In computing, a memory barrier, also called a membar, memory fence or fence instruction, is a sort of barrier instruction that causes a central processing unit (CPU) or compiler to enforce an ordering constraint on memory operations issued before and after the barrier instruction. This typically signifies that operations issued prior to the barrier are guaranteed to be carried out earlier than operations issued after the barrier. Memory boundaries are essential as a result of most fashionable CPUs employ efficiency optimizations that can lead to out-of-order execution. This reordering of memory operations (loads and stores) normally goes unnoticed within a single thread of execution, but may cause unpredictable behavior in concurrent packages and machine drivers unless carefully controlled. The precise nature of an ordering constraint is hardware dependent and defined by the structure's memory ordering model. Some architectures present multiple limitations for enforcing different ordering constraints. Memory limitations are typically used when implementing low-level machine code that operates on memory shared by a number of gadgets. Such code consists of synchronization primitives and lock-free knowledge buildings on multiprocessor techniques, and machine drivers that communicate with pc hardware.