IBMDOS.COM is the filename of the DOS kernel. It exists in DR-DOS and PC-DOS systems, with MS-DOS using MSDOS.SYS. The file is located in the root directory of the drive containing the operating system.
The kernel initializes the operating system and calls the file CONFIG.SYS, which must also be located in the root directory. A command in config.sys specifies the location of the command line interpreter, typically COMMAND.COM.