What is the trick to engineering HP calculator keys? Nobody gets keys right like the old HP calculators.
In this age of 3D printing and fast prototypes, we really ought to be able to crack this.
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).
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.
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
This is why network RFCs talk of "octets", to avoid the ambiguity. Octets are always 8 bits.
Also, let us all hate on denomals! :)
are there videos available?
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...
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!
