DoorsOS is the most popular kernel/shell for Motorola 68k Texas Instruments calculators. It supports TI-89, TI-92, TI-92+.
Without kernels like DoorsOS, the only programs that calculators could run are BASIC programs and built-in programs. With a kernel, the calculator recognizes assembly programs. This allows for a more flexible range of programs.
Because assembly and C++ are faster languages, and because the calculator processor is not fast, almost any calculation or game programs require a kernel.
Among other kernels are the older PlusShell, which was better for the older versions, like 1.00, but bad for all versions above 1.05