The Modest Mini Corporation’s best-selling computer allows at most two users to run at a time. Its only addressing architecture feature is a single page map, which creates a simple linear address space for the processor. The time-sharing system for this computer loads the page map with a set of memory block addresses before running a user; to switch to the other user, it reloads the entire page map with a new set of memory block addresses. Normally, the set of memory blocks belonging to one user has no overlap with the set of memory blocks belonging to the other user, except that memory block 19 is always assigned as page 3 in every user’s address space, providing a “communication region”.
5.3a Protection and privacy are obviously a problem with a completely public communication area, but is there any other difficulty in using the communication region for any of the following types of data?
A. The character string name of the payroll file
B. An integer representing the number of names in the current payroll file
C. The virtual memory address, within the communication region, of another data item
D. The virtual memory address of a program that lies outside the communication region
E. A small program that is designed to remain within the communication region and execute there
5.3b Ben Bitdiddle has decided that programming with page 3 always preassigned is a nuisance. He has therefore proposed that a call to the system be added that reassigns the communication region to a different page of the calling user’s address space, while not affecting the other users. What effect would this proposal have on your answers to 5.3a?
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