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In computer science, the test-and-set instruction is an instruction used to write to a memory location and return its old value as a single atomic (i.e., non-interruptible) operation. Typically, the value 1 is written to the memory location. If multiple processes may access the same memory location, and if a process is currently performing a test-and-set, no other process may begin another test-and-set until the first process is done. CPUs may use test-and-set instructions offered by other electronic components, such as dual-port RAM; CPUs may also offer a test-and-set instruction themselves. A lock can be built using an atomic test-and-set instruction as follows: function Lock(boolean *lock) The calling process obtains the lock if the old value was 0. It spins writing 1 to the variable until this occurs. Maurice Herlihy (1991) proved that test-and-set has a finite consensus number, in contrast to the compare-and-swap operation. The test-and-set operation can solve the wait-free consensus problem for no more than two concurrent processes. However, more than two decades before Herlihy's proof, IBM had already replaced Test-and-set by Compare-and-swap, which is a more general solution to this problem. == Hardware implementation of test-and-set == DPRAM test-and-set instructions can work in many ways. Here are two variations, both of which describe a DPRAM which provides exactly 2 ports, allowing 2 separate electronic components (such as 2 CPUs) access to every memory location on the DPRAM. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Test-and-set」の詳細全文を読む スポンサード リンク
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