Which Segment Are We Referring To?

Learn how the hardware is able to determine the segment to which a particular address refers.

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The hardware uses segment registers during translation. How does it know the offset into a segment, and to which segment an address refers?

Explicit approach

One common approach, sometimes referred to as an explicit approach, is to chop up the address space into segments based on the top few bits of the virtual address; this technique was used in the VAX/VMS system“Virtual Memory Management in the VAX/VMS Operating System” by Henry M. Levy, Peter H. Lipman. IEEE Computer, Volume 15:3, March 1982. A classic memory management system, with lots of common sense in its design. We’ll study it in more detail in a later chapter.. In our example above, we have three segments; thus we need two bits to accomplish our task. If we use the top two bits of our 14-bit virtual address to select the segment, our virtual address looks like this:

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