It is possible to provide more flexible protection than that in the Intel Pentium architecture by using a protection scheme similar to that used in the Hewlett-Packard Precision Architecture (HP/PA). In such a scheme, each page table entry contains a “protection ID” (key) along with access rights for the page. On each reference, the CPU compares the protection ID in the page table entry with those stored in each of four protection ID registers (access to these registers requires that the CPU be in supervisor mode). If there is no match for the protection ID in the page table entry or if the access is not a permitted access (writing to a read-only page, for example), an exception is generated.

a. How could a process have more than four valid protection IDs at any given time? In other words, suppose a process wished to have 10 protection IDs simultaneously. Propose a mechanism by which this could be done (perhaps with help from software).

b. Explain how this model could be used to facilitate the construction of operating systems from relatively small pieces of code that can’t overwrite each other (microkernels). What advantages might such an operating system have over a monolithic operating system in which any code in the OS can write to any memory location?

c. A simple design change to this system would allow two protection IDs for each page table entry, one for read access and the other for either write or execute access (the field is unused if neither the writable nor executable bit is set). What advantages might there be from having different protection IDs for read and write capabilities? (Could this make it easier to share data and code between processes?)