Figure 11-6 Queue Flushing - ARM ARM1176JZF-S Technical Reference Manual

11.3.3 Queue flushing
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When the coprocessor pipeline is flushed, in response to a command from the core, some of the
queues might also require flushing. There are two possible ways of flushing the queue:
• the entire queue is cleared
• the queue is flushed from a selected buffer, along with all data in the queue newer than the
data in the selected buffer.
The method used depends on the point when flushing begins in the coprocessor pipeline. See
Flush operations on page 11-19 for more details. A flush command has associated with it a tag
value that indicates where the queue flushing starts. This is matched with the tag carried by
every instruction.
If the queue is to be flushed from a selected buffer, the buffer is chosen by looking for a matching
tag. When this is found, the flag associated with that buffer is cleared, and every flag newer than
the selected one is also cleared. Figure 11-6 shows queue flushing.
Each buffer in the queue has a tag comparator associated with it. The flush tag is presented to
each comparator, to be compared with the tag belonging to each valid instruction held in the
queue. The flush tag is compared with each tag in the queue. If the flush tag is the same as, or
older than, any tag then that queue entry has its Full flag cleared. This indicates that it is empty.
A less-than-or-equal-to comparison is used to identify tags that are to be flushed. If a tag in the
pipeline later than the queue matches, the Flush all signal is asserted to clear the entire queue.
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Flush all Flush tag
Clear A
<=
Clear B
<=
Clear C
<=
Coprocessor Interface
Tag A A Buffer A
Tag A A Buffer A
Tag A A Buffer A

Figure 11-6 Queue flushing

11-11