Describe about MULTICS?
MULTICS is described as having a series of concentric rings, with the inner ones being more privileged than the outer ones. When a procedure in an outer ring wanted to call a procedure in an inner ring, it had to make the equivalent of a system call which is a TRAP instruction. MULTICS hardware made it possible to designate individual process as protected against reading, writing, or executing.
Describe the mechanism of upcall?
When the kernel knows that a thread has blocked, the kernel notifies the processes run time system, passing as parameters on the stack the number of the thread in question and a description of the event that occurred. The notification happens by having the kernel activate the run-time system at a known starting address, roughly analogous to a signal in UNIX.
Describe about checkpointing?
Checkpointing a process means that its state is written to a file so that it can be restarted later. The checkpoint contains not only the memory image, but also the resource state, that is, which resources are currently assigned to the process. To be most effective, new checkpoints should not overwrite old ones but should be written to new files. Whole sequences of checkpoint files are accumulated due to the process getting executed.
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