Comment by rdtsc

Yeah, the security part wasn't baked into the hardware. It relied on the OS (it ran a virtualization layer of sorts) to enforce it via traps if it set those traps.

From https://www.devever.net/~hl/ppcas

> As such, they can principally be viewed as providing a performance enhancement for the IBM i operating system, which uses these instructions to keep track of pointer validity. It is the IBM i OS which enforces security invariants, for example by always following every pointer LQ with a TXER.