The term PART OF DOS (ABBR.) can be solved by two different abbreviations: SYST (4 letters) and SYS (3 letters). SYST stands for System, which is a part of DOS (Disk Operating System). SYS stands for System Files, which are files that are part of the DOS operating system. These files are responsible for the basic functions of the computer, such as booting up the system, managing memory, and running programs.