The Blue Gene/Q Compute chip is an 18 core chip. The 64-bit A2 processor cores are 4-way simultaneously multithreaded, and run at 1.6 GHz. Each processor core has a SIMD Quad-vector double precision floating point unit (IBM QPX). 16 Processor cores are used for computing, and a 17th core for operating system assist functions such as interrupts, asynchronous I/O, MPI pacing and RAS. The 18th core is used as a redundant spare, used to increase manufacturing yield. The spared-out core is shut down in functional operation. The processor cores are linked by a crossbar switch to a 32 MB eDRAM L2 cache, operating at half core speed. The L2 cache is multi-versioned, supporting transactional memory and speculative execution, and has hardware support for atomic operations. L2 cache misses are handled by two built-in DDR3 memory controllers running at 1.33 GHz. The chip also integrates logic for chip-to-chip communications in a 5D torus configuration, with 2GB/s chip-to-chip links. The Blue Gene/Q chip is manufactured on IBM's copper SOI process at 45 nm. It delivers a peak performance of 204.8 GFLOPS at 1.6 GHz, drawing about 55 watts. The chip measures 19×19 mm (359.5 mm²) and comprises 1.47 billion transistors. The chip is mounted on a compute card along with 16 GB DDR3 DRAM (i.e., 1 GB for each user processor core).