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I'm taking an Advanced Computer Architecture course and I've come across the term "outstanding" a few times and I've searched online about what it means in the context of computer science/architecture but I can't find anything relevant. For example, on my notes there are some references such as:

"IBM Power5 allows 8 outstanding cache line misses"

"Each thread can perform 8 outstanding memory accesses"

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    $\begingroup$ It's probably used with the non-technical meaning "remaining to be paid, done, or dealt with" (Google word definition). $\endgroup$
    – Yuval Filmus
    Mar 23, 2017 at 18:22
  • $\begingroup$ So does that mean, (in the above case) that it allows 8 memory accesses before it must deal with them? $\endgroup$
    – Nubcake
    Mar 24, 2017 at 6:52
  • $\begingroup$ I have no idea. You haven't provided enough context. But it's likely that this interpretation is correct. $\endgroup$
    – Yuval Filmus
    Mar 24, 2017 at 7:55

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Outstanding means, it hasn't been resolved. In the case of cache misses, that means the data request has not been serviced yet. When a cache miss happens, it has to fetch the data from the next level cache/memory, until this data arrives, you need to hold this request somewhere. You've got two options in this situation. Either block the entire cache, until data from the next level arrives. This is the easiest to implement, but quite bad in terms of performance. On the other hand, you could save the information about this data request somewhere, and carry on servicing others. This is is much better in terms of performance. Now, coming to this "place" where you are going to store information about the pending cache miss, this is a hardware structure. So you can only have finite number of "outstanding" misses. Number of entries in this buffer, is what is being provided as the number of outstanding cache misses.

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