640k is not maximum available to applications, but the maximum amount of RAM that would fit into PC, restof the 1MB address space was reserved for ROMs and memo

640k is not maximum available to applications, but the maximum amount of RAM that would fit into PC, restof the 1MB address space was reserved for ROMs and memory mapped IO. No dos commands were normally part of ROM. In fact even command.com was designed such that large part of it could be thrown away if additional memory was required and then reloaded from disk when user program terminated (this was implemented by relocating the "transient" part onto the end of usable address space and detecting whether it was overwritten by user programs). Larger DOS file managers usually use similar approach, but simpler: the UI is implemented by separate binary that simply records what the user wants to run somewhere and then exits passing control to small stubbinary that starts requested program and restarts the UI when the program ends. Edit: MS-DOS 2.0's command.com detects that the transient part was overwritten by means of single byte checksum. The reason why it is done in this somewhat hackish way is that DOS allocates all of the available memory for most MZ EXE programs regardless of how many they actually use (MZ header specifies minimum and maximum amount of unitialized memory needed, with no maximum being the default).