dsp_gen_fir.

Hi:

I was goint through the C implementation of the dsplib function dsp_gen_fir. I want to modify it for 40 bit accumulation instead of 32 bit.

Below is a snippet from the C code. My first question is regarding the following two instructions in the snippet:

        sum_10 = _packh2(sum1 << 1, sum0 << 1);
        sum_32 = _packh2(sum3 << 1, sum2 << 1);

Why the left shift?

Also, since _hill and _loll return unsigned 32 bit integers, sum0, sum1, sum2, and sum3 are generated by additions of 32 unsigned integers, as follows:

            sum3 += _hill(r3) + _hill(r1);
            sum2 += _loll(r3) + _loll(r1);
            sum1 += _hill(r2) + _hill(r0);
            sum0 += _loll(r2) + _loll(r0);

They should infact be generated as sum of 32 bit signed integers.

Thanks a lot!

Cheers,

Mushtaq

    for (j = 0; j < nr; j += 4) {
        sum0 = 0;
        sum1 = 0;
        sum2 = 0;
        sum3 = 0;

        _nassert((int)x % 8 == 0);
        #pragma MUST_ITERATE(1,,1)
        for (i = 0; i < nh; i += 4) {
            h_3210 = _mem8_const(&h[i]);
            x_3210 = _mem8_const(&x[i + j]);
            x_4321 = _mem8_const(&x[i + j + 1]);
            x_7654 = _mem8_const(&x[i + j + 4]);

            /*-------------------------------------------------------------*/
            /* Use modified taps during the last iteration of the loop.    */
            /*-------------------------------------------------------------*/
            if (i >= nh - 4)
                h_3210 = h_3210_mod;

            h_32 = _hill(h_3210);
            h_10 = _loll(h_3210);

            r3 = _ddotpl2(x_7654, h_32);    // x6h3+x5h2, x5h3+x4h2
            r2 = _ddotph2(x_4321, h_32);    // x4h3+x3h2, x3h3+x2h2
            r1 = _ddotph2(x_4321, h_10);    // x4h1+x3h0, x3h1+x2h0
            r0 = _ddotpl2(x_3210, h_10);    // x2h1+x1h0, x1h1+x0h0

            sum3 += _hill(r3) + _hill(r1);
            sum2 += _loll(r3) + _loll(r1);
            sum1 += _hill(r2) + _hill(r0);
            sum0 += _loll(r2) + _loll(r0);
        }

        sum_10 = _packh2(sum1 << 1, sum0 << 1);
        sum_32 = _packh2(sum3 << 1, sum2 << 1);

        _mem8(&r[j]) = _itoll(sum_32, sum_10);
    }