We've repeatedly stated that AMD would offer high-end CrossFire multi-GPU solutions. In fact, AMD has a single-GPU competitor only to oppose NVIDIA GeForce GTX 260, the cheapest card in the series. We're talking about RADEON HD 4870, the fastest single-GPU solution of the HD 4800 series. The company itself mentioned on many occasions that it was going to manufacture single-GPU graphics cards for Mid-End and Lower High-End segments, and offer dual-GPU solutions for the top segment.

First of all, it was planned to oppose GeForce GTX 280 with CrossFire configurations based on two separate HD 4850 or HD 4870 cards. Then the company presented a dual-GPU solution on a single PCB, the RADEON HD 4870 X2. We've already published its preview with several tests. Now it's time for a sterling article with theoretical data, synthetic tests, and an expanded set of games.

RADEON HD 4870 X2 based on two RV770 GPUs, connected with a special bridge, uses CrossFire technology and Alternate Frame Rendering (AFR). AMD just multiplies all technical characteristics by two. We can accept 2.4 teraflops of theoretical performance with some reserve, but, for example, doubling memory capacity is quite disputable.

We repeat, even though it's convenient to design products for various price segments using varying number of the same GPUs, single-GPU solutions of similar complexity will always have a certain advantage: they will be faster in all applications, not only in those optimized for multi-GPU configurations, they won't contain excessive units in each GPU, and they will offer better power consumption and heat release. Without touching upon main problems of multi-GPU rendering in AFR mode -- draw latencies, typical of such configurations. As a result, multi-GPU systems often demonstrate high FPS accompanied by control lags and lack of smoothness.

The theoretical part about the dual-GPU card based on the known architecture cannot be long, it's just two RV770 GPUs installed on a single PCB with memory, bridge, and other elements. This dual-GPU system is based on CrossFire, implemented on the hardware level, PCI Express lanes and the bridge are installed right on the board. There is a little difference from a system based on two HD 4870 cards solely because of some architectural changes to be described below. Basically, performance differences have to do with different video memory volumes.

If you don't know about the AMD RV770 architecture, you can read all details in our baseline review. Even though it's just an improved version of the R6xx architecture, it features a lot of changes, justified hundred-per-cent -- all drawbacks of previous GPUs have been actually removed, so we've got a really powerful solution. Twice as powerful.

Before you read this article, you may want study these baseline theoretical articles, which describe various features of graphics cards and architectural peculiarities of older products from NVIDIA and AMD.

• Longhorn - Graphics Cards and Shaders for DirectX 10
• DX.Update - 3D Graphics Accelerators: Half a Step Forward
• DX.Next: The Nearest Future of Hardware Graphics Acceleration

These articles predicted the current situation with GPU architectures, and confirmed many of our assumptions about future solutions. The detailed information about AMD RV7xx and R6xx unified architectures is provided in these articles:

• AMD/ATI DirectX 10 Series: RADEON HD 2900 XT (R600)
• A Detailed Analysis of the ATI RADEON HD 2400XT/2600PRO/2600XT (RV610/630)
• ATI RADEON 3850/3870 (RV670)
• ATI RADEON 4850 512MB
• ATI RADEON HD 4870 512MB

So, let's examine detailed characteristics of dual-GPU RADEON HD 4800 graphics cards, based on the RV770 GPU.

Dual-GPU graphics cards from the RADEON HD 4800 family

• Codename: R700 (two RV770 GPUs)
• Fabrication process: 55 nm
• 2 x 956 million transistors
• Unified architecture with an array of common processors for streaming processing of vertices and pixels, as well as other data
• Hardware support for DirectX 10.1, including new Shader Model 4.1, geometry generation, and stream output
• 2 x 256-bit memory buses, four 64-bit controllers supporting GDDR3 and GDDR5
• Core clock: 750 MHz (for HD 4870 X2)
• 2 x 10 SIMD cores, including 2 x 800 scalar floating-point ALUs (integer and floating-point formats, support for FP32 and FP64 in compliance with IEEE 754)
• 2 x 10 enlarged texture units supporting FP16 and FP32 formats
• 2 x 40 texture address units
• 2 x 160 texture fetch units
• 2 x 40 bilinear filtering units that can filter FP16 textures at full speed, trilinear and anisotropic filtering for all texture formats
• Dynamic branching in pixel and vertex shaders
• 2 x 16 ROPs supporting antialiasing with programmable sample patterns (over 16 samples per pixel), including FP16 or FP32 formats of the frame buffer. Peak performance is up to 32 samples per cycle (including MSAA 2x/4x and FP16 buffers), 128 samples per cycle in Z only mode
• Writing results up to eight frame buffers simultaneously (MRT)
• Integrated support for two RAMDACs, two Dual Link DVIs, HDMI, HDTV, DisplayPort

RADEON HD 4870 X2 Specifications

• Core clock: 750 MHz
• Unified processors: 1600 (2 x 800)
• 2 x 40 texture units, 2 x 16 blending units
• Effective memory frequency: 3600 MHz (4*900 MHz)
• Memory type: GDDR5
• Memory: 2 x 1024 MB
• Memory bandwidth: 2 x 115 GB/sec
• Maximum theoretical fillrate: 2 x 12.0 gigapixel per second
• Theoretical texture sampling rate: 2 x 30.0 gigatexel per second
• CrossFireX connector support
• PCI Express 2.0 x16
• 2 x DVI-I Dual Link, 2560x1600 video output
• TV-Out, HDTV-Out, support for HDCP, HDMI, DisplayPort
• Power consumption: 286 W (8-pin and 6-pin connectors)
• Two-slot design
• Recommended price: $549

RADEON HD 4850 X2 Specifications

• Core clock: 625 MHz
• Unified processors: 1600 (2 x 800)
• 2 x 40 texture units, 2 x 16 blending units
• Effective memory frequency: 2000 MHz (2*1000 MHz)
• Memory type: GDDR3
• Memory: 2 x 1024 MB
• Memory bandwidth: 2 x 64 GB/sec
• Maximum theoretical fillrate: 2 x 10.0 gigapixel per second
• Theoretical texture sampling rate: 2 x 25.0 gigatexel per second
• CrossFireX connector support
• PCI Express 2.0 x16
• 2 x DVI-I Dual Link, 2560x1600 video output
• TV-Out, HDTV-Out, support for HDCP, HDMI, DisplayPort
• Power consumption: 230 W (8-pin and 6-pin connectors)
• Two-slot design
• Recommended price: $399

Using the 55-nm fabrication process for not-very-complex GPUs allows to produce dual-GPU solutions of such power. AMD even didn't have to reduce clock rates as it had happened in the past. Moreover, the company installed more memory on both modifications. By the way, these cards bear expected names -- X2 stands for a double kit of GPUs and video memory since HD 3870 X2. Besides, along with the top RADEON HD 4870 X2 card with two gigabytes of GDDR5 memory, the company plans to launch HD 4850 X2 with two gigabytes of less expensive GDDR3 memory in the nearest future.

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