L1 caches, each containing a two-way set associative 32-Kbyte L1 instruction cache and a four-way set associative 32-Kbyte L1 data cache. The write-through data cache did not allocate cache lines on writes. The CPU also contained ROM storing Microsoft private encrypted keys, used to decrypt game data. The heat sink implemented to cool the Xenon CPU was composed of aluminum fins with a copper base, and a heat pipe. Newer revisions, which had a smaller core, do not feature the heat pipe or copper base. The heat sink was cooled by two 70 mm fans at the rear of the console on original-style consoles, while a single fan mounted on the side of the consoles was used in Xbox 360 S consoles. There were several types of fan used in Xbox 360s, which were produced by Nidec, Sunon, and Delta Electronics. CPU data streaming During read streaming into the CPU, a custom prefetch instruction, extended data cache block touch (xDCBT) prefetches data directly to the L1 data cache of the intended core, which skips putting the data in the L2 cache to avoid thrashing the L2 cache. Writes streaming from each core skip the L1 cache, due to its no-write allocation (avoids thrashing of high-bandwidth, transient, write-only data streams on the L1 cache), and goes directly to the L2 cache. The system allows for the GPU to directly read data produced by the CPU without going to main memory. In this specific case of data streaming, called Xbox procedural synthesis (XPS), the CPU is effectively a data decompressor, generating geometry on-the-fly for consumption by the GPU 3D core. Graphics processing unit While the first Xbox's graphics processing unit was produced by Nvidia, the Xbox 360 had a chip designed by ATI called Xenos. The chip was developed under the name "C1" and "R500" was often used to refer to it. The GPU package contains two separate silicon dies, each built on a 90 nm process with a clock speed of 500 MHz; the GPU proper, manufactured by TSMC and a 10 MB eDRAM daughter-die, manufactured by NEC. Thanks to the daughter die, the Xenos can do 4× MSAA, z-buffering, and alpha blending with no appreciable performance penalty on the GPU. The GPU also houses additional capabilities typically separated into a motherboard chipset in PC systems, effectively replacing the northbridge chip. It has a theoretical peak of 240 GFLOPS. Due to