Radeon HD 4770
Specifications and Features
The new card has the following standing amidst the ATI Radeon HD 4000 family:
The RV740 is 312 million transistors more complex than the RV730. As a result, it has twice the computing capacity (640 ALUs against 320 ALUs) and twice the rasterization performance (16 RBEs against 8 RBEs). On the other hand, the new chip is 130 million transistors simpler than the RV770. Considering that the graphics cores of the Radeon HD 4830 and Radeon HD 4770 have the same configuration (640 ALUs, 32 TMUs, 16 RBEs), we can calculate the theoretical cost of the two disabled SIMD modules of the former. Easy to see, 160 ALUs (32 superscalar processors) and 8 texture processors take 130 million transistors, and one such module takes 65 million transistors.
As we’ve said above, the RV740 has eight SIMD modules, so the total amount of transistors for the computing and texture resources of the RV740 is 520 million. For the RV770 with its 10 SIMD modules the number is 650 million transistors. Thus, there is 306 million transistors left for each GPU. They are used to implement the rest of the GPU subunits: RBEs with memory controllers, task scheduler, PCIe interface, display controllers, UVD video processor and HDMI audio core.
There is only one trick here: the RV740 has a 128-bit external memory interface. Judging by the flowchart above, the new core has one memory controller with two 32-bit channels per each two RBEs (equivalent to 8 ROPs). The RV770 has four such controllers, so the RV740 is theoretically simpler.
If we get back to our mathematics, the first implementation of ATI’s unified architecture R600 incorporated 720 million transistors and had a 512-bit memory bus. The second generation employed the RV670 core with only half the memory bus width. The RV670 consisted of 666 million transistors, which means a difference of 54 million transistors. We have but rough estimates here because the second-generation Radeon HD differed from the first generation not only with the memory bus width but also with other innovations such as DirectX 10.1 support and the hardware video-processor UVD. Still, we can estimate the four 64-bit memory controllers at 40-45 million transistors. And two such controllers equal 20-22 million transistors.
This is how much simpler the RV740 should be theoretically, but we know its actual complexity. So, we can only make guesses about what the extra millions of transistors do. Do they implement some internal improvements or the new core potentially has a 256-bit memory bus that is kept secret for the time being?
Otherwise, the RV740 and the RV740-based Radeon HD 4770 seems to be a predictable and yet promising product. It is predictable because the new card is in fact a Radeon HD 4830 implemented on a new technological level. And it is promising because the core frequency of 750MHz should allow this card to beat the Radeon HD 4830 and be not much slower than the Radeon HD 4850. We need to check these suppositions in practice, though.
Just like with Radeon HD 4890, the first batch of RV740 based graphics cards will use ATI’s reference design and only a little later the market will welcome unique products with proprietary layouts. Although Advanced Micro Devices graphics division speaks of sub-$100 price point, it is not quite so. The new product MSRP is set at $109, and the $99 price is what you get after rebate. New graphics cards are going to start selling widely right after the official launch.