Well there's a bit of a GRAPHICS RACE going on between NVidia and ATI (now AMD) for quite sometime now. Whenever there was a new launch NVidia did manage to outdo ATI. After a long time ATI have come up with a new and improved warrior code-named the RV770 XT, the GPU that powers the HD4870 based cards. NVidia on the other hand (although earlier than their arch-rival) had launched the GTX280 which is one heck of a card with 240 stream processors. The new ATI cards have 800 stream processors which has caught the attention of many but what's the truth behind ATI's architecture...
The architecture followed by NVidia and ATI are quite different. In the NVidia cards, each programmable Shader can perform five operations namely ADD, SUBTRACT, MULTIPLY, DIVIDE and MODULUS whereas in the ATI cards each shader can perform only one of the above operations. So, in case of the Radeon cards five shaders are incoporated into a unit which can perform all of the above operations but suppose when an ADD instruction is sent to an unit only one of the shaders can perform that job from which it turns out that the rest four shaders are useless for the moment! This is true for any other operation also because as said above each ATI shader can perform specifically one operation and each unit has one ADD shader, one SUBTRACT shader, one MULTIPLY shader, one DIVIDE shader and one MODULUS shader.
Eight-hundred Stream Processors may sound big but the truth behind is hidden... The Effective number of shader in the RV770 is therefore 800/5 = 160 which is by far much lesser than NVidia's 240 Stream Processors on the GTX 280. But ATI does make up with it with GDDR5 memory which has a much larger bandwidth (and also lesser power consumption) than the GDDR3 memory used by the GTX 280 cards. This makes the HD 4870 have an edge over the GTX 280 by a slight margin for now. But it has been a boon for us since will experience greater price drops in lesser intervals!
For now the HD4870 is the winner but I sincerely hope that NVidia will come up with some monster from hell within the next 6 months. ATI's legacy may be short-lived...
ATI HD4850 Vs. NVidia GTX280
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