Need help to understand AMLCodec.cpp

In file xbmc/cores/dvdplayer/DVDCodecs/Video/AMLCodec.cpp in Decode() method we have this code:

Code:
// wait until we get a new frame or 25ms,
  if (m_old_pictcnt == m_cur_pictcnt)
    m_ready_event.WaitMSec(25);

  // we must return VC_BUFFER or VC_PICTURE,
  // default to VC_BUFFER.
  int rtn = VC_BUFFER;
  if (m_old_pictcnt != m_cur_pictcnt)
  {

I cant find any place in code where variable m_cur_pictcnt changed. Can anybody specify where i need to look to understand how it works?