stm8 read from sd to ring buffer

uint8_t sd_multiread_next(void)
{
    extern uint8_t dac_buf[dac_buf_size];
    extern uint32_t sd_addr;

    uint32_t tmr=0;
    uint8_t res=res_ERROR,rc=0;

  tmr = 30000;
  do {              //Wait for data packet in timeout of 100ms
    rc = sd_txrx_byte(SD_DUMMY_BYTE);
  } while (rc == 0xFF && --tmr);
  if (rc == 0xFE) {
    for (tmr = 0; tmr < SD_BLOCK_SIZE; tmr++) {
        //push data to ring buffer
        //
        dac_buf[tmr]=sd_txrx_byte(SD_DUMMY_BYTE);
        }

    sd_txrx_byte(SD_DUMMY_BYTE);//get 2 bytes of crc into /dev/null
    sd_txrx_byte(SD_DUMMY_BYTE);
  res = res_OK;
  }
if (res==res_OK)
    if (!(CardType & CT_BLOCK)) sd_addr+=SD_BLOCK_SIZE;
    else sd_addr++;
return res;
}