The Address Space

However, we have to keep those pesky users in mind, and doing so requires the OS to create an easy to use abstraction of physical memory. We call this abstraction the address space, and it is the running program’s view of memory in the system. Understanding this fundamental OS abstraction of memory is key to understanding how memory is virtualized.

Address space of a process

The address space of a process contains all of the memory state of the running program. For example, the code of the program (the instructions) have to live in memory somewhere, and thus they are in the address space. The program, while it is running, uses a stack to keep track of where it is in the function call chain as well as to allocate local variables and pass parameters and return values to and from routines. Finally, the heap is used for dynamically-allocated, user-managed memory, such as that you might receive from a call to malloc() in C or new in an object- oriented language such as C++ or Java. Of course, there are other things in there too (e.g., statically-initialized variables), but for now let us just assume those three components: code, stack, and heap.

Example

In the example in the figure below, we have a tiny address space (only 16KBWe will often use small examples like this because (a) it is a pain to represent a 32-bit address space and (b) the math is harder. We like simple math.). The program code lives at the top of the address space (starting at 0 in this example, and is packed into the first 1K of the address space). Code is static (and thus easy to place in memory), so we can place it at the top of the address space and know that it won’t need any more space as the program runs.