When a computer is first powered up, all of the valid bits in the tag array are set to 0, to record the fact that there is no data in the cache. Whenever a line is brought into the cache, the valid bit in the corresponding entry of the tag array is set to 1, recording tha the line now contains valid data. In general, once a line becomes full, it remains full, because the data that was loaded into it remains in cache until it is replaced with another line of data. The exception to this is when a program deliberately removes a line of data from the cache (most processors provide instructions to do this), in which case the line becomes empty and its valid bit is set to 0.