§15.5–15.6Operating System Issues … Summary

The hardware brought registers and exceptions; now the OS must hold up its end. Four duties, at four critical junctures:

15.5 Operating System Issues

Creation: search the free list for a slot, mark it used, set up the space. Termination: put the memory back on the free list, clean the process structures. Context switch: save and restore the base/bounds pair via the PCB — and, while a process is descheduled, the OS may even move its whole address space (copy, update saved base, resume; the process never knows). Exceptions: install handlers at boot; when a process strays out of bounds, terminate it with prejudice.

Here is the whole dance — chapter 6’s LDE protocol, upgraded with translation. Watch for the rows where the OS column is empty:

The shape is unmistakably limited direct execution: the OS sets up the hardware and gets out of the way; processes run at full speed with per-reference translation done silently by the MMU; the OS reappears only at interrupts, at switches, and when someone misbehaves.

15.6 Summary

Address translation gives the OS control over every memory access while keeping the illusion — and the speed — intact. Base and bounds is its simplest form: one addition, one comparison, real protection (without which a process could overwrite the trap table and own the machine).

But look at what the fixed slot costs us:

That waste — unused space inside the allocated unit — is internal fragmentation internal fragmentation Wasted space INSIDE an allocated unit — e.g., the unused gap between heap and stack in a fixed base/bounds slot. defined in ch. 15 — open in glossary , and it’s baked into the fixed-slot design. (A fixed-size stack region would limit recursion; not appealing either.) We need machinery that allocates closer to what’s actually used. The first attempt: generalize base and bounds into segmentation — and finally learn why it’s called a segmentation fault.