RS64

The IBM RS64 is a family of microprocessors that were used in the late 1990s in IBM's RS/6000 and AS/400 servers.

These microprocessors implement the "Amazon", or "PowerPC-AS", instruction set architecture (ISA). Amazon is a superset of the PowerPC instruction set, with the addition of special features not in the PowerPC specification, mainly derived from POWER2 and the original AS/400 processor, and has been 64-bit from the start. The processors in this family are optimized for commercial workloads (integer performance, large caches, branches) and do not feature the strong floating point performance of the processors in the IBM POWER microprocessors family, its sibling.

The RS64 family was phased out soon after the introduction of the POWER4, which was developed to unite the RS64 and POWER families.

In 1990 the Amazon project was started to create a common architecture that would host both AIX and OS/400. The AS/400 engineering team at IBM was designing a RISC instruction set to replace the CISC instruction set of the existing AS/400 computers. Their original design was a variant of the existing "IMPI" instruction set, extended to 64 bits and given some RISC instructions to speed up the more computationally intensive commercial applications that were being put on AS/400s. IBM management wanted them to use PowerPC, but they resisted, arguing that the existing 32/64-bit PowerPC instruction set would not enable a viable transition for OS/400 software and that the existing instruction set required extensions for the commercial applications on the AS/400. Eventually, an extension to the PowerPC instruction set, called "Amazon", was developed.

At the same time, the RS/6000 developers were broadly expanding their product line to include systems which spanned from low-end workstations, to mainframe competitor-large enterprise SMP systems, to clustered RS/6000-SP2 supercomputing systems. PowerPC processors developed in the AIM alliance suited the low-end RISC workstation and small server space well. But mainframe and large clustered supercomputing systems required more performance and reliability, availability and serviceability features than processors designed for Apple Power Macs. Multiple processor designs were required to simultaneously meet the requirements of the cost-focused Apple Power Mac, high-performance and RAS RS/6000 systems, and the AS/400 transition to PowerPC.

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