HIS HD6990 4GB Graphics Card Review

👤by Sahil Mannick Comments 📅17-06-11

Antilles, as the HD6990 became known, uses two Cayman GPUs on the same PCB and work via on-board CrossFireX. Cayman is the rightful heir to the successful Cypress core. For the most part, the architecture is much the same with some tweaks to increase efficiency and performance per die size. In this case, the HD6990 opts for the fully enabled XT variant rather than the Pro which found itself on the HD6950. The GPUs still make use of the 40nm manufacturing process from TSMC packing 2.64billion transistors in a 384 square mm die size. Ironically, despite AMD shifting to smaller GPU die sizes, Cayman is their largest GPU since the notorious R600.

Cayman has adopted the VLIW4 architecture over the ageing VLIW5 design dating back to when DirectX 9 was first introduced. The main reason was to increase efficiency in newer DX10/11 titles and to put more emphasis on GPGPU computing. The VLIW4 architecture introduces a narrower Stream Processing Unit, in which the 5th stream processor named the “t-unit” has been removed leaving just 4. Despite the Unit only being able to process 4 operations instead of 5, the advantage is that the space saved allows the core to have more SIMD clusters. The end result is that Cayman can have up to 24 SIMD clusters (24 on the HD6970 and 22 on the HD6950) whereas Cypress only had up to 20 SIMDs (20 on the HD5870 and 18 on the HD5850), giving Cayman a 10% improvement per mm squared compared to previous VLIW5 architecture (as used on Barts also). The architecture is now more catered towards GPU Compute than before and the higher SIMD count also gives additional texturing performance. Each SIMD from both architectures contains 4 texture units so Cayman now benefits from up to 96 units rather than the 80 available on Cypress. Another benefit of the new core is its improved dual 8th generation tessellator that bring up to 3 times the tessellation performance of the HD5870. As mentioned, the Cayman GPUs found on the HD6990 boast the full feature set and all 24 SIMD clusters for a total of 1536 stream processors on each.

With the optional AUSUM (read Awesome) feature disabled (more on this later), the cores are clocked at an impressive stock speed of 830MHz and while this is lower than the stock HD6970 single core speed of 880MHz you have to bear in mind that with two of the cores running at an albeit lower clock speed, they will kick out a serious amount of heat. Also running lower than the HD6970 is the memory speed which is set to 5GHz rather than 5.5GHz (effective). Slower speeds however are however more than compensated because of double the processing power and memory. Each GPU is married with its own 2GB framebuffer for a total of 4GB.

Unlike the GPUs on the GTX590 which end up less powerful than even the GTX570, AMD’s HD6990 does not suffer from such a significant downgrade. This means that the individual GPUs are comfortably superior to the HD6950 and in AUSUM mode, matches the HD6970 except for memory bandwidth. At around £500, the HD6990 is priced closely to two HD6970 but it remains to be seen if scaling will be good in-game.

Product Specifications

Product Code: HIS 6990 Fan 4GB GDDR5 PCI-E DVI/4xMini DP
GPU: HD6990
Core Clock: 830MHz
Memory Clock: 5000Mhz
Memory Size: 4096MB
Memory Type: GDDR5
Memory Interface: 256-bit
Interface: PCI Express x16 (PCI Express 2.1)
Max Resolution: 2560x1600 (Single Display)
DVI Output: 1 (Dual-Link DVI-I)
HDMI: None
DisplayPort: 4 mini DPs 1.2 (2 min-DP-to-DVI adapters supplied)
VGA: Supported using DVI-to-VGA adapter
Crossfire Support: Yes
Power Supply Requirement: 750W

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