VAX/VMS Virtual Memory: A Real Address Space

One neat aspect of studying VMS is that we can see how a real address space is constructed (see figure below). Thus far, we have assumed a simple address space of just user code, user data, and user heap, but as we can see above, a real address space is notably more complex.

Inaccessible zero page

For example, the code segment never begins at page 0. This page, instead, is marked inaccessible, in order to provide some support for detecting null-pointer accesses. Thus, one concern when designing an address space is support for debugging, which the inaccessible zero page provides here in some form.

Kernel mapped into each address space

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