View Full Version : IS THIS COMP REALLY 128-bit???

10-21-2005, 11:54 PM
The iBook G4, from Apple.
This phrase makes it look like the ibook has a 128 bit CPU:
"Enhancing the performance of the PowerPC G4 is its aptly named Velocity Engine. The Velocity Engine processes data in huge 128-bit chunks, instead of the smaller 32-bit or 64-bit chunks, and can perform complex calculations two to four times faster than run-of-the-mill PCs."

Is that just a trick, trying to make it look like it's got a 128 bit CPU?

10-22-2005, 12:48 AM
That's weird, I thought they had 32 bit CPUs.

10-22-2005, 03:21 AM
That's weird, I thought they had 32 bit CPUs.
Me too!

10-22-2005, 07:31 AM
Im not sure but it sounds like some kind of extension or something... The best way I can describe it is like RAM working in DDR, there no more RAM than there was before but it will be faster becuase of the increased data rate...

Then again, thats some pretty big claims and definatley makes it sound like theres a 128 bit CPU, but I highly doubt it. I think its their "Velocity Engine" thats 128 bit.... Ive never seen a 128bit cpu mentioned, let alone available to buy.

10-22-2005, 08:37 AM
Velocity Engine, VMX or more commonly known as AltiVec, is the vector processing component in PPC processors. And it's natively 128-bit. They never claimed anything else was 128-bit, even though it might seem so.

SSE, the vector processing component in x86-32/IA32 processors is 64-bit by comparison.

There are three things that determines a processor's bit-ness (in lack of a better word):
- The size of the natural native data unit (typically this means the integer).
- The size of the native pointer.
- The size of ops sent to the processor.

The last of those points was an issue on early processors. In general, 32-bit architectures didn't change to 64-bit instructions. The first point is not really important either, the only importance for it is code compability. No, pointer size is the important factor. It determines how large amount of addressable memory the processor can use. 32-bit memory space is right about where modern computers are. 64-bit is WAY larger.

64-bit systems come in a lot of different variaties (these depend on chosen compiler and operative system and not just the processor): LLP64 (long long is 64 bit, as are pointers), LP64 (long is 64 bit, as are pointers) and ILP64 (int is 64 bit, long is 64 bit, and pointers are 64 bit). These factors determine how you can cast pointers forth and back, but since pointers are 64-bit they are all considered 64 bit systems.

The G4 processor is 32-bit (pointer size). But it contains a vector processing unit that is 128-bit (data chunk size). This is probably the best vector processing unit seen in any of the desktop/workstation processor lines, and there are 256-bit ones that it's competing with.

As for 128-bit processors, there are processors with 256-bit instructions, of the VLIW type, but none using over 64-bit address space to my knowledge. There is simply no need for larger address spaces than that today or for plenty of years into the future.

11-12-2005, 01:38 AM
Bits express more than graphics. You could be looking a t a 128 bit hard drive for all we know.

11-14-2005, 07:23 PM
Great explanation liorean. Thanks.

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