I designed a nibble-oriented CPU in Verilog to build a scientific calculator

gdevic

The core question: how did HP's scientific calculators actually work at the gate level? That rabbit hole led to building one from scratch. The architectural decision everything else follows from: a decimal calculator should store numbers as BCD — one decimal digit per 4-bit nibble. A standard byte-oriented CPU (Z80, 6502) fights that layout constantly. So I designed a small custom CPU in Verilog where 4 bits is the natural data width and memory is nibble addressable. What the project covers: - Custom CPU: Harvard architecture, 12-bit ISA, 8-state execution FSM, hardware stack guard with a FAULT state for microcode debugging - CORDIC for trig functions, verified to 14 significant digits - Two-pass assembler in Python (~700 lines) - Verilator + Qt framework: same Verilog source runs in simulation, as a desktop GUI debugger, as WebAssembly, and on real hardware - Scripting language on top of the microcode for adding functions without touching hardware - Custom PCB (EasyEDA/JLCPCB), battery, charging circuit Write-up: https://baltazarstudios.com Hackaday: https://hackaday.com/2026/05/13/build-the-cpu-then-build-the...

drob518

My dad worked for HP from the mid-1970s through the mid-1990s. Needless to say, I used HP calculators in high school and college. The best things about having an HP calculator were the solid physical construction (the buttons on the 11C and 15C were awesome), the accuracy, and the fact that whenever your classmates asked to borrow your calculator they would recoil in horror when you asked them whether they knew RPN. Nobody borrowed my calculator. Anyway, I love this project.

wewtyflakes

If the CPU is nibble-oriented, wouldn't that mean that that is its byte size?

fitsumbelay

Love this and love seeing people building their own hardware/software tools. I hope to carve out the time soon to be one. Calculators are a perfect project are there videos available?

NetMageSCW

This is a brilliant project. (My DM42 returned 9 exactly.) Blog post 6 had an error where the picture of a HP-71B (I have one and used its Forth/Assembler ROM manual to write the first HP-48 ROM decoder) where the caption says it is a 48GX (both used a Saturn CPU).

gdevic

There is a WebAssembly version running online here, with and without debugger panel: https://baltazarstudios.com/files/calculator-d/Calculator.ht... https://baltazarstudios.com/files/calculator/Calculator.html This WebAsm code is compiled using Qt and Verilator so it runs the "hardware" and its microcode inside the simple UI shell that provides the calc interface. In the debug version you can list the ucode, set breakpoints, see regs etc.

russdill

Cool project Really hoping that ghidra can add support for non-byte aligned memory regions some day. So many cool 4-bit architectures out there and attempting to shoehorn them into ghidra produces not great results

random__duck

I wish I could give this project more respect than simply saying it is awesome and contraz. Also, let us all hate on denomals! :)

ajxs

One of the coolest projects I've seen in a while! Amazing work! In case anyone missed the write-up^1, it's very well-written. I really enjoyed the chapter about designing the instruction set. 1: https://baltazarstudios.com/calculator-introduction/

wg0

It is... Unbelievably exciting project. There's a lot and lot learn and explore. I'll be reading all of it.