How does the OS detect B&B violation from hardware?
When an invalid access is detected, the #BR exception is triggered, the CPU will switch to privilege mode and load the corresponding vector (Bounds range exceeded) from the IVT. The CPU also pushes the faulty EIP (as well as other things) to the kernel stack and updates the BNDSTATUS register to describe the fault.

Note: the #BR exception saves other purposes as well because Base &amp; Bound works like paging: there is a 2-level hierarchy to store B&amp;B values (since the CPU can only store 4 of them in registers), so it works a bit like a TLB but for B&amp;B.