The rest, the majority of the actual problem, was due to the kernel performing tasks such as checking the message for port access rights. While it would seem this is an important security concern, in fact, it only makes sense in a UNIX-like system. For instance, a single-user operating system running a cell phone or robot might not need any of these features, and this is exactly the sort of system where Mach's pick-and-choose operating system would be most valuable. Likewise Mach caused problems when memory had been moved by the operating system, another task that only really makes sense if the system has more than one address space. DOS and the early Mac OS have a single large address space shared by all programs, so under these systems the mapping did not provide any benefits.