Saturation support

 

One important point of MMX is the support of saturated operations. If a operation would cause an overflow, the value stays at the highest or lowest possible value for the data type: If you use byte values you get normally 250+12=6. This is very annoying when doing color manipulations or changing audio samples, when you have to do a word add and check if the value is greater than 255. The solution is saturation: 250+12 gives 255. Saturated operations are supported by the MMX unit. If you want to use them, you have simple turn the switch saturation on: $saturation+

Here is an example:

Program SaturationDemo;
{
  example for saturation, scales data (for example audio)
  with 1.5 with rounding to negative infinity
}

var
   audio1 : tmmxword;

const
   helpdata1 : tmmxword = ($c000,$c000,$c000,$c000);
   helpdata2 : tmmxword = ($8000,$8000,$8000,$8000);

begin
   { audio1 contains four 16 bit audio samples }
{$mmx+}
   { convert it to $8000 is defined as zero, multiply data with 0.75 }
   audio1:=tmmxfixed16(audio1+helpdata2)*tmmxfixed(helpdata1);
{$saturation+}
   { avoid overflows (all values>$7fff becomes $ffff) }
   audio1:=(audio1+helpdata2)-helpdata2;
{$saturation-}
   { now mupltily with 2 and change to integer }
   audio1:=(audio1 shl 1)-helpdata2;
{$mmx-}
end.