flash读取ID怎么理解？

//读取芯片ID W25X40的ID:0XEF12
u16 SPI_Flash_ReadID(void)
{
	u16 Temp = 0;	  
	SPI_FLASH_CS=0;				    
	SPIx_ReadWriteByte(0x90);//发送读取ID命令	    
	SPIx_ReadWriteByte(0x00); 	    
	SPIx_ReadWriteByte(0x00); 	    
	SPIx_ReadWriteByte(0x00); 	 			   
	Temp|=SPIx_ReadWriteByte(0xFF)<<8;  
	Temp|=SPIx_ReadWriteByte(0xFF);	 
	SPI_FLASH_CS=1;				    
	return Temp;
}   		    
中间的读写数据都是00是怎么回事