Upon BOOT, the original MS-DOS system would first load settings from a file called CONFIG.SYS, and then execute any commands found in the AUTOEXEC.BAT file. Commonly the AUTOEXEC.BAT file would define the system PATH, load device drivers, and launch Windows. Because DOSBox already emulates most of what a CONFIG.SYS, and AUTOEXEC.BAT would do, there isn't much need to define anything other than your mountable drives.
A Common AUTOEXEC.BAT
@ECHO OFF MOUNT C C:\DOSGAMES C: CLS ECHO Hello world...
(The message that MS-DOS printed immediately before loading the CONFIG.SYS and AUTOEXEC.BAT).
MOUNT C C:\oldgames C: