Why does Round(-0.0066219357357) return -1?

Question

I want to Round(-0.0066219357357) and it gives -1.

Isn't it supposed to be 0? And what can I use to round it correctly?

UPDATE: the number is a result of (LineDirection.X/distance); where LineDirection.X is an integer and Distace is double.

Answer 1

Looking into System.pas from XE, all work is done by FISTP instruction:

    procedure _ROUND;
    asm
      ...
            FISTP   qword ptr [ESP]
      ...
    end;

According to the Intel's Instruction Reference, value "is rounded to an integer value, according to the rounding mode specified by the RC field of the FPU control word"

So, I would advise you to check if this RC field of FPU control word is set to the rounding you need.

You can do this by working with whole control word - see Set8087CW, and related xxxx8087CW procedures/functions, with Default8087CW variable.

Or you can try Math.SetRoundMode, as @Uwe Raabe suggested. Your case sounds like either rmUp or rmTruncate rmDown was used.

Theoretically, your CPU can be the reason as well, however, it hardly so.