The Elektronika B3-34 is a Soviet-made programmable RPN calculator. It may not look like much, but it's very robust and would probably survive getting driven over by a tank.
It has a custom architecture (not cloned!) with a 4-register stack and 14 addressable registers, each of which can store an 8-digit mantissa with 2-digit exponent (range from 1e-99 to 9.9999999e+99). There are 98 bytes of program memory available as well, however, the program memory is volatile and is erased whenever the calculator is powered off.
- Калькулятор "Электроника Б3-34" — an interesting website with low-level description of the Б3-34's architecture (in Russian).