RV770: AMD ATI Radeon HD 4870 - RV770: The Architecture

By Edward Chester



  • Editors choice
RV770: AMD ATI Radeon HD 4870


Our Score:


Much as nVidia did with GT200, ATI has taken the basics of its previous generation architecture and added a bit more here, taken away a bit there, and generally optimised the whole lot to better suit modern, real world, performance needs. And, just as was the case with GT200, explaining the differences requires delving fairly deep into how the whole chip works. So, let's start from the bottom and work our way up.

The core grunt work of RV770 is performed by its Stream Processing Units (SPU). These are largely similar to the Stream Processors used in nVidia's latest architecture - they each contain three basic Arithmetic Logic Units that can power through simple floating point mathematical calculations like add and multiply. Even here there are differences in exactly how the two manufacturers perform these calculations but they're similar enough to compare. It's as we zoom further out that the two architectures really start to diverge.

The next step up for the RV770 is what I'm calling the Multi Stream Processing Unit (MSPU). This contains four of the basic SPUs along with a fifth, Special Function Unit (SFU) that can do everything the other SPUs can do plus perform transcendental calculations like logarithms and trigonometrics.

Although the MSPU incorporates a number of processing units it is in many ways only the equivalent of a single Stream Processor from nVidia's architecture. This is because, while an MSPU can perform up to five calculations per cycle, all those calculations have to be on the same thread. So unless the thread can be efficiently broken down the whole MSPU might only be as fast as one of nVidia's SPs. Conversely, this is why nVidia runs its shaders at a much higher speed than ATI - to counter those situations where ATI's many processors can perform calculations faster.

ATIs SIMD core

Zooming out another step we see a Single Instruction Multiple Data (SIMD) core that from the outside looks similar to nVidia's Streaming Multiprocessor (SM). Both contain a cluster of processing units, a thread sequencer and a few data stores. However, there are a number of places where the two differ.

nVidias Streaming Multiprocessor (SM)

For a start, ATI includes (four) texture processing units within the SIMD whereas nVidia does texturing further up the ladder. Each texture unit can perform one address lookup and one filtering operation per clock cycle. While this is essentially the same as RV670 (indeed the whole chip is largely identical up to this point) a few hidden improvements have been made. ATI claims performance per mm^2 has increased by 70 per cent through some untold tweaking. Also, additional L1 texture cache memory has also been dedicated to each SIMD making for a significant increase in texture cache bandwidth.

Varis Vitols

July 5, 2008, 2:51 pm

Yeah, it was about time!

Matthew Bunton

July 5, 2008, 5:43 pm

This is the best bang for buck right now, good to see the ATI cards fighting back. Though it is still too early to upgrade in my opinion.


July 6, 2008, 1:18 am

if ati get cuda... i will get this card from sapphire(it have 2 (4 pin to 6 pin conector)(my power suply just have one 6 pin conector) but only if it get cuda, because i like to play games but i also do some video editing and transcoding videos to my zune and it take like 4hrs to covert a dvd to h.264 in my conroe


July 6, 2008, 2:10 am

Your comment on the CoD4 results is incorrect. The HD 4870 beats the GTX 280 comfortably.

It simply fails to be the double card GX2.

Varis Vitols

July 6, 2008, 3:58 am

It depends on what u upgrade from...


July 6, 2008, 6:14 am

Probably not worth upgrading from a 8800GT to a 4850 but the 4870 looks like it could be a possibility. Hopefully ATi will have OpenCL to compete against CUDA but I'm not bothered by the lack of support for it.


July 7, 2008, 3:35 pm

anybody know the minimum power supply rating? looking to install in Dual-Xeon Dell workstation with 700w PSU??


July 7, 2008, 4:06 pm

My X1950XTX is looking a bit long in the tooth now. Getting that itchy upgrade feeling.


July 7, 2008, 9:08 pm


I wouldn't look at it like that, it completely depends on what else you're running in your system. At peak the card will draw 150 - 200W, if you've got that much headroom in your current supply then you'll be fine. Otherwise you may need to upgrade. Easy way to check is with a plug-in power consumption meter. They're only about 㿀.

What card are you upgrading from?


July 8, 2008, 12:18 am

Hmmm, thanks ED. The Dell manual's state GPU's upto 150W(supported) so I may be pushing it if you say it draws upto 200W. Upgrading Dell PSU's I don't think is that easy as your probably aware most components are besopke (got to love Dell).


July 8, 2008, 1:09 pm

Ah, that could be a bit of a problem. Dells are an absolute pig to work with.


July 14, 2008, 5:22 pm

^^ wrt the Dell PSU prob - not sure about the Xeon's, some of the midi-tower cases still have the same footprint as a regular PSU, but just proprietary gaps for the kettle lead... in which case you can buy a normal PSU and get out the metal cutters and drill... I've got "creative" with a few Optiplex's over the years... :o)


July 14, 2008, 5:24 pm

and thanks for the great review!!

As per DMN's comment, that X1950XTX-GDDR4 (me too!) is suddenly looking really long in the tooth :o)))


February 15, 2009, 7:58 pm

i have got to get me one of these...

Joseph Middleton

August 24, 2009, 3:43 pm

This Radeon 4870 by HIS... It's not mentioned how much on board memory it has...

512MB or 1GB?

comments powered by Disqus