There's some confusion here. The Arthur OS ROMs, starting with 0.2 on EPROM, that's why Acorn wanted them back on the upgrade to 0.3 on ROM, were written in ass

There's some confusion here. The Arthur OS ROMs, starting with 0.2 on EPROM, that's why Acorn wanted them back on the upgrade to 0.3 on ROM, were written in assembler, including `modules' like the filing systems, BASIC, and the Wimp that gave SWIs like Wimp_CreateIcon. More as a demo of WIMP rather than anything useful there was a Desktop program stored in the ROM, written in BASIC, that used the Wimp module to present a crude selection of calculator, etc. Nothing like the later drag-and-drop RISC OS desktop; no other programs could run with the Arthur version. So there was some BASIC in the OS but only as one stores sprites and other data files too. The source of RISC OS was opened up. https://www.riscosopen.org/viewer/view/castle/RiscOS/Sources/Programmer/BASIC/s/Lexical?rev=1.6;content-type=text%2Fx-cvsweb-markup https://www.riscosopen.org/viewer/view/castle/RiscOS/Sources... shows part of the BASIC module which has pedigree going back to when Sophie Wilson, designer of the ARM instruction set, wrote it.