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Thus far, we have assumed that an address space is unrealistically small and fits into the physical
memory. In fact, we have been assuming that every address space of ervery running process fits
into memory. We will now relax these big assumptions, and assume that we wish to support many
concurrently-running large address space. To do so, we require an additional level in the memory
hierarchy. Thus far, we have assumed that all page tables in physical memory. However, to support
large address space, the OS will need a place to stash away portions of address spaces that currently
are not in great demand. In general, the characteristics of such a location are that it should have more
capacity than memory; as a result, it is generally slower (if it were faster, we would just use it as
memory, no?). In modern systems, this role is usually served by a hard disk dirve. Thus in our memory
hierarchy, big and slow hard drive sit at the bottom, with memory just above. And thus, we arrive
at the crux of the problem: How to go beyond physical memory? How can the OS make use of a larger,
slower device to transparently provide the illusion of a large virtual address space ? One question you
might have: why do we want to support a single large address space for a process ? Once again, the
answer is covenience and ease of use. With a large address space, you do not have to worry about
if there is room enough in memory for your program‘s data structure; rather, you just write the program
naturally, allocating memory as needed. It is a powerful illusion that the OS provides; and makes your
life vastly simpler. You are welcome! A contrast is found in older systems that used memory overlays,
which require programmers to manually move pieces of code or data in and out of memory as they
were needed. Try imagining what this would like: before calling a function or accessing some data,
you need to first arrange for the code or data to be in memory. Yuck!
Operating System: Three Easy Pieces --- Beyond Physical Memory: Mechanisms (Note)
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原文地址：http://www.cnblogs.com/miaoyong/p/4858185.html
