Check this guy out!

[jklsadf]

Have a look at https://github.com/jjj11x/stc_rpncalc which fits a (decimal) floating point library on an 8-bit 8051 microcontroller with 13k of ROM. The transcendental functions are relatively accurate, other than the trigonometric functions based on the Sinclair Scientific algorithms.

If your goal is to write your own decimal floating point library, an 8-bit microcontroller with a little more flash would be sufficient, although honestly if I were starting a design from scratch, I'd just go with a 32-bit ARM microcontroller. The free GCC or clang compilers are very good. There are also open-source decimal floating point libraries available, for instance see https://sourceforge.net/projects/wp34s/ which uses the IBM decNumber library https://sourceforge.net/p/wp34s/code/HEA...decNumber/

The 15C ROM is incredibly space efficient, but was also coded in assembly targeting a 4-bit microcontroller which was specifically designed for a calculator.