Vampire 2.7 FPU Part 2

Posted by ALB42 on 3. März 2018No Comments

I played a little bit more with the Vampire FPU the yesterdays example shows an other interesting effect, the calculation of a * 1.3 shows an incorrect result. There is some problem in the last places when comparing with 68882 A1200 or 68060 UAE.
Let’s dig a little bit deeper into that, test how well the double precision calculation works. MUIMapparium needs Double precision everywhere, I tested to make it single but the calculation really get very wrong results (Eiffel tower somewhere near London, such stuff). To test double precision we multiply a very small value to 1 very often to see if it correctly handle the bits. for example like this:

program testfpu;
{$mode objfpc}
  i: Integer;
  b: Double;
  b := 1;
  for i := 1 to 100000 do
    b := b * 1.0000000001;

as you can see we multiply the 1 in b with 1 and a very little bit more, and this little bit more is just over the single precision limit. How the result look like:

Vampire 2.7 1.0000000000000000E+000
Amiga 1200 68882 1.0000100000502183E+000
Linux x86_64 1.0000100000494698E+000

ehm.. no Vampire thats wrong. Seems it does calculate all FPU calculations in single precision, so even they repair the rounding problem MUIMapparium still would be not usable.
Btw. I tried the same thing with the FEmu back in the days and it worked, as expected.
So maybe thats also the reason for the very good benchmark results… you make the calculations in single instead of double of course you can be much faster (and the original 68k FPUs calculate everything in extented so even more precise). I really think about to go back to Gold 2, it was slower but reliable.
As always the downloads: TestFPU3, TestFPU3.pas

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