was far more impressive than if the chip had made its originally scheduled spring/summer release. Like its predecessor, the X850, the R520 chip carries 4 "quads", which means it has similar texturing capability at the same clock speed as its ancestor and the NVIDIA 6800 series. Unlike the X850, the R520's shader units are vastly improved: they are Shader Model 3 capable, and received some advancements in shader threading that can greatly improve the efficiency of the shader units. Unlike the X1900, the X1800 has 16 pixel shader processors and equal ratio of texturing to pixel shading capability. The chip also increases the vertex shader number from six on the X800 to eight. With the 90 nm low-K fabrication process, these high-transistor chips could still be clocked at very high frequencies, which allows the X1800 series to be competitive with GPUs with more pipelines but lower clock speeds, such as the NVIDIA 7800 and 7900 series that use 24 pipelines. The X1800 was quickly replaced by the X1900 because of its delayed release. The X1900 was not behind schedule, and was always planned as the "spring refresh" chip. However, due to the large quantity of unused X1800 chips, ATI decided to kill one quad of pixel pipelines and sell them off as the X1800GTO. X1900 and X1950 series The X1900 and X1950 series fixed several flaws in the X1800 design and added a significant pixel shading performance boost. The R580 core is pin-compatible with the R520 PCBs, which meant a redesign of the X1800 PCB was not needed. The boards carry either 256 MB or 512 MB of onboard GDDR3 memory depending on the variant. The primary change between the R580 and the R520 is that ATI changed the pixel shader processor-to-texture processor ratio. The X1900 cards have three pixel shaders on each pipeline instead of one, giving a total of 48 pixel shader units. ATI took this step with the expectation that future 3D software will be more pixel shader intensive. In the latter half of 2006, ATI introduced the Radeon X1950 XTX, which is a graphics board using a revised R580 GPU called R580+. R580+ is the same as R580 except it supports GDDR4 memory, a new graphics DRAM technology that offers lower power consumption per clock and offers a significantly higher clock rate ceiling. The X1950 XTX clocks its RAM at 1 GHz (2 GHz DDR), providing