Instruction translation for an experimental S/390 processor

The IBM™ S/390™ architecture is a complex architecture, which has grown over a long period of time. Typical implementations use microcode to cope with the more complex instructions and facilities of S/390. Current IBM S/390 processors even contain two levels of microcode.We report on an experimental S/390 processor based on a RISC processor kernel employing superscalar, out of order execution of instructions. S/390 instructions have to be translated into internal sequences of RISC instructions. Actually two closely coupled internal sequences - one for register based execution and one for storage based execution are generated. The translation is a straight-forward mapping in most cases with some flexibility for special instructions.The paper introduces the hardware mechanisms used for mapping S/390 instructions to internal sequences. The facilities, which provide a greater degree of flexibility are discussed. The interactions of the low-level mapping scheme with the microcode levels is examined. Finally we discuss our experiences with this type of implementation of a CISC architecture on a RISC processor kernel.