Well, it’s probably no that I have an Elektronika MK-85 calculator (the links points to a Russian page at Wikipedia because there is no English one). The calculator is actually a small PDP-11 computer. It may look like an overkill for a calculator but it works (actually it works well) and additionally in creates some possibilities to expand the machine.
The computer is not as underpowered as it looks. There is available a calculator comparison page which lists the MK-85 on several places. In the default mode it is not much better than the poor old Casio FX-700P (the 1982 calculator from what the Soviets copied the look and interface for the MK-85). But it has a Fast Mode which makes it much better (about 5x faster). And it’s a PDP-11 so if one can write the code in the PDP-11 assembler then it is possible to make it comparable with much more recent pocket computing devices. The fast mode is nothing special – it just makes the machine to rung at about 5x higher CPU frequency (so about 2.0 MHz).
These 6 kB of the RAM are probably enough for most uses. In my opinion no one will write long programs for a relatively low power machine.
Anyway, it can be hacked to be even better.
The first thing is a memory expansion. From the beginning there has been a MK-85M version which has 6 kB of the RAM (instead of default 2 kB). It simply has a three memory chips instead of just one. It’s not a default setup because of cost and power consumption (please remember that battery technology was somewhat less advanced in 1980s and that Soviet batteries were propably somewhat weaker than the Western ones).
There is a page which lists the necessary steps to make it use a more modern memory chip (note that it also requires a new ROM). The RAM can be up to 32 kB. It’s not much, isn’t it? But it is comparable with HP 48G (a 1990s thing) and it is probably more than one can use.
The second thing (actually the first that have to be done) is a ROM chip (and content) upgrade.
But one can do even a little more: add a real time clock or add an I2C-compliant storage device. In any case I can recomend to visit the Polish MK-85 page where are all these cool thing described.
And what about software?