So a lot of that is overstated, IMO.
The N64 was in most cases the first system with a modern memory hierarchy game developers had come across. From my experiments, rdram is dozens of cycles away from the CPU at least, so it's pretty easy to be memory bound without coming close to saturating the little more than ~200MB/sec memory bandwidth the system is capable of sustaining. Remember too the context of the mid 90s, where the previous Nintendo console had single cycle access to main memory[0]. So you'd see crazy stuff like loop unrolling into 16KB of straight code with no branches despite the CPU only having a 16KB instruction cache, guaranteeing that you just flushed everything else.
Similarly the GPU seemed really hampered by it's small FIFO at basically the ROP stage which meant that the RMW of both color and z buffers meant a lot of pipeline stalls. I think a lot of the benefit of switching to z sort late in the system's cycle had less to do with overall memory bandwidth, but instead the fact that you don't have to wait on memory just to then blit out the pixel. There's no z to check against, so you know that yes, as soon as the pixel hits the ROP stage, the GPU can just write it to memory (assuming alpha isn't involved).
And that's not to shit of the devs in question; it wasn't until probably the last half of the PS2 era that the industry as a whole really internalized how to approach true long tail memory hierarchies. I certainly hadn't despite being able to regurgitate the textbook definitions involved.
I guess what I'm saying: hey demo scene coders, there's a bunch of untapped power in this system with weird hangups. : )
[0] Yes, the memory system of the SNES is hard to talk about in broad strokes like this with FastROM/SlowROM, wait states on cart mem accesses, etc, but 'single cycle' is the right order of magnitude for this discussion.
I’m a little skeptical that there’s “huge” untapped power in this system, given the performance of some later games like World Driver Championship (1999). There’s certainly a lot of processing power across the CPU, RSP, and RDP, but given how heterogeneous the system is, and how many weird hangups there are, I have doubts that we’re going to see something much better come out of the demoscene community. You need a lot of appetite for a long-term project in order to make something impressive on the N64, and while we have better emulators and compilers now, it’s hard to compete against someone in the 1990s who got to spend multiple years on the system full-time, with the support of a team and from the console developers.
There are some tricks I can imagine using, like spending more time with the RDP in single-cycle mode, or rendering just the fields to get 480i at the cost of 240p, but there are just so many thorny problems to deal with.
This is speaking as someone who participates both in the demoscene (I was just in Boston for @party), and N64 homebrew (you can find me on the Discord).
The N64 is a unified memory system, and memory stalls triggered by the CPU will slow RDP down.
The N64's memory controller appears to be very simple. As I understand it, if one sub-component attempts to access a DRAM row that is closed (DRAM rows are 2KB long), then the entire memory controller stalls for ~100ns as the RDRAM chip closes the previous row (writing out any dirty changes) and opens the new row.
The controller doesn't appear to do any reordering to optimise access patterns. If multiple components are accessing the same 1MB bank simultaneously, you can hit pathological bad cases were the entire system slows down, as the memory controller is continually stalling for 100ns at a time while the memory chips close and reopen rows.
Which is why some games can enable a high resolution mode when the memory expansion pack is present. They often don't need the extra 4MB of ram, but simply being able to strategically spread their data across eight different banks instead of four banks can massively improve performance.
Yes, memory access by the CPU will slow the RDP down. But the RDP is plenty slow even when the CPU is idle. The reason that we are seeing such improvements with SM64 is because SM64 was in such bad shape to begin with—something to be expected, given the novelty of 3D hardware in 1996 and problems with compiler bugs.
That said, I don't doubt SM64 was in bad shape, this was really Nintendo's 1st attempt at doing 3D at that scale and almost everything was experimental. But even with that, it is amazing to see just how well a lot of it works despite this. But seeing some of the later stuff released on N64, it did seem like a really tapped out resource. Mind you looking at the Portal 64 coming along, it is neat to see that folks are still trying to push it just a little bit more.
Back to SM64, one thing I still find really cool is when you get Mario on a rotating platform, the rotation position and angle is calculated as it should be for both location and angle. That is just slick to see on something that old.
Another huge problem in rasterization was that pretty much all graphics resources had to be in a 4kb texture cache to be rasterized, and unless you micromanaged that cache really effectively (and it was cut in half if you wanted certain RDP features!) that cache constantly had the wrong data, and this would slow everything down. If they had given it just a bit more cache for textures, it likely would have been much closer to it's claimed peak performance in normal usage.
Your premise is correct but your conclusion is wrong.
The CPU and the "GPU" (the RSP, Reality Signal Processor, was not actually a GPU, but for the purposes of this discussion it's close enough) shared memory bandwidth. If the CPU was using memory bandwidth, it was stealing memory bandwidth from the GPU, slowing down rasterization.
The thing to make better was to reduce the total amount of bytes the CPU sends to/from main memory. If you could reduce the number of bytes the CPU sends to/from RAM, that gave the GPU more memory bandwidth, and that would improve GPU performance. CPU time was irrelevant.
This changes a lot of what you think about when you think about performance optimization. -Os is substantially faster than -O2, for instance. Loop unrolling is always a loss, even for small fixed size loops. Temporary variables are bad; if you can do a thing by manipulating an existing variable and then de-manipulating it, and that prevents it from spilling onto the stack, that will improve performance. Lots of optimizations a normal programmer and/or the compiler does will make the CPU faster but the extra RAM bandwidth used will slow down the GPU.
> After recompiling with optimizations enabled, and maybe after swapping in newer versions of the RSP microcode, my guess is that further improvements to CPU efficiency would have very limited returns.
He's actually done this, it is not a hypothetical. The original game got 20FPS. (ballpark) He recompiled with optimizations enabled and was getting 30FPS. (ballpark) He performed a bunch of other code optimizations and got this up to 60FPS. So there were enormous returns still on the table.