如果数据或指令没有出现在高速缓存中，或者如果高速缓存线路无效的时候，CPU通过从主存储器中读数据来更新它的高速缓存。
If the data or instruction is not present in the cache, or if the cache line is invalidated, the CPU updates its cache by reading the data from the main memory.
换句话说，如果当货物抵达欧洲后，出具的证书仍在使用旧的无效的指令，那这批货物就将被扣押或拒收。
In other words when the products arrive to Europe with the Certificate showing the old Directives, the products are seized and rejected because the Directives are NOT the valid ones.
在读操作中，如果发出了这个指令，那么两个时钟周期后，读出数据无效，数据总线进入高阻状态。
During a READ operation, When this command is issued, data outputs are disabled and become high impedance after two clock delay.
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