the Model 20) was replaced with the compatible System/370 range in 1970 and Model 20 users were targeted to move to the IBM System/3. (The idea of a major breakthrough with FS technology was dropped in the mid-1970s for cost-effectiveness and continuity reasons.) Later compatible IBM systems include the 4300 family, the 308x family, the 3090, the ES/9000 and 9672 families (System/390 family), and the IBM Z series. Computers that were mostly identical or compatible in terms of the machine code or architecture of the System/360 included Amdahl's 470 family (and its successors), Hitachi mainframes, the UNIVAC 9000 series, Fujitsu as the Facom, the RCA Spectra 70 series, and the English Electric System 4. The System 4 machines were built under license to RCA. RCA sold the Spectra series to what was then UNIVAC, where they became the UNIVAC Series 70. UNIVAC also developed the UNIVAC Series 90 as successors to the 9000 series and Series 70. The Soviet Union produced a System/360 clone named the ES EVM. The IBM 5100 portable computer, introduced in 1975, offered an option to execute the System/360's APL.SV programming language through a hardware emulator. IBM used this approach to avoid the costs and delay of creating a 5100-specific version of APL. Special radiation-hardened and otherwise somewhat modified System/360s, in the form of the System/4 Pi avionics computer, are used in several fighter and bomber jet aircraft. In the complete 32-bit AP-101 version, 4 Pi machines were used as the replicated computing nodes of the fault-tolerant Space Shuttle computer system (in five nodes). The U.S. Federal Aviation Administration operated the IBM 9020, a special cluster of modified System/360s for air traffic control, from 1970 until the 1990s. (Some 9020 software is apparently still used via emulation on newer hardware.) Table of System/360 models Model summary Six of the twenty IBM System/360 models announced either were never shipped or were never released. Fourteen of the twenty IBM System/360 models announced shipped. Technical description Influential features The System/360 introduced a number of industry standards to the marketplace, such as: The 8-bit byte (against financial pressure during development to reduce the byte to 4 or 6 bits), rather than adopting the 7030 concept of accessing bytes of variable size at arbitrary bit addresses. Byte-addressable memory (as opposed to bit-addressable or word-addressable memory) 32-bit words The Bus and Tag I/O channel standardized in FIPS-60 Commercial use of microcoded CPUs