It is important in understanding the system, to distinguish between EXDOS and ISDOS.
EXDOS is a disk extension to the EXOS operating system, contained in ROM on the disk controller card, which is automatically linked in when the Enterprise is switched on. It provides an EXOS disk device and also various extension commands which are available to all applications programs through the normal scan system extensions EXOS call (for example coloncommands from ISBASIC).
ISDOS is an EXOS applications program which can be loaded from disk either automatically when EXDOS starts up, or afterwards by an explicit command from the user. Once it is loaded, ISDOS controls the machines in much the same way that ISBASIC normally does. It provides a command line environment similar to MSDOS in which the user can type commands to ISDOS or load transient programs which run and then return to ISDOS.