AMD Radeon HD 7970 & HD 7950 - Heading to Southern Islands with Tahiti

By Wong Chung Wee - 13 Feb 2012

AMD Radeon HD 7000 - Heading to the Southern Islands

AMD Radeon HD 7000 - Departure from the Northern Islands

On 9th January this year, AMD officially launched its next generation family of graphics cards, the Radeon HD 7900 series. The successor and flagship offering from this series is the Radeon HD 7970. This graphics card has been touted as the successor and heir to the Radeon HD 6970 who hails from the 'Northern Islands' Radeon HD 6900 series.

The Radeon HD 7000 series of GPUs mark a departure from the architecture of the Northern Island family. AMD has introduced their next generation GPU architecture, Graphics Core Next (GCN). Its latest generation of graphics cards, based on 28nm GPUs built with GCN architecture, will hail from the Southern Island series.

The two major chip manufacturers, AMD and NVIDIA are consistently revising their GPU architectures; hence, the introduction of a new one can be considered a momentous event and AMD is first off the blocks this time round to unveil GCN.

The block diagram of the 'Taihiti' GPU based on AMD's next generation GCN architecture. Take note of the number of GCN Compute Units which can number as many as 32 on a single GPU.


What is GCN?

From the diagram above, we can picture a GPU, built with AMD's GCN architecture, to be made up of basic building blocks called GCN Compute Units.

A block diagram of a GCN Compute Unit (CU). According to AMD, it features four 16-wide SIMD engines which allows it to process four different instruction sets in simultaneously.

Each GCN Compute Unit (CU) can be described as a processor with a scalar and  four vector units (each of which has 16 processing element)  that are built on a non-VLIW (Very Long Instruction Word) instruction-set architecture. According to AMD, the GCN CU is able to execute instructions with dependencies efficiently as its own hardware scheduler is able to assign instructions according to the various available computation units within the CU to avoid dependency bottlenecks. This allows the GPU as a whole to handle more instruction-sets per clock cycle.

In contrast, the Northern Island GPUs have its basic building blocks (VLIW4) based on VLIW instruction-set architecture. In return, these GPUs were not able to process instructions that have dependencies efficiently. This results in the graphics processors using up more clock cycles until the dependency issues were resolved. The VLIW architecture was suited for 3D processing; however, it could not handle the scalar processing requirements of generic computing applications efficiently.

AMD's departure from the Northern Island architecture can be seen as its efforts to introduce GPUs that strike a balance between 3D graphics and general purpose computing. AMD's GCN architecture is expected to handle a wider spectrum of compute workloads as AMD expands its GPU repertoire to target both graphics and professional computing requirements. Another important feature of the GCN architecture is its support for PCIe 3.0 x16 bus interface, making the AMD Radeon HD 7900 graphics cards one of the first PC components launched  that is compliant with this new standard.


Meet the Radeon HD 7900 Series

In keeping with tradition, the AMD Radeon HD 7900 series was launched with two SKUs - the Radeon HD 7970 and Radeon HD 7950. These two entities share the same Tahiti GPU core; however, the latter features less active GCN compute units that number 28. The more premium Radeon HD 7970 GPU sports a full complement of 32 active GCN compute units as supported by the Tahiti core.

Considering each GCN CU has quad 16-processing element vector units, that brings a total of 64 stream processors per GCU block. This means the number of stream processors on the GPU of Radeon HD 7970 is 2,048 while Radeon HD 7950's GPU features 1,792 of such stream processors. To accompany them, the number of texture mapping units (TMU) have been pegged to 128 for the Radeon HD 7970, while the the lesser equipped Radeon HD 7950 sports 112 TMUs. Compared with the Radeon HD 6970 and 6950 respectively, the new Tahiti core based Radeon HD 7900 series are looking favorably better - from a specs perspective.

Other basic differences for the 7970 and the 7950 are GPU and memory clock speeds. The AMD Radeon HD 7970 has its GPU core clock rated at 925MHz (marginally better than the 6970 it superceded) and its memory clock speed is 5500MHz. Its younger brother, the 7950 has its GPU core clock rated at 800MHz and its memory clock speed is lowered to 5000MHz (which sort of match the clock speeds of a default Radeon HD 6950). 
The Radeon HD 7900 series also sport a much wider local memory bus at 384 bits wide - applicable for both the Radeon HD 7970 and the 7950 SKUs. This should bring significant performance gains from the previous 256 bits wide bus. This brings the Radeon HD 7970's total memory bandwidth to 264GB/s while Radeon HD 7950's memory bandwidth is expectedly lower at 240GB/s considering clock speed diifferences.

In the end, these changes between the two Radeon HD 7900 SKUs aren't drastic and are the usual level of differentiation applied for getting out a slightly different SKU (or in other words, maximizing the produced GPU cores that probably can't meet the requirements to pass off as the 7970). For a casual observer looking over the reference card, the AMD Radeon HD 7950 looks almost the same as its elder brother. More on that on the following pages, but here's how the new AMD Radeon HD 7900 series stack up against its closest competitors.

AMD Radeon HD 7900 GPUs and competitive SKUs compared


AMD Radeon HD 7970

AMD Radeon HD 7950 AMD Radeon HD 6970 ATI Radeon HD 6870 NVIDIA GeForce GTX 580 NVIDIA GeForce GTX 570
Core Code  Tahiti XT  Tahiti Pro Cayman XT Barts XT GF110 GF110
Transistor Count  4300 million  4300 million 2640 million 1700 million 3000 million 3000 million
Manufacturing Process 28nm 28nm 40nm 40nm 40nm 40nm
Core Clock 925MHz 800MHz 880MHz 900MHz 772MHz 732MHz
Stream Processors  2048 Stream processing units 1792 Stream processing units 1536 Stream processing units 1120 Stream processing units 512 Stream processing units 480 Stream Processors
Stream Processor Clock  925MHz 880MHz 880MHz 900MHz 1544MHz 1464MHz
Texture Mapping Units (TMU) or Texture Filtering (TF) units  128 112 96 56 64 60
Raster Operator units (ROP)  32 32 32 32 48 40
Memory Clock  5500MHz GDDR5 5000MHz GDDR5 5500MHz GDDR5 4200MHz GDDR5 4000MHz GDDR5 3800MHz GDDR5
DDR Memory Bus  384-bit 384-bit 256-bit 256-bit 384-bit 320-bit
Memory Bandwidth  264GB/s 240GB/s 176GB/s 134.4GB/s 192.4GB/s 152GB/s
PCI Express Interface  PCIe ver 3.0 x16 PCIe ver 3.0 x16 PCIe ver 2.0 x16 PCIe ver 2.0 x16 PCIe ver 2.0 x16 PCIe ver 2.0 x16
Molex Power Connectors  1 x 6-pin, 1 x 8-pin 1 x 6-pin, 1 x 6-pin 1 x 6-pin, 1 x 8-pin 2 x 6-pin 1 x 6-pin, 1 x 8-pin 2 x 6-pin
Multi GPU Technology  CrossFireX CrossFireX CrossFireX CrossFireX SLI SLI
DVI Output Support  1 x Dual-Link 1 x Dual-Link 2 x Dual-Link 1 x Dual-Link, 1 x Single-Link 2 x Dual-Link 2 x Dual-Link
HDMI 1 1 1 1  1 (mini-HDMI) 1 (mini-HDMI) 
DisplayPort 2 (version 1.2 HBR2) 2 (version 1.2 HBR2) 2 (version 1.2) 2 (version 1.2)  None None
HDCP Output Support  Yes Yes Yes Yes Yes Yes
Street Price  Launch Price: US$549 ~US$449 ~US$349 ~US$238 ~US$509 ~US$349
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