A fine-grained memory protection system and technique provide computer memory protection at least to a word granularity. A permissions table having permission values associated with a computer memory is arranged as protection domains. The permissions table can be cached in a protection lookaside buffer (PLD) and/or in sidecar registers. A software calls across protection domains (a cross-domain call) can be facilitated with a switch gate and a return gate. In some embodiments, a gate table is provided to store the switch gates and return gates, each having gate values. In some embodiments, a stack permission stable allows stack frames to be associated with the cross-domain call.