I never said it can't be done either. Nor even that you need to put strong restrictions on resources (after all, '51 is Turing-complete, that means -everything- can be written on it - the only limiting factors are available memory (16M? 4G? Or terabytes over serial protocol?) and speed.) But just these limiting factors make multitasking a bad thing, and unrestricted multitasking even worse thing.

As to I/O, if you write a kernel, you should take care to write proper IO routines library, with exclusive locking, accessiblity and permissions checking etc. '51 has no memory protection so you can't stop tasks from accessing the "restricted area" around the limitations, but programs written using the kernel IO calls instead of direct IO write should behave just fine. Just slow...