The GameCube CPU

While the Xbox was a PC turned game-console, Nintendo's GameCube was designed as a game console from the ground up. Nintendo's needs for this next-generation console were very clear: the chip would have to be powerful, cheap to manufacture, and run cool enough that it could fit in a very small enclosure. Nintendo ended up contracting IBM to handle the production of the CPUs for the GameCube based on their well-known PowerPC 750CXe processor.

However details on this processor are sketchy at best but the information we've been able to gather points at a relatively unmodified PowerPC 750CXe microprocessor with the addition of close to 40 new instructions (potentially SIMD FP) designed to specifically aid in game performance. Followers of the PowerPC architecture will quickly realize that these additional instructions do not comprise all of the instructions provided by Motorola's AltiVec SIMD instruction set. It is possible that only a subset of AltiVec was implemented into this processor, using instructions heavily geared towards the tasks that it would be handling.

The basics of this PPC 750CXe derivative (codenamed Gekko) are fairly simple; the PowerPC core features a 4-stage basic integer pipeline which is mostly responsible for the very low clock speeds the core is able to achieve. Most important for gaming performance however are more precise floating point calculations and the Gekko's floating point pipeline is 7 stages long. Since the Gekko is a native RISC processor it does not suffer the same fate as its Xbox counterpart in that it doesn't have to spend much time in the fetch/decoding stages of the pipeline. Immediately upon fetching the RISC instructions to be executed, they are dispatched and one clock cycle later, they are ready to be sent to the execution units.

The PowerPC architecture is a 64-bit architecture with a 32-bit subset which in the case of the Gekko processor, is what is used. The CPU supports 32-bit addresses and features two 32-bit Integer ALUs; separate to that is a 64-bit FPU that is capable of working on either 64-bit floats or two 32-bit floats using its thirty two 64-bit FP registers. This abundance of operating registers is mirrored in the 32 General Purpose Registers (GPRs) that the processor has, dwarfing the Xbox's x86-limited offering (8 GPRs).

Although both the Gekko and the Intel CPU used in the Xbox are built upon advanced 0.18-micron processes, the Gekko is held back by its relatively short pipeline limiting it to generally no higher than 500MHz. The Gekko does use Copper interconnects which are superior to their Aluminum counterparts (used in the Xbox CPU for example) in that they more efficiently conduct electricity, but this advantage is still not able to result in a higher clock speed for the CPU. In the case of the GameCube, the CPU is clocked at 485MHz, or 3 times its 162MHz FSB frequency. The benefit of a shorter pipeline is of course, an increased number of instructions that can be processed in those limited number of clocks. However from all of that data that we have seen comparing the PowerPC 750 to even the desktop Intel Celeron processor, it does not seem that the Gekko can compete, performance-wise.

Your experience in the PC hardware world however should have taught you that CPU performance does not matter when it comes to games as long as you are bottlenecked elsewhere in the system, so theoretically Gekko could be more than enough for the GameCube but we have a feeling it's not.

Instead of being a processing powerhouse, Gekko was actually chosen for its physical characteristics. Although it does have a larger on-die L1 & L2 cache than the Xbox CPU (64KB/256KB vs. 32KB/128KB) and is composed of more transistors (over 21 million vs. approximately 9 million for the Xbox CPU), Gekko's die is under 45 mm^2. For comparison, the processor used in the Xbox has a die measuring approximately 100 mm^2.

The Gekko is actually a very cool running CPU, dissipating around 5W at its 485MHz operating frequency. Again, when compared to the Intel CPU used in the Xbox, you're looking at roughly three times more being produced by the X-CPU than by the GameCube's Gekko.

So while isn't as powerful at the Xbox CPU, Gekko's smaller die and cooler operation provide for lower manufacturing costs and a smaller sized console which fit Nintendo's goals perfectly.

Gekko does have more FSB bandwidth at its disposal than the X-CPU, simply because its FSB is running at 162MHz vs. the 133MHz FSB frequency that is within the limits of Intel's AGTL+ spec. This results in a 1.3GB/s connection between Gekko and the North Bridge, which like in the case of the Xbox's nForce-based platform, is integrated into a single chip along with the graphics core.

It's time to meet a close friend of Gekko; we call him Flipper.

Index A glimpse into ATI's future?
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  • cubeguy2k5 - Monday, December 20, 2004 - link

    feel that anandtechs article on xbox vs ps2 vs gamecube didnt go in depth enough, guessed at too many things, and intentionally got others wrong, not sure where to discuss this at, would like to get a thread going.....

    "However details on this processor are sketchy at best but the information we've been able to gather points at a relatively unmodified PowerPC 750CXe microprocessor " - where did they gather this from? gekko isnt a PPC 750CXE or it would be marked as such.

    "The Flipper graphics core is a fairly simple fixed function GPU aided by some very powerful amounts of memory bandwidth, but first onto the architecture of the graphics core. Flipper always operates on 4 pixels at a time using its 4 pixel pipelines; each of those pipelines is capable of applying one texture per pipeline which immediately tips you off that the ArtX design wasn't influenced by ATI at all. Since the Radeon and GeForce2, both ATI and NVIDIA's cores have been able to process a minimum of two textures per pixel in each of their pipelines which came quite in handy since none of today's games are single textured anymore." - who told them that gamecube only has one texture unit per pipeline? it wasnt nintendo, i could just as easily say it has 2, doubling texel bandwidth....... who said it was fixed function?

    "Planet GameCube: In a recent IGNinsider article, Greg Buchner revealed that Flipper can do some unique things because of the ways that the different texture layers can interact. Can you elaborate on this feature? Have you used it? Do you know if the effects it allows are reproducible on other architectures (at decent framerates)?

    Julian Eggebrecht: He was probably referring to the TEV pipeline. Imagine it like an elaborate switchboard that makes the wildest combinations of textures and materials possible. The TEV pipeline combines up to 8 textures in up to 16 stages in one go. Each stage can apply a multitude of functions to the texture - obvious examples of what you do with the TEV stages would be bump-mapping or cel-shading. The TEV pipeline is completely under programmer control, so the more time you spend on writing elaborate shaders for it, the more effects you can achieve. We just used the obvious effects in Rogue Leader with the targeting computer and the volumetric fog variations being the most unusual usage of TEV. In a second generation game we’ll obviously focus on more complicated applications."

    The TEV pipeline is completely under programmer control, so the more time you spend on writing elaborate shaders for it, the more effects you can achieve. COMPLETELY UNDER PROGRAMMER CONTROL MEANS NOT FIXED FUNCTION, and on fixed function GPUs you cannot do advanced shader effects in realtime can you? rogue leader and rebel strike use them EXTENSIVELY.... anandtech.... wheres your explanation?

    ill provide more examples later....



    "Julian Eggebrecht: Maybe without going into too much detail, we don’t think there is anything visually you could do on X-Box (or PS2) which can’t be done on GameCube. I have read theories on the net about Flipper not being able to do cube-mapped environment maps, fur shading, self-shadowing etc... That’s all plain wrong. Rogue does extensive self-shadowing and both cube-maps and fur shading are not anymore complicated to implement on GameCube than on X-Box. You might be doing it differently, but the results are the same. When I said that X-Box and GameCube are on par power-wise I really meant it. " looks like a PROVEN DEVELOPER just proved anandtech is WRONG... nice..... factor5 was involved in the creation of cube, they know it better than ANYONE else, including anandtech....


    come on anandtech, i know you see this article... what about this?

    you clearly state that you believe xbox is ageneration ahead of gamecube technically, when you COULD NOT do any of the shader effects nor the amount of bumpmapping thats in rogue leader even, on a pre GF3 GPU, let alone rebel strike..... what about the water effects in rebel strike, mario sunshine, waverace, i do believe that in 2001, not one game had water even on pc, even CLOSE to waverace in terms of how it looked, and the physics behind it, and in 2002 there wasnt one game close to mario sunshine as far as water goes, wow!..... what about all the nice fully dynamic lighting in RE4, and rebel strike? you couldnt pull that off on a fixed function gpu could you? apparently they cant even pull it off on xbox, when halo2 has massive slowdown, mostly static lighting, an abysmal polygon count, coupled with lod pop in, and various other problems/faked effects.... nice, what about ninja gaiden ? same story, good character models, very bad textures, non existant lighting, shadows that seem to react to non existant lightsources that exist inside of walls..... cute.....

    http://www.geocities.com/cube_guy_2k5/ng3.jpg

    nice textures and lack of lighting... low polycount and invisible lightsources that seem to only allow ryu to cast shadows, not the environment, wow.... what bout the faked reflections used in the game?... neat
    Reply
  • Arkz - Saturday, September 17, 2011 - link

    "the other incorrectly labeled digital AV (it's still an analog signal) for component connections."

    wrong, its purely digital. the component cable has a DAC chip in the connector block. technically they could make a DVI cable for it.
    Reply
  • Arkz - Saturday, September 17, 2011 - link

    and gc cpu is 485 not 500 Reply
  • ogamespec - Thursday, August 08, 2013 - link

    Actually Gekko speed is 486 ( 162 x 3) MHz.

    And Gamecube GPU (Flipper) TEV is fixed stage. No custom shaders.
    Reply

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