Section 2 ½ Memory Hierarchies
and Multiple Processes 229
Table 1 Storage Hierarchy Dimensions
Nonresident
computation occurs on z to obtain the actual address in Mp. This computation is part of the Pc, just as is an automatic indexing or indirect-addressing calculation. It takes as input not just the virtual address z but information on where the program is located in Mp. The latter information is called the map, and a program's map information is determined when it is placed into Mp on a given run. Thus, using ISP notation, and calling the address calculation f, we get
This whole scheme is built to permit programs to be placed in Mp's in various ways, e.g., relocated or scattered around, and still make it possible to run the program. Any such scheme brings a solution to the protection problem, namely, that for some values of z the above calculation cannot take place or is invalid (i.e., there is no mapping for z). This can correspond to a violation of protection, which can then be prevented. All calculations may even be permissible, but f is usually arranged so that it never produces an address in anyone else's part of Mp.
The memory map is part of each user's program. With many