ISSUE: how do I get decode yuv420 data

hi,all,

My env: dm8168 and dvr rdk 4.0  my chains:

:    

I wate to use ipcFrame link to get yuv420 data,then ,save this data to a file(test.yuv).

my issue:

(1) The test.yuv file can not be played.but display is right.

(2) when change some links，use the ipcframeLink after swLink (out data format is yuv422  )  ,so my chains:DEC-SW-IPCFRAME-DIS .the test.yuv file is OK.

My main code:

while (FALSE == ipcFramesCtrl.thrObj.exitFramesInOutThread)

            {

                          status =  Vcap_getFullVideoFrames(&bufList,0);

                          OSA_assert(0 == status);

                          if (bufList.numFrames)

                          {

                                        for(frameId=0;frameId<bufList.numFrames;frameId++)

                                        {

                                                      pBuf = &bufList.frames[frameId];

                                                      //frameSize = (pBuf->frameWidth * pBuf->frameHeight) << 1; //for yuv422

                                                      frameSize = (pBuf->frameWidth * pBuf->frameHeight) *1.5;   //yuv420                                                        

                                                      UInt32 pMemVirtAddr;

                                                      pMemVirtAddr = 0;

                                                      VcapVenc_mMap((UInt32)(pBuf->phyAddr[0][0]), frameSize, &pMemVirtAddr);

                                                      fwrite((Ptr) pMemVirtAddr,1,frameSize,YUV_Fp);                                                                                           

                                                      VcapVenc_unmapMem();

                                        }

                          }

                          status = Vdis_putFullVideoFrames(&bufList);

                          OSA_assert(0 == status);

    }

my question:

         To get  yuv422 data is right , How to get the correct yuv420 data?

Need your help,thank you.