Virtual Memory for persistent store

 
Figure 3.7: The segment map after running the claim algorithm.

If one is to use virtual addresses for persistence, an advantageous technique is to reserve a range of virtual addresses that will constitute the persistent store. There exists no explicit system call to do this under DPMI, but there does exist a call to claim an individual segment selector. The effect of claiming a range of segments can be had by the following algorithm. Suppose we wish to claim 512 segments ( 32 megabytes) starting at segment 4000, then we:

1. Call the allocate selector procedure 4512 times, recording in a bitmap which selectors were allocated. Since some n selectors will already have been reserved by other applications before our code starts this will have reserved all segments up to 4511+n.
2. Check that all segments between 4000 and 4511 have been reserved. If not free all the claimed selectors and abort.
3. Free all the segments that were claimed below 4000 and above 4511.

• Segment fault handler
• Backing store organisation