[X360 bandwidth] where does this come from?

[patroclus02]

Hello you all

It is well known that CPU-GPU bandwidth is 2x10,8Gb/s
But, does anyone know how many bits the bus is? or what frequency does it operate at?

Also, it is well known that its GDDR3 bandwidth is 22,4Gb/s.
But, as far as I know, the bus is 128 bits, and operating at 700MHz it would deliver 11,2Gb/s...
Maybe the 700Mhz value is related to the internal clock? so it is an effective 2x700 = 1400MHz??
for example, I own a Geforce 6800 GT. It has GDDR 1000MHz, and here, the bandwidth is calculated as

256bits / 8 *1000MHz = 32Gb/s

where am I getting wrong??

thank you!

 

[see colon]

a nice chart explaining the bandwidth between cpu and gpu can be found in b3d's own xenos article.

http://beyond3d.com/articles/xenos/index.php?p=03

i think the 700mhz is the real clock speed of the GDDR3, and since it's double data rate, it gives you double the bandwidth per clock.

 

[TurnDragoZeroV2G]

Yes, the GDDR3's effective clock rate is 1400MHz.

 

[patroclus02]

thanks! of course, I already read that article, but it was not clear to me.

What about the 10,8Gb/s for CPU-GPU ??
If you suppose it is a 64 bit bus you'd get a 1350 operating freq... and I don't know where does it comes from.

 

[Darkon]

thanks! of course, I already read that article, but it was not clear to me.

What about the 10,8Gb/s for CPU-GPU ??
If you suppose it is a 64 bit bus you'd get a 1350 operating freq... and I don't know where does it comes from.

what about it ? that's the bandwidth from GDDR (10,8 GB/s read + 10,8 GB/s write) if that's what your asking

 

[dlm]

Yes, the GDDR3's effective clock rate is 1400MHz.

Whew, that kind of statement is asking for trouble. The clock speed, effective or not, is 700 MHz by the definition of a clock. I'm sure you know the difference, and it's tempting to say these sorts of things, but they're still entirely wrong. The only accurate thing to say is that it transmits two bits per clock.

 

[Acert93]

what about it ? that's the bandwidth from GDDR (10,8 GB/s read + 10,8 GB/s write) if that's what your asking

I believe there is also the XPS line buffer (at Gamerfest they mentioned realworld peak write speeds currently reaching the 3.8GB/s range).

 

[patroclus02]

what about it ? that's the bandwidth from GDDR (10,8 GB/s read + 10,8 GB/s write) if that's what your asking

total GDDR3 bandwidth is 22,4Gb/s.
It does not explain WHY CPU-GPU bandwidth is 10,8Gb/s in each direction.
For these speed, there shpuld be a 64 bit bus running at 1350MHz

64 * 1350 / 8 = 10800 Mb/s

But I'm just gessing...

 

[ERP]

total GDDR3 bandwidth is 22,4Gb/s.
It does not explain WHY CPU-GPU bandwidth is 10,8Gb/s in each direction.
For these speed, there shpuld be a 64 bit bus running at 1350MHz

64 * 1350 / 8 = 10800 Mb/s

But I'm just gessing...

It's just the Bus between the CPU and GPU.
Bear in mind that the GPU is the memory controller, so CPU accesses have to go through it.
The GPU can also access the CPU L2 cache directly on this channel.

 

[patroclus02]

yeah, I know that, but that bus must be of some width, 32, 64, 128 bits... and it also must work at a certain freq, isn't it? that's what I'm trying to find...

 

[pegisys]

Whew, that kind of statement is asking for trouble. The clock speed, effective or not, is 700 MHz by the definition of a clock. I'm sure you know the difference, and it's tempting to say these sorts of things, but they're still entirely wrong. The only accurate thing to say is that it transmits two bits per clock.

DDR memory is always labeled by the effective clock, or double the actual speed, and it is not misleading to speak as such, since that is the standard

anyway I think he is thinking about the FSB, IIRC the 360's FSB is around 1300Mhz, so his math might be right

EDIT: yep your math is right

On the CPU side, this interfaces to a 1.35GHz

Link

 

[patroclus02]

thanks! there it is.
A 1350MHz bus, with 2 x 64 bits lanes in the CPU side, and 1 x 128 bits liane in the GPU side.
that delivers 1350 * 128 / 8 = 10800 MB/s