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...
Very impressive, and obviously a labour of love! As a calculator and SystemVerilog enthusiast, it's wonderful to see a project such as this come to fruition - congratulations!
I'm holding in my hand a 4-bit Von Neumann Mostek MK50310N that my father and I used to use to build calculators long ago. Although Mostek made chips for HP (such as the HP-35), they weren't commercially available, but the 50310 was. We could only dream of a project such as yours. I was happy when the "open source" NumWorks was released, but this project aligns more with my interests.
Will definitely install the Qt simulator - would be even better to build one IRL!
Re Numworks, at $125 for a calculator, you've got to really love the idea.
I didn't think it was that much, but I checked my receipt from 2017, and sure enough it was!
I have in my hand (I guess I like using that phase today), my father's original receipt for the HP-45 (it's with the box and manual). $299.00 in 1975, which is $1,850 today(!!!).
Relatively speaking, electronics are very, very cheap today compared to what they used to be. Still appreciate that my $30 CASIO does 90% of that the NumWorks can do, but I'm happy to support upstarts such as NumWorks.
At least the 6502 has a BCD mode built in!
I'd like to amplify this. Decimal math using nibbles containing 0 through 9 was standard operating procedure, one of the things that made the 6502 nice. Unfortunately you had to use a short loop to convert between 2s complement and BCD.
Ironically the Z80 is a nibble ALU. That's why its so slow compared to the competition, an 8 bit add on a "2 MHz" Z80 takes as much clock time as a 8 bit add on a "1 MHz" 6809.