mainframes which are the direct descendants of the B6500, which was influenced by the B5000, and still have the MCP operating system after 40 years of consistent development. This architecture is now called emode (for emulation mode) since the B6500 architecture has been implemented on machines built from Intel Xeon processors running the x86 instruction set as the native instruction set, with code running on those processors emulating the B5000 instruction set. In those machines, there was also going to be an nmode (native mode), but this was dropped, so you may often hear the B6500 successor machines being referred to as "emode machines". B5000 machines were programmed exclusively in high-level languages; there is no assembler. The B5000 stack architecture inspired Chuck Moore, the designer of the programming language Forth, who encountered the B5500 while at MIT. In Forth - The Early Years, Moore described the influence, noting that Forth's DUP, DROP and SWAP came from the corresponding B5500 instructions (DUPL, DLET, EXCH). B5000 machines with their stack-based architecture and tagged memory also heavily influenced the Soviet Elbrus series of mainframes and supercomputers. The first two generations of the series featured tagged memory and stack-based CPUs that were programmed only in high-level languages. There existed a kind of an assembly language for them, called El-76, but it was more or less a modification of ALGOL 68 and supported structured programming and first-class procedures. Later generations of the series, though, switched away from this architecture to the EPIC-like VLIW CPUs. The Hewlett-Packard designers of the HP 3000 business system had used a B5500 and were greatly impressed by its hardware and software; they aimed to build a 16-bit minicomputer with similar software. Several other HP divisions created similar minicomputer or microprocessor stack machines. Bob Barton's work on reverse Polish notation (RPN) also found its way into HP calculators beginning with the 9100A, and notably the HP-35 and subsequent calculators. The NonStop systems designed by Tandem Computers in the late 1970s and early 1980s were also 16-bit stack machines, influenced by the B5000 indirectly through the HP 3000 connection, as several of the early Tandem engineers were formerly with HP. Around 1990, these systems migrated to MIPS RISC architecture but continued to support execution of stack machine binaries by object code translation or direct emulation. Sometime after 2000, these systems migrated to Itanium architecture and continued to run the legacy stack machine