IS, bits [23:0] IMPLEMENTATION DEFINED syndrome information that can be used to provide additional information about the SError interrupt. Only valid if bit[24] of this register is 1. If bit[24] is 0, this field is RES0.

表示IMPLEMENTATION DEFINED(即各硬件厂商根据自己的实现定义)的异常信息，也就是说这部分信息需要具体的硬件厂商来翻译，或者需要根据具体的芯片手册来。我的环境中暂时无法确认。