lock prefix
The x86-64 prefix byte (f0) that makes the memory update of the instruction after it atomic, as in lock add.
The lock prefix (0xF0) makes a read-modify-write instruction with a memory destination (add, sub, inc, and, or, xor, xadd, cmpxchg, bts ...) atomic, by holding the cache line in exclusive state for the duration; it also acts as a full memory barrier. xchg with memory is implicitly locked.
Examples
lock add qword ptr [rdi], 1is an atomic increment on x86-64.
See also