest. I experimented with various counter systems, and all worked as expected. I initially wrote the code on paper and calculated the DIP switch positions manually based on the book's instruction set.

To simplify this process, I developed a small web-based utility using JavaScript that converts assembler source code directly into DIP switch positions. This allowed me to iterate quickly and try out more complex logic. You can find this TD4 Assembler utility here: github.com/dilshan/td4-assembler.

The logic levels of this system can easily monitor using logic probe.

This is one of the counter programs I tested with the CPU. This specific routine reads the input from the DIP switches and increments the value with every clock cycle. Once the counter reaches 0xF, it resets back to the user-specified starting value and resumes counting.:

in b
out b
add b, 1
jnc 1
jmp 0
Address bus and C-flag states of above code listing when input = 0x8.

The TD4 is the perfect "Hello World" for computer architecture because it strips away the complexity of modern processors. There are no hidden layers-no microcode, no complex cache, and no operating system. By building it, you see exactly how a bit moves from a switch, through a multiplexer, and into a register. It demystifies the relationship between physical hardware and software logic, making it an invaluable educational tool for anyone interested in embedded systems or electronic engineering.

Details for How to Build a CPU can be found at the publisher's support site. A TD4 emulator (Windows only) is also available for download at the same link.

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