W5100_S0_RX_RD
offset = w5100_read16(priv, W5100_S0_RX_RD(priv));
w5100_write16(priv, W5100_S0_RX_RD(priv), offset + rx_buf_len);
w5100_write16(priv, W5100_S0_RX_RD(priv), offset + 2 + rx_len);