The UNIVAC Array Processor, or UAP, was produced in even more limited numbers that the IOC. It was a custom-built, stand-alone math coprocessor to the 1108A system. The UAP, at its most basic level, consisted of four 1108A arithmetic units, and associated control circuitry, contained in a standalone cabinet almost identical to the 1108A CPU. The UAP was physically and logically situated between two 1108A multiprocessor systems. It was capable of directly addressing and interfacing to the four 65K core memory cabinets of two independent 1108A systems. It was capable of executing a number of array-processing instructions, the most important being fast Fourier transform (FFT). At a simplified level, one of the 1108A CPUs would move data arrays into core memory, and send the UAP an instruction packet, containing the function to be executed, and the memory address(es) of the data array(s), across a standard I/O channel. The UAP would then perform the operation, totally independent of the CPU(s), and, when the operation was complete, "interrupt" the originating CPU via the I/O channel. A very small number of UAPs were built, with Shell Oil Company being (likely) the only customer. The UAPs were installed in Shell's Houston Data Center, and were used to process seismic data.