The hardware should provide special instructions to modify the base and bounds registers, allowing the OS to change them when different processes run. These instructions are privileged; only in kernel (or privileged) mode can the registers be modified. Imagine the havoc a user process could wreak* if it could arbitrarily change the base register while running. Imagine it! And then quickly flush such dark thoughts from your mind, as they are the ghastly stuff of which nightmares are made.