Hi
The athlon 64 only have 200 MHz system bus, so will it affect the performance?
The Athlon64 doesn't HAVE a system bus in the classic sense of the
term. Instead it has a separate memory bus and Hypertransport link
for all I/O. The memory bus runs at 200MHz DDR and either 64-bits
wide (Socket 754 chips) or 128-bits wide (Socket 939 or Socket 940
chips). Total theoretical memory bandwidth is either 3.2GB/s or
6.4GB/s.
The Hypertransport link runs at 800MHz DDR or 1000MHz DDR and is
16-bits wide in either direction. The Opteron adds on to this by
having 3 of those Hypertransport links instead of just one. Total I/O
bandwidth is 3.2GB/s in either direction. In total this gives the
Athlon64 a theoretical maximum of 12.8GB/s of aggregate bandwidth.
For comparison, the P4 has only a single bus that is shared between
all memory and I/O requests for the processor. It runs at 200MHz QDR
on more current P4 chips (some older P4's and Celeron D chips run
their bus at 133MHz QDR, and the next generation of new P4's will bump
it up to 266MHz QDR). This bus is 64-bits wide, for a total
theoretical bandwidth of 6.4GB/s.
Perhaps a quick ASCII graphic might illustrate this better. First a
more traditional design like the AthlonXP or P4 (note: you may need to
adjust the type of font used for this to come out properly):
------- ----------- -------
| | System | | Memory | |
| CPU | <------> | Chipset | <--------> | RAM |
| | Bus | | Bus | |
------- ----------- -------
| -------
| I/O | |
\--------> | I/O |
Bus | |
-------
And now, for comparison, we have the Athlon64 design:
------- ------- ----------- ------
| | Memory | | Hypertransport | | I/O | |
| RAM | <--------> | CPU | <--------------> | Chipset | <-----> | I/O |
| | Bus | | | | Bus | |
------- ------- ----------- ------
By I/O I mean all I/O stuff, including AGP, PCI, PCI Express, ATA,
etc. etc. Most chipsets of this style actually have one bus for AGP
(or PCI Express 16x) and another for all other I/O.
As you can see, the layout is rather different. In this way, the
Athlon64 is clearly superior. While it's a somewhat more complex and
expensive design, it improves bandwidth and dramatically reduces
latency. Other than the additional complexity, there are really
little to no downsides to this design (the only one that jumps to mind
is that making a chipset with integrated video is a little bit
trickier).
Will it affect the memory performance too?
The Athlon64, in the Socket 939, has by far the best memory
performance of any x86 chip on the planet. It's real-world bandwidth
is slightly higher than the very latest and greatest P4 chips, and
much more importantly, the latency is 20-30% lower.
It's likely that eventually Intel will be forced to mimic AMD's
design, because right now they are fighting with one arm tied behind
their back due to their inferior bus design. However for the time
being the advantage for AMD is small enough that they don't need to
worry too much.
