关于LCD横屏的问题

请教原子： LCD_WriteReg(0X4F, 319-Xpos);//没有明白要319-Xpos

void LCD_SetCursor(u16 Xpos, u16 Ypos)
{
#ifdef USE_HORIZONTAL
if(DeviceCode==0X8989)
{
  LCD_WriteReg(0X4E, Ypos);
  LCD_WriteReg(0X4F, 319-Xpos);//没有明白要319-Xpos
    }else
{
  LCD_WriteReg(R32, Ypos);
  LCD_WriteReg(R33, 319-Xpos);
}         
#else
if(DeviceCode==0X8989)
{
  LCD_WriteReg(0X4E, Xpos);
  LCD_WriteReg(0X4F, Ypos);
    }else
{
  LCD_WriteReg(R32, Xpos);   
  LCD_WriteReg(R33, Ypos);
}         
#endif
}