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Topic Title: What is Better?
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Created On: 08/13/2004 12:13 PM
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 08/13/2004 12:13 PM
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Fynris
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Joined: 08/13/2004

I just had some questions. I just bought a AMD Athlon 64 3000+. I was wondering If the processor is running at 2.00 Ghz and the FSB is running at 1600 Mhz. Does that make it faster than a Pentium 4 3.40 Ghz running at 800 Mhz. I don't understand the whole myth behind. FSB speed is the speed of your computer and the faster the FSB the faster your computer? If someone could clear this up for me, that would be great!
 08/13/2004 01:33 PM
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Greyhound
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On an Athlon 64 platform, there is no 'FSB' in the 'classical' sense anymore.

The 'classical' Frontside Bus is the connection between the CPU and the chipsets' Northbridge which contains the memory-controller and connects to the rest of the system(via the Southbridge).
If you define the FSB as the connection between the CPU and memory-controller, then it does not really exist anymore on an Athlon 64-platform since the memory-controller is integrated into the CPU...in this case you could say the 'FSB' is running at full core-clockspeed.

However, if you define the FSB as the connection between the CPU and the rest of the system(regardless of whether a memory-controller is involved or not), you could say the CPUs' Hypertransport-link replaces the FSB(or it IS the FSB if you like).

The Hypertransport-link is different from a 'classical' FSB - it is not really a Bus, but a fast, packet-based point-to-point interconnect.
The HT-link comprises an upstream-(to CPU) and a downstream-part(from CPU), each 16-bits wide and clocked at 800MHz(Socket 754) using a DDR-technology for an effective 1600MHz clockspeed - this results in a total bandwidth of 6.4GB/s bidirectionally.

The Opteron 2xx- and 8xx-series have additional HT-links for communication between multiple CPUs.

You can find more info about Hypertransport here' ">http://www.hypertransport.org/faqs.html#b


Both the Athlon 64's Hypertransport-link and the current Pentum 4's 800MHz(QDR) FSB have a total bandwidth of 6.4GB/s - 200MHz x4(QDR) x8(Bus width: 64-bits=8 Bytes)) = 6400MB/s for the P4 and 800MHz x2(DDR) x4(HT-link width: 16-bits up- and 16-bits downstream = 32-bit total = 4 Bytes) = 6400MB/s for the A64.

Now where's the difference?(apart from the architectural differences of the connection between the processor and chipset)

On a Pentium 4 platform, the memory-controller is integrated into the chipsets' Northbridge - this means both memory- and I/O-access have to go through the FSB.
Since modern P4-chipsets have dual-channel memory-controllers, this leaves technically no room(bandwidth-wise) for I/O accesses - dual DDR400(PC3200)-channels have a maximum theoretical bandwidth of 6.4GB/s.
Additionally, having the memory-controller integrated into the chipset causes added latencies when accessing memory.

In contrast, an Athlon 64 has an on-die memory-controller - this means the memory-controller is integrated right into the CPU-core and has it's own dedicated path to the memory itself which is completely independent from the HT-link.
This leaves the HT-links entire bandwidth available for I/O.
Also, the on-die memory-controller significantly reduces latencies when accessing memory, which noticably improves efficiency and performance.

Pentium 4 platform:

CPU
I
I>[FSB(6.4GB/s)]
I
Northbridge(also connects to AGP - 2.1GB/s)-------[dual DDR-channels(6.4GB/s)]-----Memory(DDR400/PC3200)
I
I
Southbridge-----Rest of system('I/O'

Athlon 64 platform:

CPU-----[single DDR-channel(Socket 754, 3.2GB/s)]-----Memory(DDR400/PC3200)
I
I>[HT-link(6.4GB/s)
I
Northbridge(also connects to AGP - 2.1GB/s)
I
I
Southbridge-----Rest of system('I/O'

More info about the 'FSB' in general here' ">http://forums.amd.com/index.php?showtopic=11141

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