ARM website contains an explanation of a feature called "HUM" (hit-under-missing).
It seems that "under" can be interpreted as "after" or "follow", meaning that the previous access is a miss, and the subsequence access is a hit. Is this understanding correct? And if, I am wondering is there special context for using the word "under" (i.e., use spatial-relationship instead of time-relationship).
>Solution :
The word "under", in this case, possibly means "during", as in "hit during (under the circumstances of) recovering from a previous miss".
The intent is to let the load/store unit continue accessing memory freely while it waits for a linefill to recover from a previous miss. Provided it’s not the "same" memory that you’re linefilling, there’s no need to stall.
But, if it tries to access the same memory that is currently being retrieved by a linefill, it will stall until the linefill is complete.
hit hit miss linefill------------------------> carry-on
hit hit hit hit ..............>
If the entire second line above are hits, they will carry on in parallel. A stall will only happen if a miss occurs. At that point, no further action will take place until the first linefill completes, at which point the stall will "un-stall" and re-issue the memory request.