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Keil usb audio questions

Hi

if studied the keil usb-audio example: and there are two things I didn't understand. Maybe someone of you could give me some hints / explanations to these two code lines

DataIn &= B_S - 1;                    /* Adjust Data In Index */
if (((DataIn - DataOut) & (B_S - 1)) == (B_S/2)) {
     DataRun = 1;                        /* Data Stream running */
}

B_S = 1024 | DataIn shows the received data of the B_S pkt. But what means "adjust datain index? and why is the stream only running if the data pkt count size is B_S/2?

void USB_EndPoint3 (uint32_t event) {
#if USB_DMA
  USB_DMA_DESCRIPTOR DD;

  if (event & USB_EVT_OUT_DMA_EOT) {
    /* End of Transfer */
    if (USB_DMA_BufAdr(0x03) != ((uint32_t)DataBuf + 2*DataIn)) {
      /* Data Available */
      DataIn += P_C*P_S;                    /* Update Data In Index */
      DataIn &= B_S - 1;                    /* Adjust Data In Index */
      if (((DataIn - DataOut) & (B_S - 1)) == (B_S/2)) {
        DataRun = 1;                        /* Data Stream running */
      }
    } else {
      /* No Data */
      DataRun = 0;                          /* Data Stream not running */
      DataOut = DataIn;                     /* Initialize Data Indexes */
    }
  }
  if (event & (USB_EVT_OUT_DMA_EOT) | (USB_EVT_OUT_DMA_NDR)) {
    /* End of Transfer or New Descriptor Request */
    DD.BufAdr  = (uint32_t)DataBuf + 2*DataIn; /* DMA Buffer Address */
    DD.BufLen  = P_C;                       /* DMA Packet Count */
    DD.MaxSize = 0;                         /* Must be 0 for Iso Transfer */
    DD.InfoAdr = (uint32_t)InfoBuf;            /* Packet Info Buffer Address */
    DD.Cfg.Val = 0;                         /* Initial DMA Configuration */
    DD.Cfg.Type.IsoEP = 1;                  /* Iso Endpoint */
    USB_DMA_Setup (0x03, &DD);              /* Setup DMA */
    USB_DMA_Enable(0x03);                   /* Enable DMA */
  }
#else
  event = event;
#endif
}

best regards
Jan