The DDR Thread: Let's Settle This.

Hemingray

macrumors 68030
Original poster
Jan 9, 2002
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Okay, guys. Being as I honestly don't know that much about the whole DDR bus thing, I (and seemingly many others) would like to get this out in the open and discuss *JUST* this issue:

Is the new PowerMac's System Bus DDR or not?

Present your views on why or why not you think it is/isn't. There have been many people claiming both sides of the coin here.
 

job

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Jan 25, 2002
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taken from xlr8yourmac.com

Clarification on new G4s - they do not have a DDR frontside CPU Bus - I'm getting lots of mails on this but as you can clearly see from the Apple PowerMac G4 specs page - they note the system bus bandwidth at 1.3GB/sec - clearly indicating that is *not* a DDR system/CPU (frontside) bus. Just like the Xserve, the new G4s use a single data rate system/cpu bus and a DDR memory bus. If the frontside CPU/system bus was DDR it would have much higher bandwidth specs noted. (There is no shipping PowerPC I know of with a DDR FSB yet.)

(quote from http://www.apple.com/powermac/specs.html)

"Up to 167MHz system bus supporting over 1.3GBps data throughput

256MB or 512MB of PC2100 or PC2700 DDR SDRAM main memory supporting up to 2.7GBps throughput "

Note the DDR memory bandwidth is 2x the system bus bandwidth - because the latter is SDR vs DDR for the memory bus. (The system/cpu bus is not transferring data on the rising and falling edge of the clock pulse as the DDR memory bus does. The system bus and memory bus are separated by the system controller as shown in Apple's own docs.) I don't know why some people refuse to believe this... Ask Apple if you don't believe me or their own system specs. Do the math - bus width in bytes times clock speed and you'll see it's SDR on the system bus. Hopefully next year there will be DDR frontside bus G4 (or G5) CPUs, but they're not shipping yet. (Apple would have used them I'm sure if they were available.)

Although the CPU is the primary "data pump" for applications, etc., the extra memory bandwidth should benefit other I/O and devices that use DMA. The system improvements are welcome changes and I only wanted to clarify my comments earlier today that noted no DDR frontside/system bus to prevent any more emails from people that were mistaken about the fact there is no DDR frontside (system/cpu) bus in the new G4s.
There you go....

The controller between the RAM and the CPU may be "true" DDR, but as stated there is no true DDR front side bus...yet...
 

Sun Baked

macrumors G5
May 19, 2002
14,874
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Probably Not - Clearly states it's using the same controller as the XServe, which everybody agrees is not DDR between the controller and CPUs.

From the PowerMac G4 Technology Overview.

Faster system bus. A new system controller, first introduced in the Xserve, integrates all the components of the Power Mac G4 and transfers data among them. The speed of the system bus sets the pace for data transfer speed across the entire system. Power Mac G4 customers can choose a 133MHz system bus or a 167MHz system bus for high-speed performance.

Double Data Rate (DDR) main memory. New Power Mac G4 systems feature Double Data Rate synchronous dynamic random access memory (DDR SDRAM). DDR SDRAM allows the system to read and write data to and from memory on both the rising and falling edge of each clock cycle. It provides twice the throughput of Single Data Rate (SDR) SDRAM, which reads and writes only on the rising edge of the clock cycle. The resulting throughput between main memory and the system controller is 2.7 GBps, more than double the throughput of the previous dual 1GHz Power Mac G4.

DDR SDRAM also increases memory bandwidth to the processors and all other elements of the system. Direct memory access (DMA) allows system elements, such as a hard drive controller or a graphics processing unit, to send and receive data directly from main memory, without going through the processors. The added memory bandwidth allows system elements to function independently at high data rates, boosting total system performance.
 

Hemingray

macrumors 68030
Original poster
Jan 9, 2002
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Well, there you have it. I think Hitman's post from xlr8yourmac.com said it best.

And here's a quote from MOSR:

The new 1.25GHz chips are, unfortunately, just upclocked PowerPC 7455 processors, not the PPC 7470s hoped for by the grapevine. This is a dangerous flaw, because the PPC 7455 cannot take advantage of DDR memory -- as on the Xserve, the new PowerMacs will be limited to 1.3GB/s actual processor bandwidth despite 2.7GB/s memory bandwidth on the two high-end models.
That's what I was afraid of. Sounds like the Yikes incident all over again.
 

topicolo

macrumors 68000
Jun 4, 2002
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Ottawa, ON
Oh Gawd. Not another DDR post. It has been done to death in the other powermac thread. I've explained it 5 times myself. All these new macs are just overclocked XSERVES!!! It's "fake" DDR, and it's a stopgap until the G5s arrive.
 

jefhatfield

Retired
Jul 9, 2000
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Originally posted by topicolo
Oh Gawd. Not another DDR post. It has been done to death in the other powermac thread. I've explained it 5 times myself. All these new macs are just overclocked XSERVES!!! It's "fake" DDR, and it's a stopgap until the G5s arrive.
stopgap is what was expected until g5...but let's see benchmarks:)
 

Hemingray

macrumors 68030
Original poster
Jan 9, 2002
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Ha ha haaa!
Originally posted by topicolo
Oh Gawd. Not another DDR post. It has been done to death in the other powermac thread. I've explained it 5 times myself.
Why do you think I made a new thread about it? There was enough discussion to warrant a spin-off thread, so I made one. And enough people were preaching the wrong info that I wanted this settled once and for all.

Sheesh, it's not like I started another PDA thread. :p
 

jefhatfield

Retired
Jul 9, 2000
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Originally posted by Hemingray



Sheesh, it's not like I started another PDA thread. :p
i think the pda has this yuppie image where i live and many have turned their anger from yuppie pager owners to yuppie cell phone users to yuppie pda owners

a pda is just a tool, not a show off item like a bentley or a million dollar house (five million in silicon valley and surrounding areas);)
 

job

macrumors 68040
Jan 25, 2002
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Originally posted by topicolo
a stopgap until the G5s arrive.
And I assume that the G5s will eventually become a stop-gap until the "G6" arrives right. :rolleyes: :D

Just kidding man.

In all seriousness, I would not mind purchasing the baseline tower even with the whole DDR RAM and FSB issue. The base line seems to represent extreme value for performance, especially with a BTO 80GB HD and a Radeon 9000.
 

topicolo

macrumors 68000
Jun 4, 2002
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Ottawa, ON
ahh, well that's something interesting. It's good to see that at least somebody is producing new info and not regurgitating the same old stuff repeatedly