round result of floating point calcualtion

That looks like you actually need arbitrary precision arithmetics A rough outline: recall how you learned to do multiplication and division on multi-digit numbers in elementary school. Apply same principles to very large numbers, using bytes or ints instead of decimal digits.

Let's say we use unsigned 16-bit ints for "digits". That makes your dividend a 4-digit number, and all your divisors 2-digit numbers.

So you would divide pairwise, compute remainders and accumulate results.

There are multiple precision arithmetics packages out there that help with this --- I'm not aware of any working on 8051s though.

That looks like you actually need arbitrary precision arithmetics A rough outline: recall how you learned to do multiplication and division on multi-digit numbers in elementary school. Apply same principles to very large numbers, using bytes or ints instead of decimal digits.

Let's say we use unsigned 16-bit ints for "digits". That makes your dividend a 4-digit number, and all your divisors 2-digit numbers.

So you would divide pairwise, compute remainders and accumulate results.

There are multiple precision arithmetics packages out there that help with this --- I'm not aware of any working on 8051s though.

You will find a set of 64-bit functions using only 32-bit variables here:

ftp.embedded.com/.../crenshaw98.txt

I have solved the problem.
Here is the solution which I use. I hope I will get no problem with the timing.

unsigned long test_value;
unsigned int result;
unsigned long test_freq;


         dds_word.long_dds = 0;
         test_value = 0x04C4B400;
         for (i=0; i<7; i++)
            {
            result = test_value / test_freq;
            test_value = test_value - (result * test_freq);
            test_value = test_value << 4;
            dds_word.long_dds += result;
            dds_word.long_dds = dds_word.long_dds << 4;
            }
         result = test_value / test_freq;
         dds_word.long_dds += result;