# Velocity Engine 128bit ???????

Discussion in 'Macintosh Computers' started by alexlai5050, Apr 22, 2003.

1. ### alexlai5050 macrumors member

Joined:
Apr 9, 2003
#1
Hi eveyrone
may i ask what is the difference between the 970 procceseur and the G4 i thought the G4 runs at 128 bit
Thats what they say in the apple website
Thanks
Alex

2. ### Remus macrumors member

Joined:
Jan 30, 2002
Location:
Newcastle, Washington (Bill's backyard)
#2
The Velocity Engine is 128 bits. Most of the rest of the G-4 Chip is 32 bit. The 970 Velocity Engine is reported to be the same 128 bit while the rest of the chip is 64 bit. The Velocity Engine is only part of the CPU chip.

3. ### KingArthur macrumors regular

Joined:
Jun 15, 2001
Location:
Marion, Ohio
#3
lol. The "Velocity Engine" (a vector processing unit) isn't actually even true 128-bit. Rather, it is a SIMD (single instruction multiple data) unit that can process up to 128-bits of information each cycle but the largest chunk of data it can process is 32-bits. SIMD means that given 4, 32-bit pieces of data (128-bits in all) that have to have the exact same instruction applied to them, rather than having each one go through the processor in its own turn, the SIMD unit can just apply the same rule to each one. Say that you have co-ordinates for the four corners of a box: {(3,4), (3,8), (5,4), (5,8)}. Now, you move this box along the x-axis and shift it over 4. Your resulting box would now be {(7,4), (7,8), (9,4), (9,8)}. Now, in a traditional non-SIMD fashon, the processor would first have to add (3,4) + (4,0) to get (7,4) and then have to procede to the next data piece (3,8) so on and so forth until it finished it. Now, the "Velocity Engine" just applies the same "+ (4,0)" to evey piece at the same time in one processor cycle rather than having to do each one in turn. This has just saved you 3 cycles of the processor.

The "Velocity Engine", though, can do more than just apply it to four 32-bit operations. It could also scale to do eight 16-bit, sixteen 8-bit, and I think it scales even smaller than that. So, if you were having to do the same thing to sixteen 8-bit pieces of data, by using the "Velocity Engine", you would be saving the processor an extra 15 processor cycles in which it could be doing something else.

The only problem with this is that usually the only time you have SIMD instructions are in graphical manipulations b/c when you shift a box or something like that, you are shifting all the points of that object and all of the points connecting the corners of the object. But we all know that graphical manipulation isn't the only thing the computer is used for, technically, the "Velocity Engine" isn't worth much most of the time. It is better than nothing, though. Intel has the MMX, SSE, and SSE2 instruction sets for its SIMD operations which deal more with streaming media than graphical manipulation.

Note: the example I gave will not be entirely accurate to all of you who know about vector processing, but I am merely trying to put it into easier terms to understand. lol. Also, I may not be on the boards all that often, but I hope the info I provide is useful and worth the breaks in attendance

4. ### alexlai5050 thread starter macrumors member

Joined:
Apr 9, 2003
#4
Cheers i didnt actually understand all dat but hey it still helped me
alex

5. ### KingArthur macrumors regular

Joined:
Jun 15, 2001
Location:
Marion, Ohio
#5
lol. I guess in laymans terms, all I am saying is that the G4 is 32-bit. Even the "velocity engine" is 32-bit. The 970 will be 64-bit with (we hope) a vector processing engine similar to the "velocity engine" in the G4s. It is a bunch of complicated mumbo-jumbo with companies trying to make things sound better than they really are. lol. Just like Intel trying to tell us it is using 800Mhz system bus when all it is doing is making a 8x pumped FSB, not system bus. Remember, don't believe everything you hear to be the truth. Then again, if you did that, you wouldn't be asking us here about it, so you're good in my book.