请教大家：这2句代码是什么意思？

  ///////////////////////////////////////////////////////////////////////////////////
// Reason for ISR
//
typedef enum _PLX9056_ISR_REASON
{
    Plx9056ReasonNone = 0,
    Plx9056ReasonDoorbell = 1,
    Plx9056ReasonPciAbort = 2,
    Plx9056ReasonLocalInput = 4,
    Plx9056ReasonDma0 = 8,
    Plx9056ReasonDma1 = 16,
    Plx9056ReasonOutboundPost = 32

} PLX9056_ISR_REASON;



// There is no |= operator for enum type
//请教大家：这下面2句代码是什么意思？
inline PLX9056_ISR_REASON operator |= (PLX9056_ISR_REASON& a, PLX9056_ISR_REASON b)
{
    return (a = (PLX9056_ISR_REASON)(ULONG(a) | ULONG(b)));
}


代码是DS例子里的，我一直没有看明白。谢谢拉

|= 运算符重载.