TTL 8-Bit CPU better than Z80

Description

I started this project to understand how real CPUs work. After making my first 4-bit CPU I started reading about the 8085, z80, and the 6052. I wanted to design a faster CPU than those. Right now my CPU can run instructions in less cycles, but it can't run on a super fast clock and doesn't have some of the powerful block transfers that the z80 has. I designed the CPU from scratch using TTL chips and a ROM for the instruction decoder.

The CPU has a custom instruction set with over 60 instructions. Its ALU can do over 8 operations including: ADD, SUB, Complement, compare, shift, etc. It has 4 general purpose registers and 4 for special purposes. The CPU can address 128 kilobytes of RAM and ROM. Its stack can store sixteen 16 bit wide data so it can call 16 times and be able to return.

If you want more hardware info, check out my github link and my youtube channel. This project is still in progress as I am making the ALU part. Logic designs simulated using Digital by H. Neemann.

Files

instr.txt

Instruction set and microcode

plain - 4.73 kB - 02/06/2026 at 22:56

Download

CPU_4.0.dig

Main CPU

dig - 326.08 kB - 02/06/2026 at 22:56

Download

ALU_V2.dig

main ALU

dig - 83.19 kB - 02/06/2026 at 22:56

Download

instr-data.hex

The stored hex format of the

hex - 67.56 kB - 02/06/2026 at 22:56

Download

Stack_FOR_MIT.dig

Stack for the CPU

dig - 22.27 kB - 02/06/2026 at 22:56

Download

Project Logs

Collapse

ALU works!
Bohan Xu • 03/15/2026 at 21:10 • 0 comments

YES! The FPGA works! It didn't work because I used one of the MODE pins as a normal I/O and when I would boot it it would not boot my program. I also changed the switches to just wires because that way I wouldn't have to worry about pull-down resistors.
Please comment or follow! Thanks!
Testing ALU
Bohan Xu • 03/13/2026 at 21:41 • 0 comments

So the ALUI doesn't exactly work. I took the FPGA I was using and put it into a testing circuit. I'm testing it and it doesn't work. Trying to fix it hopefully I will find a solution sometime soon! If you have anything to say please comment! Thanks! Here's a picture:
Finished building ALU!
Bohan Xu • 02/18/2026 at 01:22 • 2 comments

I finally finished building the ALU part! Here's a picture. The FPGA in the middle does addition, subtraction, complement, and, or, xor, and shifts. The rest off the chips are buffers, comparaters, registers, and the A register. The eight LEDs are connected to the data bus. If you have any suggestions please comment! Thanks!