Prescott: The Last of the Mohicans? (Pentium 4: from Willamette to Prescott). Part II

Here I would like to omit all the details of our endless search, but I have to admit that the idea to study carefully Intel’s patents turned out very fruitful in the long run. It is there that we found a pretty abstract description of a special system for repeated micro-operations processing aka replay. This system is intended for repeated execution of micro-operations that have already left the scheduler but were executed incorrectly for some reason.

This way, we revealed the details about a mysterious sub-system of the Pentium 4 processor, which has hardly been described in any Pentium 4 documentation, or any articles and reviews devoted to this processor micro-architecture. Well, this is very unexpected windfall! Especially, keeping in mind that we didn’t plan going rally that deep into details at this point.

Of course, we were very curious to find out how this system affects the processor performance. Moreover, it is always a challenge to investigate the features and peculiarities of a subsystem that hasn’t been known to the general public before. We should finally be able to explain the weird behavior of our CPU, anyway.

So, we were really excited about getting this mysterious replay thing. And I have to confess that we managed to achieve some really impressive results. I would like to stress that this article is the world’s first detailed discussion of the replay feature and its functioning peculiarities. Of course, the first thing we supposed that replay will be the key to the revealed deviation of the actual processor performance from the theoretical one.

Later we arrived at the following conclusion: it looks like we found much more than we were actually looking for. This Replay turned out very interesting as a not very well-known feature of the Pentium 4 micro-architecture which clarifies some of its mysteries as well as affects the processor performance.