Radeon HD 4870: What’s Improved?
|GPU||HD 4850||HD 4870||GTX 260||GTX 280|
|GPU frequency||625 MHz||750 MHz||576 MHz||602 MHz|
|ALU frequency||625 MHz||750 MHz||1242 MHz||1296 MHz|
|Memory frequency||1000 MHz||900 MHz||999 MHz||1107 MHz|
|Memory bus width||256 bits||256 bits||448 bits||512 bits|
|Memory quantity||512 MB||512 MB||896 MB||1024 MB|
|Number of ALUs||800||800||192||240|
|Number of texture units||40||40||64||80|
|Number of ROPs||16||16||28||32|
|Shading power||1 TFlops||1.2 TFlops||715 GFlops||933 GFlops|
|Memory bandwidth||64 GB/s||115.2 GB/s||111.9 GB/s||141.7 GB/s|
|Number of transistors||956 million||956 million||1400 million||1400 million|
|Die surface area||260 mm²||260 mm²||576 mm²||576 mm²|
|Shader Model supported||4.1||4.1||4.0||4.0|
The difference between the Radeon HD 4870 and its little brother the 4850 can be summed up in two characteristics: theoretical power, increased by 20% solely via higher clock speeds (the number of ALUs remain unchanged, unlike Nvidia’s approach), and above all memory bandwidth, which has almost doubled (+ 80%). This change is the result, as we’ve seen, of the use of GDDR5, with an equivalent frequency that’s nearly doubled compared to the GDDR3 used on the Radeon HD 4850. This type of memory is expensive, but less so than using a 512-bit or at least a 448-bit bus, which would have been necessary to achieve an equivalent bandwidth using GDDR3 memory as Nvidia has chosen to do — not to mention higher power consumption (memory chips + controller). The result is that the Radeon HD 4870’s bandwidth is similar (in fact 3% higher) to the GeForce GTX 260’s.
By the way, the theoretical superiority of the 4870’s raw processing power over the GTX 260’s is particularly impressive considering that the 4870’s GPU has a die surface area that’s barely 45% of the Nvidia card’s!
On the other hand, we can’t help but mention the obvious limitation that emerges from a reading of these specs – the memory quantity, still limited to 512 MB on the 4870. That’s only slightly over half as much as the GTX 260, and even when you consider that the Radeon cards suffer much less than the GeForce ones when using central memory in cases of frame buffer limitations, we’ll need to keep a close eye on how performance and rankings evolve as resolution increases. Also note that certain manufacturers, such as PowerColor, have already announced 1-GB versions of the Radeon HD 4870, but they won’t be available to do that before the end of July.