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The three system files that are absolutely required to Boot to a DOS prompt are:

io.sys
msdos.sys
Command.com

A portion of IO.SYS is loaded at the completion of POST and is referenced as the boot strap loader. It calls on the operating system or returns the ‘non-system boot disk error’. IO.SYS may be considered a low level hardware manager, since its responsibility is low level I/O manager.
The boot sector also has the partition loader which identifies the beginning and end sector of the partitions.

MSDOS.SYS is loaded if IO.SYS finds success in reaching the partition table and is responsible for drive, directory and file management.
MSDOS.SYS will look for a config.sys file, and if it is present that file will be run. If no config.sys file is present, then MSDOS.SYS will locate Command.com and load it.

COMMAND.COM is the translator (command interpreter).

These three files are absolutely required to boot to a Non NT Microsoft operating system. Furthermore they all have to be from the same vintage. I.e. DOS 5, DOS 6 or Windows 98. You cannot copy com-mand.com from a Windows 98 machine and expect it to work on a Windows 95 machine or vice versa. To move these three files to a disk, the sys command is required. An example would be making a floppy disk bootable from a DOS prompt within Windows 98 (In real mode). C:>\sys a: System transferred

(The system transferred message indicates that the three core files have been transferred.)