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__usada8 intrinsic

This intrinsic inserts a USADA8 instruction into the instruction stream generated by the compiler. It enables you to perform four unsigned 8-bit subtractions, and add the absolute values of the differences to a 32-bit accumulate operand.

unsigned int __usada8(unsigned int val1, unsigned int val2, unsigned int val3)

Where:

val1

holds the first four 8-bit operands for the subtractions

val2

holds the second four 8-bit operands for the subtractions

val3

holds the accumulation value.

The __usada8 intrinsic returns the sum of the absolute differences of the following bytes, added to the accumulation value:

  • the subtraction of the first byte in the second operand from the first byte in the first operand

  • the subtraction of the second byte in the second operand from the second byte in the first operand

  • the subtraction of the third byte in the second operand from the third byte in the first operand

  • the subtraction of the fourth byte in the second operand from the fourth byte in the first operand.

Example:

unsigned int subtract_add_diff_accumulate(unsigned int val1, unsigned int val2, unsigned int val3)
{
  unsigned int res;

    res = __usada8(val1,val2,val3); /* absdiff1 = val1[7:0] - val2[7:0]
                                       absdiff2 = val1[15:8] - val2[15:8]
                                       absdiff3 = val1[23:16] - val2[23:16]
                                       absdiff4 = val1[31:24] - val2[31:24]
                                       sum = absdiff1 + absdiff2 + absdiff3
                                        + absdiff4
                                       res[31:0] = sum[31:0] + val3[31:0]
                              */
    return res;
}

See also