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: $saturation+
Here is an example:
Program SaturationDemo;
{
example for saturation, scales data (for example audio)
with 1.5 with rounding to negative infinity
}
uses mmx;
var
audio1 : tmmxword;
i: smallint;
const
helpdata1 : tmmxword = ($c000,$c000,$c000,$c000);
helpdata2 : tmmxword = ($8000,$8000,$8000,$8000);
begin
{ audio1 contains four 16 bit audio samples }
{$mmx+}
{ convert it to $8000 is defined as zero, multiply data with 0.75 }
audio1:=(audio1+helpdata2)*(helpdata1);
{$saturation+}
{ avoid overflows (all values>$7fff becomes $ffff) }
audio1:=(audio1+helpdata2)-helpdata2;
{$saturation-}
{ now mupltily with 2 and change to integer }
for i:=0 to 3 do
audio1[i] := audio1[i] shl 1;
audio1:=audio1-helpdata2;
{$mmx-}
end.
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