First off, this project was a death march. What Paul Bettner (formerly Ensemble, now at Playful Corp) publicly said years ago is true: Ensemble Studios was addicted to crunching. I lived, breathed, and slept that codebase. We had demos every 4-8 weeks or something. This time in my life was beyond intense. I totally understand why Microsoft shut us down, because we really needed to be put out of our collective misery.
I was more or less addicted to crunch at Ensemble. I remember working so much, and being so consumed with work on this game, that the muscles in my neck would basically "lock up". Working on all those demo milestones was a 3 year adventure. That team was so amazing, and we all got along so well. I could never do it again like that unless lives depended on it.
Anyhow, the engine/tools team on that project built a low-level, very 360-specific "game OS" in C++ for the simulation team. Why did we build a whole new engine from the ground up? Because the Age3 engine just completely melted down after Billy Khan and I ported it to 360. (That was 4 months of the most painful, mind numbing full-time coding, porting and debugging I've ever done.)
The Age3 360 port ran at ~7 FPS, on a single thread, and took 3-5 minutes to load. After I got the net code working on 360 (no easy task, because Age3 used the Win32 window message-based Winsock API's), we played a few brutally slow multiplayer games on the 360. It was pretty bad.
Of course, we could have spent months trying to optimize and thread this engine to get it above 30Hz. But Billy and I just rolled off Age3, where we spent months working on optimizing and tuning the engine to run well on PC's. I also had a bunch of new 360-specific rendering features I wanted to implement, and doing this in the old PC-centric codebase would have been a nightmare.
The HW1 engine consisted of many global managers, very heavy use of synchronous/asynchronous cross-thread messaging, and lightweight platform-specific wrappers built on top of the Win32 and D3D API's. The renderer, animation, sound, streaming, decompression, networking, and overlapped I/O systems were heavily multithreaded. (Overlapped I/O actually worked properly on Xbox 360's OS.) We used 360-specific D3D9 extensions that allowed us to compose command buffers from multiple threads, and we carefully managed all GPU physical memory ourselves just like a driver would. There are lots of other cool things we did on HW1 that I'll cover here on rainy days.
The original idea for using message passing for most of our parallelism in our next engine was from Bill Jackson, now CCO at Boss Fight Entertainment in Dallas. I implemented it and refined the idea before I really understood how useful it was. It was inspired by message passing and concurrency in Erlang. It worked well and was really fun to use, but was hard to debug. Something like 5,000 intra and inter thread messages were involved in loading a map in the background while Scaleform UI was playing back on its own core. We also had a simple job system, but most of our concurrency was implemented using message passing. (See this article on a similar Message Passing system by Nicholas Vining.)
We tried to follow our expression of the Unix philosophy on this game: Lots of little objects, tools, and services interacting in an ecosystem. Entire "game OS" services were designed to only send/receive and process messages on particular 360 CPU cores.
My manager and I created this powerful, highly abstracted virtual file I/O system with streaming support. The entire game (except the 360 executable) could quickly load over the network using TCP/IP, or off the hard drive or DVD using package files. Hot reloading was supported over the network, so artists could watch their textures, models, animations, terrain, and lights change in real-time. We had the entire company (artists, designers, programmers) using this system.
Something like singletons made no sense for the managers. These services were abstracting away one specific global piece of hardware or global C API, so why bother. I've been told the C-based Halo codebases "followed not strictly the same philosophy, but of the same mind".
This codebase was very advanced for its time. It made the next series of codebases I learned and enhanced feel 5-10 behind the times. I don't talk about it because this entire period of time in my life was so intense.