Program Code

So far we have been putting the entire address space of each process in memory. With the base and bounds registers, the OS can easily relocate processes to different parts of physical memory. However, you might have noticed something interesting about these address spaces of ours: there is a big chunk of “free ” space right in the mid-dle, between the stack and the heap. As you can imagine from Figure 15.1, although the space between the stack and heap is not being used by the process, it is still tak-ing up physical memory when we relocate the entire address space somewhere in physical memory; thus, the simple approach of using a base and bounds register pair to virtualize memory is wasteful. It also makes it quite hard to run a program when the entire address space doesn’t fit into memory; thus, base and bounds is not as flexi-ble as we would like. And thus, a problem: THE CRUX: HOW TO SUPPORT A LARGE ADDRESS SPACE How do we support a large address space with (potentially) a lot of free space between the stack and the heap? Note that in our ex-amples, with tiny (pretend) address spaces, the waste doesn’t seem too bad. Imagine, however, a 32-bit address space (4 GB in size); a typical program will only use megabytes of memory, but still would demand that the entire address space be resident in memory.