This is an emulator for the old Busicom 141-PF.
It might not behave as you'd expect from a modern calculator.

Here's some links to get you started:
Unicom 141 Series Calculator Operating Instructions
The horrible uncommented source code
The amazing fully commented firmware
Busicom 141-PF on the Vintage Calculators Web Museum
MCS-4 Users Manual
4004.com

And some examples:

2 + 3 - 4 + = : 3
5 x 6 x 7 =   : 210
8 √           : 2.8…
90000 x = =   : OVF

Made by Chen Steenvoorden
dutchen18@gmail.com