The following is edited from (and translated from) an E-mail I sent to my sister. My sister claimed that GameCube was said to be an 128-bit system, while I was thinking that since it's a PowerPC system, it can't possibly do more than 32 bits - and who cares if it can't do more than that, anyway, I've been a happy user of 32-bit computers for quite a time now, all of them have been quite good for their time. Note that I'm not a hardware guy and last time programmed in Assembler for C64, so this might not be 100% accurate either.)
After some research (consulting Nintendo's specs and the Wikipedia), The GameCube's PowerPC "Gecko" processor is indeed a 128-bit one, though this isn't for all kinds of oprations.
How many bits a processor has is, these days, not very relevant. The most important factor when considering the processor speeds nowadays is the clockspeed measured in megahertz, though even that isn't comparable between two different kinds of architectures. In other words, a 1GHz Intel Pentium4 isn't as fast as 1GHz PowerPC or even a 1GHz AMD processor, even when P4 and the AMD are mostly compatible with each other.
"Bitness" measures one thing in the processor: How much data the processor can handle at a time? Specifically, how much data the processor can handle with a single programming instruction?
The problem is that modern processors don't process all information in equal sized chunks. The larger chunks you process, the slower it is, so different kinds of hardware should only be processed at the precision that is absolutely necessary.
The GameCube processor can handle
- 32-bit integer operations,
- 64-bit decimal operations,
- 128-bit SIMD operations, that is, processing a large dataset in parallel, or some other voodoo stuff, please don't ask me for details.
So, GameCube is 128-bit, but only for certain types of operations. This is, however, better than the competition (see below).
Most part of the code will use (the 32-bit) integer mathematics anyway, since it's the fastest to do. But let's not forget that the processor can do 128-bit operations. Therefore, it's justified to call the console 128-bit.
On the other hand, we can just say that Nintendo 64 is 64-bit but only for certain operations. Playstation 2 is 128-bit if you do vector operations - but not otherwise. Atari Jaguar isn't exactly 64 bit since it can do 32-bit stuff in two different processors. Well, with that logic, PS2 isn't 128-bit either, because vector processor unit is separate. And XBox has an almost ordinary 32-bit Intel processor, which, due to cost, has been made less powerful than an equivalent processor found in PC component stores.
Also, what's noticeable: Nobody needs more precise operations than this. 32 bit integers will take you far, and the program can, if necessary, simulate a larger number range with programmatic tricks (hell, people did 64-bit calculations on ye olde Commodore 64, which clearly couldn't do more than 8 bits itself! And that processor didn't have any damned decimal operations either!)
So, let's just let the people who wait for the era of "256-bit" consoles. The technical progress is breathtaking, but the ambition is restrained by practicality - 64 bit integer operations (and 128-bit floating point operations, once the <acronym title="Institute of Electrical and Electronics Engineers">IEEE invents and standardizes them) might appear on the popularily used processors in near future, but no one in their right mind is going to make a processor that handles 256-bit integer operations! It is more likely that a new data processing technique using 256 bits at a time (similar to the SIMD mentioned above) is invented, added to the processors, thus justifying to call them 256-bit processors.
Not to even mention the size of the address bus - in other words, it might be pretty cool to use a computer that can have 1.15*1053 yottabytes of memory - nice how we've run out of SI prefixes for powers of ten long ago - but no game can require that kind of memory space. Well, I don't think there's even enough atoms in this universe to build that kind of memory...
So, what do we learn from this? The marketing people are always pulling the engineers to the dirt by telling about things that aren't exactly true - because explaining the whole truth might take some time and the "masses" wouldn't understand them anyway. Someone ought to tell the amrketing people that it doesn't matter, they should rather make sure the games are better than the competition's games =)