NX bit
In computer security, executable-space protection marks memory regions as non-executable, such that an attempt to execute machine code in these regions will cause an exception. It makes use of hardware features such as the NX bit (no-execute bit), or in some cases software emulation of those features. However technologies that somehow emulate or supply an NX bit will usually impose a measurable overhead; while using a hardware-supplied NX bit imposes no measurable overhead.