The X1900 series doesn’t introduce any significantly new technologies to the table but makes up for it by adding a whole lot of punch. Pixel shading punch. As speculated, ATI has boosted the number of pixel shader processors to 48, which is three times the amount found in the X1800 line. This accounts for the largest change from the X1800 and also for the increase in the amount of transistors. The X1900 has over 380 million transistors, an increase of 20% from the X1800, and draws approximately 150W of power! The maturation of the 90nm process technology allowed ATI to pack more transistors into the same area.

The significance of pixel shader power with respect to performance is still relatively vague in comparison to using the number of pipelines (ROPs) as a traditional indicator. A jump of three times in magnitude of ROPs would’ve meant substantial performance improvement. What does such an increase in pixel shader power mean?

The use of pixel shaders in games has steadily increased over the past few years. ATI research shows that all new titles in 2006 will use pixel shaders in some way. Not only are they used more but they are becoming increasingly more complex, so that pixel shading is fast becoming the main performance bottleneck. This observation easily explains ATI’s design consideration to up the number of pixel shader processors. Old games already run plenty fast on today’s high end cards. Games released recently that use fairly simple pixel shader programs will see a noticeable improvement and future games that use even more PS will see substantial gains.

The X1900 uses the same pixel shader core from the X1800. Each one is capable of performing up to two 3-component vector operations and two scalar operations each clock cycle at full FP32 precision. Each core is equipped with a dedicated Branch Execution Unit that allows flow control to occur seamlessly. Of course ATI’s Ultra-Threaded Pixel Shader Engine has carried over to ensure that each PS Core is always working. (To read more about the Ultra-Thread Pixel Shader Engine check out our X1800 review)

The next improvement again deals with shading but this time on the texturing side. With the X1900 ATI has added Fetch4 capability to the texture units. Fetch4 actually isn’t something new as the X1300 and X1600 have had the capability ever since launch but it hasn’t been widely publicized due to the X1800 lacking the feature. Fetch4 is a new texture sampling method that is useful for shadow map acceleration. Normally texture units sample a colour texture where one colour value consists of four components (red, green, blue and alpha) and each texture unit is only able to sample one value at a time. A shadow map (depth texture) on the other hand is made up of only one component value. Fetch4 takes advantage of this situation and allows a texture unit to sample four values from adjacent address at a time, increasing the texture sampling rate by four. Taking advantage of this feature requires additional programming on the applications part but ATI noted that it is fairly easy to implement. 

Fetch4 Chart

High resolution users will be happy with the final improvement which benefits them considerably. ATI increased the Hierarchical Z on-chip memory by 50% to handle resolutions such as 1920x1200 all the way up to 2560x1600. In prior cases there would be a large performance impact if the memory wasn’t enough to handle it but this has been eliminated.

Everything else about the X1900 is carried over from the X1800 including its advanced 512-bit ring-bus memory controller and industry-leading AVIVO technology. From a paper stand point the X1900 filled in the one weak point the X1800 had which was less pixel shader processors compared to the competition. The X1900 reverses that weakness and makes it one of its strengths.