The R520 (codenamed Fudo) is a graphics processing unit (GPU) developed by ATI Technologies and produced by TSMC.

The Radeon X1800 video cards that included an R520 were released with a delay of several months because ATI engineers discovered a bug within the GPU in a very late stage of development.

With a large number of threads per quad, ATI created a very large processor register array that is capable of multiple concurrent reads and writes, and has a high-bandwidth connection to each shader array, providing the temporary storage necessary to keep the pipelines fed by having work available as much as possible.

There is a fifth, significantly less complex stop that is designed for the PCI Express interface and video input.

This design allows memory accesses to be quicker though lower latency from the smaller distance the signals need to move through the GPU, and by increasing the number of banks per DRAM.

The pixel shader engines are actually quite similar in computational layout to their R420 counterparts, although they were heavily optimized and tweaked to reach high clock speeds on the 90 nm process.

ATI has been working for years on a high-performance shader compiler in their driver for their older hardware, so staying with a similar basic design that is compatible offered obvious cost and time savings.

ATI's development of their "digital superstar", Ruby, continued with a new demo named The Assassin.

It showcased a highly complex environment, with high-dynamic-range lighting (HDR) and dynamic soft shadows.

However, using HDCP requires external ROM to be installed, which were not available for early models of the video cards.

This modular design allows ATI to build a "top to bottom" line-up using identical technology, saving research, development time, and money.

Due to the programming content of available games, the X1600 is greatly hampered by lack of texturing power.

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.

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.

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.

The X1900 and X1950 series fixed several flaws in the X1800 design and added a significant pixel shading performance boost.

[15] In the latter half of 2006, ATI introduced the Radeon X1950 XTX, which is a graphics board using a revised R580 GPU called R580+.

[16] The X1950 Pro was released on October 17, 2006, and was intended to replace the X1900GT in the competitive sub-$200 market segment.

[17] The following table shows features of AMD/ATI's GPUs (see also: List of AMD graphics processing units).

Note that ATI X1000 series cards (e.g. X1900) do not have Vertex Texture Fetch, hence they do not fully comply with the VS 3.0 model.