The Radeon HD 4850 & 4870: AMD Wins at $199 and $299
by Anand Lal Shimpi & Derek Wilson on June 25, 2008 12:00 AM EST- Posted in
- GPUs
One, er, Hub to Rule them All?
With R500 AMD introduced its first ring bus, a high speed, high bandwidth bus designed to move tons of data between consumers of memory bandwidth and the memory controllers themselves. The R600 GPU saw an updated version of the ring bus, capable of moving 100GB/s of data internally:
On R600 the ring bus consisted of two 512-bit links for true bi-directional operation (data could be sent either way along the bus) and delivered a total of 100GB/s of internal bandwidth. The ring bus was a monster and it was something that AMD was incredibly proud of, however in the quest for better performance per watt, AMD had to rid itself of the ring and replace it with a more conventional switched hub architecture:
With the ring bus data needed to be forwarded from one ring stop to the next and all clients got access to the full bandwidth, regardless of whether or not they needed it. For relatively low bandwidth data (e.g. UVD2 and display controller data), the ring bus was a horrible waste of power.
With the RV770 all that exists is a simple switched hub, which means that sending data to the display controller, PCIe and UVD2 (AMD's video decode engine) traffic are now far less costly from a power standpoint. Another side effect of ditching the ring bus is a reduction in latency since data is sent point to point rather than around a ring. With the move to a hub, AMD increased their internal bus width to 2kbits wide (which is huge). Maximum bandwidth has increased to 192GB/s (in 4870) but this depends on clock speeds.
With nearly double the internal bandwidth and a point to point communication system, latency between memory clients should be decreased, and huge amounts of data can move between parts of the chip. Certainly getting enough data on to the GPU to feed 800 execution units is a major undertaking and AMD needed to make a lot of things wider to accommodate this.
The CrossFire Sideport
Although AMD isn't talking about it now, the CrossFire Sideport is a new feature of the RV770 architecture that isn't in use on the RV770 at all. In future, single-card, multi-GPU solutions (*cough* R700) this interface will be used to communicate between adjacent GPUs - in theory allowing for better scaling with CrossFire. We'll be able to test this shortly as AMD is quickly readying its dual-GPU RV770 card under the R700 codename.
One thing is for sure, anything AMD can do to assist in providing more reliable consistent scaling with CrossFire will go a long way to help them move past some of the road blocks they currently have with respect to competing in the high end space. We're excited to see if this really makes a difference, as currently CrossFire is performed the same way it always has been: by combining the output of the rendered framebuffer of two cards. Adding some sort of real GPU-to-GPU communication might help sort out some of their issues.
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StevoLincolnite - Wednesday, June 25, 2008 - link
Of course it can, There are benchmarks isn't there?Seriously ANY Direct X 9 card can run Crysis, The Quality and Performance is a different matter.
Inkjammer - Wednesday, June 25, 2008 - link
I have a 9800 GX2 in my primary gaming rig, but I've been debating on what card to drop into my Photoshop/3DS Max art rig. I've been waffling over it for some time, and was going to settle on an 8800GT... but after seeing this, my mind's set on the 4850. It definitely appears to offer more than enough power to handle my art apps, and allow me to use my second PC a gaming rig if need be... all without breaking the bank.This'll mark my return to buying ATI hardware since the X800 was king.
weaksideblitz - Wednesday, June 25, 2008 - link
this is a welcome development although im only buying a 4850 :)Locutus465 - Wednesday, June 25, 2008 - link
Very much so, actually from where I sit I think all AMD really needs to do is get a SAM2+ CPU out there that can compete with intel at least similarly to how this card competes with nvida and they'd have one hell of a total platform solution right now. As for upgrading my vid card... I just finished upgrading to the Phenom 4x and Radeon 3870 so I'll be sticking with that for a while. Quite honestly that platform can pretty much run anything out there already as it is, so I'm feeling pretty confident my current setup will last a couple years at least.Lifted - Wednesday, June 25, 2008 - link
Ditto. If I can get a 4850 for ~$150 or so, that's what I'm doing as well.billywigga - Friday, August 29, 2008 - link
where are you getting it from best buy or somethingClauzii - Wednesday, June 25, 2008 - link
That leaves 50 for a better cooler ;)Lifted - Wednesday, June 25, 2008 - link
Is there any reason the first pages of benchmarks have SLI setups included in the charts, but you wait until the end of the article to add the CF? I'd think it would make the most sense to either include both from the start or hold both until the end.Anand Lal Shimpi - Wednesday, June 25, 2008 - link
The original idea was to format it like the 4850 preview, keep things simple early on but offer SLI/CF graphs later in the article for those who wanted them.It looks like in the mad rush to get things done it didn't work out that way, I'll see if it's possible to clean it all up but right now we've got a lot of other minor touchups to do first :)
Take care,
Anand
TechLuster - Wednesday, June 25, 2008 - link
Anand,I really like your idea of "keeping things simple early on" by only including configurations that us mere mortals can afford at first (say, all single-GPU configs plus "reasonable" multi-GPU configs less than ~$400 total), and then including numbers for ultra high-end multi-GPU configs at the end (mainly just for completeness and also for us to drool over--I doubt too many people can afford more than one $650 card!).
Anyway, great job on the review as always. I think you and Derek should get some well-deserved rest now!
-TL