Hello, I have one question and I can't find any logical answer for it:

Why a number value which is 1000 bits long is hard or even impossible to display on the screen?

And the answer isn't: You have small resolution on your monitor etc

Is it something with processor registers or something else?

In Pascal is variable "exetended" ("long double" in c++) and it size is 10 bytes max (so 80 bits am I right?).

It is possible.

Typical modern hardware uses 32-bit or 64-bit for integers of floating point. They're easy because you already have plenty of functions for that kind of variables.
There is no builtin support for 1000 bit numbers on computers, so you'd just need to do them manually.

In EE there is support for 512-bit numbers via Number class.
It can convert a string to 512-bit number, I haven't implemented method for converting 512-bit to string, but it wouldn't be hard.

Thank you for answer