The first high performance floating point unit to support both IBM 360 hexadecimal based floating point architecture and the IEEE 754 Standard binary floating point architecture is described. The S/390 G5 floating point unit supports the new S/390 architecture which includes hexadecimal based short, long, and extended precision formats and IEEE 754 standard single, double, and quad formats. This floating point unit is part of the microprocessor chip on the S/390 G5 mainframe computer introduced in 1998 and generally available at 500 MHz speeds. The S/390 G5 represents the current state of the art in CISC processor design. The paper describes the S/390 architecture enhancements, the internal format of the FPU, and the modifications to the FPU dataflow.
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