Consider the ring protection scheme in MULTICS.If we were to implement the system calls of a typical operating system and store them in a segment associated with ring 0,what should be the values stored in the ring field of the segment descriptor? What happens during a system call when a process executing in a higher-numbered ring invokes a procedure in ring 0?
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The ring should be associated with an access bracket (b1,b2),a limit value b3,and a set of designated entry points.The processes that are allowed to invoke any code stored in segment 0 in an unconstrained manner are those processes that arecurrently executing in ring i where b1≤i ≤b2 .Any other process executing within ring b2 < i≤b3 is allowed to invoke only those functions that are designated entry points.This implies that we should have b1= 0 and set b2 to be the highest ring number that comprises of system code that is allowed to invoke the code in segment 0 in an unconstrained fashion.We should also store only the system call functions as designated entry points and we should set b3 to be the ring number associated with user code so that user code can invoke the system calls.