IBMDOS.COM

Loaded and initially invoked by the DOS BIOS in IBMBIO.COM during the boot process,[1] it contains the hardware-independent parts of the operating system, including the embedded FAT12, FAT16 and, in newer versions, the FAT32 file system code, as well as the code to provide the DOS API to applications.

[2] As IBMDOS.COM is a binary image containing executable code rather than a true COM-style program, the hidden attribute is set to keep the file from being accidentally invoked at the command prompt, which would lead to a crash.

This is not necessary for DR-DOS 7.02 and higher, because under these systems the file is a fat binary also containing a tiny COM-style stub just displaying some version info and exiting gracefully when loaded inappropriately.

[3][1] In Digital Research terminology, the kernel component of the operating system is called the BDOS (Basic Disk Operating System),[1] a term originally coined by Gary Kildall in 1975 for CP/M,[4] but which is continued to be used in all other DRI operating systems (except for Concurrent DOS 286 and FlexOS).

The FAT file system specific code is called the FDOS in DRI terminology.