Improve precision of float numbers

[agarza]

(04-11-2019 10:10 AM)deetee Wrote:  Hi all!

I like to build small RPN calculators (ScArY, SCOTT, ARC) with AVR microcontrollers (which are easy to program).

Unfortunately these microcontrollers support float numbers with 4 byte single precision (due IEEE-754) only. That means they support a precision of 6 to 7 decimal digits.

Now I would like to improve this precision to at least 9 decimal digits. The only idea I have is to define a new number format (struct) to separate mantissa and exponent:

Code:

struct real {
  long m;
  int8_t e;
};

But now I have to "reinvent" every mathematical operation like adding two numbers (the following code works, but is far from beeing efficient):

Code:

void realadd(real * res, real a, real b) {
  if (a.e >= b.e) {
    b.m /= _pow10(a.e-b.e);
    res->m = a.m + b.m;
    res->e = a.e;
  }
  else {
    a.m /= _pow10(b.e-a.e);
    res->m = a.m + b.m;
    res->e = b.e;
  }
}

Looking forward to transzendent functions or complex numbers I feel overstrained.

Do you have any other idea how to raise the precision with less effort?

Thanks for any idea.
deetee

What I did was to to use uint64_t (which gcc does support) and map it to a new float-64 type.
(see post: https://www.hpmuseum.org/forum/thread-12761.html)