mcontext_t提供了有关在系统出问题时可以找到的所有寄存器值的数据；这些寄存器值可以作为信号发送给这个进程。
McOntext_t provides data about all the register values found in the system at the time of the fault; the register values will in turn be delivered to the process as a signal.
因为大多数设备是分开的CPU总线，慢得多的发送数据跨比写信给CPU寄存器或内存(缓存)。
Because most devices are separated from the CPU by a bus, which is much slower to send data across than it is to write to CPU registers or (cached) memory.
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