The POWER1 is a multi-chip CPU developed and fabricated by IBM that implemented the POWER instruction set architecture (ISA). It was originally known as the RISC System/6000 CPU or, when in an abbreviated form, the RS/6000 CPU, before introduction of successors required the original name to be replaced with one that used the same naming scheme (POWERn) as its successors in order to differentiate it from the newer designs. History The POWER1 was introduced in 1990, with the introduction of the IBM RS/6000 POWERserver servers and POWERstation workstations, which featured the POWER1 clocked at 20, 25 or 30 MHz. The POWER1 received two upgrades, one in 1991, with the introduction of the POWER1+ and in 1992, with the introduction of POWER1++. These upgraded versions were clocked higher than the original POWER1, made possible by improved semiconductor processes. The POWER1+ was clocked slightly higher than the original POWER1, at frequencies of 25, 33 and 41 MHz, while the POWER1++ took the microarchitecture to its highest frequencies — 25, 33, 41.6, 45, 50 and 62.5 MHz. In September 1993, the POWER1 and its variants was succeeded by the POWER2 (known briefly as the "RIOS2"), an evolution of the POWER1 microarchitecture. The direct derivatives of the POWER1 are the RISC Single Chip (RSC), feature-reduced single-chip variant for entry-level RS/6000 systems, and the RAD6000, a radiation-hardened variant of the RSC for space applications. An indirect derivative of the POWER1 is the PowerPC 601, a feature-reduced variant of the RSC intended for consumer applications. The POWER1 is notable as it represented a number of firsts for IBM and computing in general. It was IBM's first RISC processor intended for high-end applications (the ROMP was considered a commercial failure and was not used in high-end workstations), it was the first to implement the then new POWER instruction set architecture and it was IBM's first successful RISC processor. For computing firsts, the POWER1 would be known for being the first CPU to implement some form of register renaming and out-of-order execution, a technique that improves the performance of superscalar processors but was previously reserved for mainframes. The POWER1 was also the origin for the highly successful families of POWER, PowerPC and Power ISA processors that followed it, measuring in hundreds of different implementations. The open source GCC compiler removed support for POWER1 (RIOS) and POWER2 (RIOS2) in the 4.5 release. Microarchitecture The POWER1 is a 32-bit two-way