Going back a few pages here, but I was doing a bit of reading yesterday, and thought I might reply to this:
What with the OoO execution, short pipeline and large cache, it really looks like the kind of CPU you'd end up with if you wanted the best pathfinding performance possible within very small die size and thermal limits.
...or probably one of the newer ARM cores at this point. Or Intel's mobilized Atom coming from the other end and presumably AMD's low end cores. But I'm guessing other competitive solutions would cost more and not have the benefit of BC (without another added cost) and familiarity, which may have avoided some additional development transition costs.
Actually, the 750 series has a short pipeline even by the standards of short pipeline CPUs. Here's a (partial) set of data on a few of the chips you mentioned (and Xenon for comparison):
Espresso - Wii U
Cores - 3
Multithreaded - No
Clock Speed - 1.25GHz
Pipeline length - 4 stages (+2 for floating point)
L2 Cache - 2MB (Core 1), 512KB (each for Cores 0 & 2)
Out of order execution
Jaguar - AMD (speculated for PS4 and possibly next XBox)
Cores - 2 or 4 (in theory a custom chip could be higher)
Multithreaded - No
Clock speed - up to ~2GHz
Pipeline length - 14 stages (+3 for floating point, not sure about SIMD)
L2 cache - 512KB per core (shared)
Out of order execution
Atom - Intel
Cores - up to 2
Multithreaded - Yes (2 per core)
Clock speed - up to 2.13GHz
Pipeline length - 16 stages
L2 cache - up to 1MB (shared)
In order execution
Cortex-A15 - ARM
Cores - up to 4
Multithreaded - No
Clock speed - up to 2.5GHz
Pipeline length - 15 stages (+up to 10 for floating point/SIMD)
L2 cache - up to 4MB (shared)
Out of order execution
Xenon - XBox 360
Cores - 3
Multithreaded - Yes (2 per core)
Clock speed - 3.2GHz
Pipeline length - 23 stages (+4 to +14 for SIMD)
L2 Cache - 1MB (shared)
In order execution
Corrections from those more knowledgable than me would be appreciated, but hopefully this helps illustrate why Espresso's clock speed is as low as it is. Longer pipelines allow higher clocks, but also mean fewer instructions per clock (IPC) and higher penalties for branch misprediction and code with a large degree of dependencies between close operations (which cause pipeline bubbles). Since the XBox360 was released there's been a trend from the long-pipeline, high-clock architecture of Pentium 4 style chips to shorter-pipeline, high-IPC designs. Even by the standard of short-pipeline designs of today, though, Espresso has an
exceptionally short pipeline. It's actually about as short a pipeline as you could possibly get, and even though it's only running at 1.25GHz, Espresso is probably the highest-clocked CPU with a 4 stage pipeline you're ever likely to see.