An explanation of HyperTransport, if you please?

[samgorney101]

I am looking for a comprehensive explanation on how HyperTransport works. The FSB of my A64 is 200mhz. But the FSB is supposed to be 1600mhz, correct. Is it bi-directionally quad pumped like the P4?
Someone please lay out the story here. Thanks

 

[Mik3y]

athlon 64's dont technically have an fsb. it has a 2 way bus system: 800mhz one way, 800Mhz the other way. also, i guess you can call it quad pumped. i'd explain more, but i'm too lazy.

 

[imported_MrT]

OK, so the athlon64 doesn't have an FSB. So why do AT reviews of athlon64 motherboards mention "FSB" in the overclocking sections? (eg Overclocking and Stress Testing: MSI K8N Neo Platinum) Granted that they may be using "FSB" to normalise comparisons between intel and amd chips, but how does this impact on the HyperTransport setting?

Here's a quote from that example review: "HyperTransport could be maintained at the 4X (800) setting up to a 260 to 265 FSB. Above this point to the BIOS maximum of 300FSB, we needed a 3X HT setting"

I gather from this that as the FSB is ramped up, the HyperTransport (or more specifically the multiplier) needs to be ramped down. What is the optimal relationship between FSB and HyperTransport settings?

 

[Yanagi]

FSB = HTT

HTT plus the multiplyer = Hypertransport Bus speed

HTT = 4 x 200 = 800

When they upped the HTT they needed to lower the multipyer in order for it to be stable. Just like oyu need to do with the multiplyer on normal systems..

So in A64s you have 2 multiplyers while other only have one..

Meh. Its hard to explain *sob*

 

[Tiamat]

So, what are the implications of "260fsb, 520DDR, 4x hypertransport (2080mhz ht)
compared to "280fsb, 560DDR, 3x hypertransport (1680mhz ht)

Did the system gain performance because the ram is faster (assume 1:1) or did it lose performance because the hypertransport dropped by 400mhz. Or is it moot? Does hypertransport bandwidth do anything? Is there a real world difference between 1200mhz ht, 1600mhz ht, 2000 mhz ht?

I know almost nothing about how hypertransport works, but if you can compare the dropping of the ht multiplier while increasing the fsb to "dropping the cpu multiplier while increasing the fsb" in the MSI case, arent you losing performance? In my experience, albeit limited, performance only increases when the total mhz stays the same, or increases. In this case, it clearly decreases when the ht multiplier is lowered to "3"...

So what does this all mean?

 

[Nyati13]

In an Athlon 64 there is a Clock Generator circuit. It is not a Bus (as it carries no data across itself) so calling it the FSB or the HT bus or anything else is false.

There are 3 multipliers in an A64.
1. The logic core multiplier (CPU multiplier).
2. The memory controller multiplier (RAM)
3. The HyperTransport multiplier.

I'll use my A64 3200+ 1MB as the example.
Clock Generator = 200MHz
CPU clock = 200MHz X 10 = 2GHz
RAM clock = 200MHz X 1 = 200MHz(DDR) = 400MHz (PC3200)
HTT clock = 200MHz X 4 = 800MHz(unidirectional) = 1600MHz

That's all there is to it. People still use the term FSB because it is just a habit to think that.

Jeremy

 

[klah]

http://support.intel.com/desig...pix/865G_schematic.gif

vs

http://www.viaarena.com/htmlimages/K8T800BD.jpg

--------

http://www.hypertransport.org/faqs.html

http://www.mindshare.com/hyper...nsport_books_main.html

 

[zephyrprime]

Originally posted by: Nyati13

In an Athlon 64 there is a Clock Generator circuit. It is not a Bus (as it carries no data across itself) so calling it the FSB or the HT bus or anything else is false.

The reason it's not correct to call it a bus is because it's a point to point link not because it uses a clock generator. The clock generator is the clock generator but there's gotta be data lines there also. It's perfectly possible to use a clock generator with a bus. A bus is by definition only a bus if multiple devices can connect to that bus ("ride" the bus so to speak).

 

[Nyati13]

If the HT Link was the source of the clock generator then that would be true, but the clock gen is not part of the HT link, it is part of the core.

Jeremy

 

[Gamingphreek]

Originally posted by: Nyati13

In an Athlon 64 there is a Clock Generator circuit. It is not a Bus (as it carries no data across itself) so calling it the FSB or the HT bus or anything else is false.

There are 3 multipliers in an A64.
1. The logic core multiplier (CPU multiplier).
2. The memory controller multiplier (RAM)
3. The HyperTransport multiplier.

I'll use my A64 3200+ 1MB as the example.
Clock Generator = 200MHz
CPU clock = 200MHz X 10 = 2GHz
RAM clock = 200MHz X 1 = 200MHz(DDR) = 400MHz (PC3200)
HTT clock = 200MHz X 4 = 800MHz(unidirectional) = 1600MHz

That's all there is to it. People still use the term FSB because it is just a habit to think that.

Jeremy

Excelent post.. also HT is not only like FSB but it is how fast the CPU can address the other components of the system.

-Kevin

 

[icarus4586]

The way I understood it, the Hypertransport Bus is an interconnect between the CPU and the AGP controller, and/or other CPU's. The FSB is the interconnect between the CPU and the memory. All the system clocks are derived from the Hypertransport clock.