TI-COS (standing for Texas Instruments Calculator Operating System) is the simple CP/M-like operating system used on Texas Instruments' scientific calculators, such as the TI-83 & TI-84 Plus lines. The name is likely a reference to MS-DOS and the abbreviation for cosine, cos. Despite all being under the same naming umbrella of "TI-COS", the operating system each calculator uses can vary dramatically, with the versions of TI-COS included on TI-84 Plus models prior to the Color Edition being noticeably much more primitive and the version for the Nspire being the most sophisticated. Algebraic Operating System (AOS) and Equation Operating System (EOS) are versions of TI-COS customized by Texas Instruments for specific models of their calculator line. TI-COS is capable of running programs written in TI-BASIC, assembly language, C, Lua (Nspire), and, as of the TI-84 Plus CE Python Edition and Nspire CX II, Python. Starting in OS version 5.5.1, released in 2020, support for programs written in assembly and C was removed due to concerns with students using exploits to bypass the OS' exam mode.