hi i have just been reading about OS and i have a question. it says here that multiprogramming and timesharing require that the memory and the i/o of the OS and user processes be protected against each other. most PCs do not support this kind of protection.
my question is why dont they support that kind of protection in most PCs? why do they need protection against each other?

Actually, most current processors support process segregation, but the OS has to utilize it. Most do these days, including Unix, Linux, and Windows (since Windows 2000). You don't want processes to be able to access memory and other resources in other processes as that would allow them to, accidentally or otherwise, corrupt those processes.

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