your code have been audited?

for (i = c = d = 0; i < 32; ++i) {
if (d < 6) d += 8, c <<= 8, c |= *fld++;
*ptr++ = bits2mime[c >> (d -= 6) & 0x3F];

}

Don't you think it would be better…

for (int i,c,d; i = c = d = 0; i < 32; ++i) {
        if (d < 6) {
                               d += 8;
                               c <<= 8;
                               c |= *fld++;
                                                     }
               d -= 6;
        *ptr++ = bits2mime[(c >> d) & 0x3F];

      }

And many places use assembly language, why not use functions?  This is not a singlechip.