Before the protection unit is enabled, valid protection regions must be programmed. If they are not programmed, the ARM940T can enter a state that is recoverable only by reset. Setting bit 0 of the CP15 register 1, the control register, enables the protection unit.

When the protection unit is disabled, all instruction fetches are non-cacheable and all data accesses are non-cacheable and non-bufferable. This results in very poor system performance, so software should define memory regions and enable the protection unit soon after reset.