Test-and-set

An atomic read-modify-write that returns a flag's old value while setting it. In a sequential consistency model with the flag initially $0$, exchange(flag, 1) permits entry only when it returns $0$; an owner resets the flag to $0$ on release. Between releases only one exchange can observe $0$. By contrast, separate read(flag) and write(flag, 1) steps allow both contenders to read $0$ before either writes $1$. The indivisibility of the exchange is an assumed primitive, not something proved by writing its steps as ordinary assignments.^[ref]