True DDR?

bousozoku

Moderator emeritus
Original poster
Jun 25, 2002
13,951
3
Gone but not forgotten.
For all of those complaining about the lack of true DDR on Apple's systems due to a hack, you should take a look at this article on ExtremeTech and the excerpt I'm posting here:



"AMD Breaks 2GHz Barrier"

What hasn't changed is the frontside bus clock, which still runs at 133MHz (266MHz DDR). We had been hoping the new Thoroughbreds would offer faster FSBs, but alas, it's not to be this time around.


The article labeled can be found at:
http://www.extremetech.com/article2/0,3973,477743,00.asp
 

cb911

macrumors 601
Mar 12, 2002
4,119
3
BrisVegas, Australia
Re: True DDR?

Originally posted by bousozoku
For all of those complaining about the lack of true DDR on Apple's systems due to a hack...
huh? who's complaining? and what this about lack of true DDR and a hack? i've never heard of any of this?:confused: what's going on?
 

barkmonster

macrumors 68020
Dec 3, 2001
2,122
12
Lancashire
True DDR is like this :

Athlon XP : 133Mhz FSB x 2 = 266Mhz DDR

RDRAM is like this :

Pentium 4 : 100Mhz FSB x 4 = 400Mhz RDRAM
Pentium 4a : 133Mhz FSB x 4 = 533Mhz RDRAM

The way it works on a mac at present is like this :

G4 : 133/167Mhz FSB - the G4 cpu (PPC7455) doesn't support a DDR FSB

System Controller : 133/167Mhz x 2 = 266/333Mhz DDR

Even though the system controller is DDR, the cpu is still starved, PCs have cpus that support either single channel or dual channel DDR, even though the Front Side Bus is only 133Mhz, the memory controller can move data to and from the cpu at either 2 or 4 times the FSB, hence the 266 - 533Mhz of memory bandwidth.

The reason they're complaining about the 133Mhz DDR on the new Athlon is because a 2Ghz chip with only 266Mhz of memory bandwidth is starved of bandwidth because the RAM speed is less than 1 7th of the clockspeed.

That's why the DDR implementation on the G4s and Xserve has been called a "Hack", the system controller has no benefit to the cpus, especially seeing as both cpus have got to share a 133Mhz bus. The system controller is there for faster I/O performance. Disks, networking and graphics cards benefit from the extra bandwidth, the cpus still only have the same 133Mhz of bandwidth they had on the older 133Mhz SDRAM motherboard used in the quicksilver G4s. The top 2 models have a 167Mhz FSB so they offer more bandwidth than the old motherboards did even though it's only 33Mhz extra instead of the 200Mhz extra you'd assume it had because of the 333Mhz DDR on the motherboard.