In the BASIC type the
:ISDOS
when the ISDOS is started, then it is looks similar as the MSDOS command prompt. For loading programs type the name such as in the MSDOS.
It is can possible to make disk image with autostart the program. Need a EXDOS.INI file, one line: ISDOS
And a AUTOEXEC.BAT with the program name.
If this two file exist on the disk, then the EXDOS read the EXDOS.INI and start the ISDOS, and the ISDOS read the AUTOEXEC.BAT and run the specified program.
ISDOS can be started without the ROM version (it is originaly a disk program), copy the IS-DOS.SYS to the disk. If the EXDOS not found it in the ROM, then load it from the disk.