DSP instruction for turbo decoding

A structure for performing a MAX star function of a first operand and a second operand comprising:

• a subtractor having a first input receiving said first operand and a second input receiving said second operand for subtracting said second input from said first input and yielding a difference having either a positive or a negative sign result;
• a multiplexer, having a first input receiving said first operand, a second input receiving said second operand, a control input receiving said difference from said subtractor and an output, said multiplexer selecting for output said first input if said difference has a positive sign result and said second input if said difference has a negative sign result;
• a look-up table having an input receiving said difference from said subtractor including both said positive or negative sign result and an output, said look-up table storing at locations corresponding to each possible input an approximation of the function ln(1−e−|D|), where D is said difference of said subtractor, and recalling an entry corresponding to said input;
• and
• an adder having a first input receiving said output of said multiplexer, a second input receiving said output of said look-up table and an output, said adder for summing said output of said multiplexer and said output of said look-up table to produce a MAX Star output.