Crashman is right. They both suck. That's one of the reasons I came to this board in the first place. I was trying to figure out what the hell was going on. I was tripping over the P4's 400Mhz FSB. Thought I had died and gone to heaven...
<font color=red>This is a forum, not a chat room. You aren't going to find a date here.</font color=red>
March 14, 2001 11:30:56 AM
Correct me if I'm wrong but,
AMD has been saying 200mhz fsb for years. Duel Pumped mobo and CPU architecture with 100mhz SDRam = 200mhz fsb. Athlon B and Duron run at 100mhz external Clock speed.
So they came out with the C version of the Tbird cpu which supports 133mhz external clock speed also duel pumped, equaling 266mhz fsb with 133mhz ram. Tricky ain't they.
Now the RDRAM driven Intel P4 is using 100mhz RDRAM with quad pumped CPU and mobo architecture yeilding 400mhz fsb.
even trickier huh!
No we come to the real fly in the ointment, DDR Ram. Or better yet the sugar in your lemonaide someday when DDR2 arrives.
DDR ram can be addressed by the CPU 2 times in one clock cycle, while the SDRAM flavor can only be addressed once per cpu clock cycle.
This in theory gives you ddr ram that is 2 time faster at communicating with your cpu then good old SDRAM hence 1/2 the latency of SDRam or 2 times the band width of Sdram depending on your out look.
This would in a sense, equal 266mhz fsb as the Tbird C version is running at an external clock speed of 133 mhz and able to address the DDR ram 2 times per clock cycle equaling 266mhz fsb.
Well Thats the theory anyway
Have any of the new DDR motherboards realised DDR's full pontential?
Not that I have heard of, seems there are still some latency issues.
Is a DDR mobo running PC2100(133mhz DDR) that much faster than a kt133a running 133mhz SDram . Both with tbirds c versions running at 133mhz External clock speed?
Yes the DDR setup is faster then the SDram setup but not buy very much.
All in All the best bang for the buck right now is the kt133a coupled with a C version Tbird in my opinion.
Now if PC2100 DDR ram mobo's realy gave ya 2 times the performance of a kt133a 133mhz sdram mobo's it would be awsome but sad to say its just not there yet. We can all hope DDR2 Ram will become mainstream sooner rather than later, then you may see DDR ram mobo's really shine!!!
Hope i have confused you as much as i confused myself
This is what I'm talking about. The board is currently being sold as a 266 fsb product, but AMD has removed it from their recommended list! Think about how many shady PriceWatch companies are gonna have to deal with unsatisfied customers...
March 14, 2001 8:52:25 PM
How can windows 98 installation program says
More powerful and stable than windows 95, if it can't run correctly on any computer ?
March 17, 2001 11:53:45 PM
I think I figured all this out last week. Here is how I summarized it for a colleague. How close did I come to your view of the way things really are:
Several months ago (November?) - way back in my PC knowledge infancy - I read that AMD was not going to be able to take advantage of DDR because the memory bandwidth was the same as the processor bandwidth. I didn't understand his gist and asked about it as well as I could phrase it at that time. I got an answer like 'those numbers are just peak burst rates -
they don't mean that much - it doesn't matter'. Now I can say this would be a pretty good answer to a question about ATA 100 vs ATA33, but I now see what the guy was saying and it is borne out by all the testing/reviews that show DDR does not deliver on it's promise (2x SDRAM), but Rambus does deliver when code is optimized to take advantage of the P4. It's amazing how much you can learn in a few months - actually just 6-10 articles if you don't count the ones that don't help.
We were touching on the topic today with the question of abandoning the C chip when you go with SDR instead of DDR. It's easier to see if you convert the frequency numbers into time, see below. [I am running with your statement that the interface between the North bridge and the CPU actually runs at 266 MHz - 'something' in the system has to run at 266 to keep up with DDR]
Bottom line, with 133 MHz DDR, you get 2100 Bps and that's the capacity of the FSB. I assume the FSB is isolated from the processor internal to the CPU chip, so there are still processor cycles while the FSB is bogged down. If not, then the CPU has to stop to access memory. So if the memory runs wide open, the PCI & AGP get nothing.
It's kind of analogous to the LAN hub vs switch situation: 200 Mbps at each port of a switch instead of 100/n Mbps at each hub port.
Today's northbridge is a hub. If it was a switch, it could/would run the PCI, AGP & DDR buses concurrently, and then the system would fully benefit from the RAM speed.
Since it isn't/doesn't, then there will be intervals where the process waits for somebody to get his 2-bits in. This is why we only see 10-20% improvements with DDR over SDR (216 vs 236 [10%], in one of Tom's reviews). The processor can't USE the available data rate. In P4 systems with the "400 MHz" bus, even with dual banks of RDRAM, there is FSB bandwidth available to the system - or else the point of tie up is 2x (400 vs 266) farther down the frequency pipe, so performance is better.
What does AMD need to capitalize on DDR:
RAM 8 Bytes @ 266 MHz or 2100 Bytes per microsecond (double data rate)
PCI 4 Bytes @ 66 MHz or 264 Bytes per microsecond (half word width)
AGP 8 Bytes @ 264 MHz or 2112 Bytes per microsecond (quad-pumped for 4xAGP)
total 4476 Bytes per microsecond for 4x AGP
3420 Bytes per microsecond for 2x AGP
2892 Bytes per microsecond for 1x AGP
(More than these, if I left out some significant resource on the FSB.)
If this 'new AMD chip' sticks with 64' width, 4476/8= 560 MHz - close to P4's 400 MHz!!!
So what is P4 actually doing:
400*8 = 3200 MBps. Take away 2100 for RAM and 264 for PCI, then Intel has 836 bytes per microsecond for AGP & everybody else.
This is what the unknown guy I first read was trying to say. AMD will have to have a new processor to take advantage of DDR. It will need a 4000-5000 MBps FSB. [that's the sentence he left out of his article] Is this beast in AMD's hammer line? I haven't the foggiest. But at least, now, I know where we are and why.
Tom wonders why P4 excels at Quake 3 (and SSE- II 'optimized' MPEG4 & nothing else). I'll bet it's because that program has some tight loops, where the game runs from the cache and memory is only accessed for data. That's what the 'streamlined' P4 is designed to do. Intel said that the Flask MPEG4 conversion could be optimized more if it was rewritten with the P4 in mind. I'll bet it could: say limit the processing to loops that fit in the cache and process a block of data. Then load another loop and process the same data again. Do it again ... until that data is complete. Then repeat the process for another block of data. That way you might load actual processing code into the CPU once! per data block instead of once per pixel. Even if there was 50-100% code overhead in breaking the process into such small jobs, there might still be orders of magnitude improvement in the overall, hours long job.
If this last thought is true, imagine Intel's frustration at having such a powerhouse on the market and no field where it can run....
The same thing could be done with the AMD processor as the target, but since Athlon's cache is undecoded instructions and P4's is fully decoded register control bits, P4 would still be faster, even if the loops were the same, since there would be no latency for instruction decode.