Code of TMS320F28335 CCS4v120DSP2833x_exampleseqep_pos_speedExample_posspeed.c文件中有这么一段是计算机械角度的：
   pos16bval=(unsigned int)EQep1Regs.QPOSCNT;     // capture position once per QA/QB period
      p->theta_raw = pos16bval+ p->cal_angle;        // raw theta = current pos. + ang. offset from QA

   // The following lines calculate p->theta_mech ~= QPOSCNT/mech_scaler [current cnt/(total cnt in 1 rev.)]
   // where mech_scaler = 4000 cnts/revolution
1     tmp = (long)((long)p->theta_raw*(long)p->mech_scaler);   // Q0*Q26 = Q26
2     tmp &= 0x03FFF000;                                       
3     p->theta_mech = (int)(tmp>>11);           // Q26 -> Q15
4     p->theta_mech &= 0x7FFF;   

1~4为什么要这样进行位操作，请解释一下，多谢！