以太网转串口这几句代码完全不懂，求解释，小弟不胜感激

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*/
unsigned char SPI1_ReadWrite(unsigned char writedat)
{
/* Loop while DR register in not emplty */
while(SPI_I2S_GetFlagStatus(SPI1,SPI_I2S_FLAG_TXE) == RESET);
/* Send byte through the SPI1 peripheral */
SPI_I2S_SendData(SPI1, writedat);
/* Wait to receive a byte */
while(SPI_I2S_GetFlagStatus(SPI1, SPI_I2S_FLAG_RXNE) == RESET);
/* Return the byte read from the SPI bus */
return SPI_I2S_ReceiveData(SPI1);
}