What is memory interlock?

Recently I came across the term "Memor开发者_开发技巧y interlocked test and set instruction?". I am not able to understand the term.. Can anyone explain me?

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It is a basic atomic instruction in other to do many stuff in parallel programming. See this

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At the lowest level of the process management, the hardware must provide a memory interlocked test-and-set instruction. The test-and-set instruction must allow two operations to be done on a main-memory location—the reading of the existing value followed by the writing of a new value—without any other processor being able to read or write that memory loca- tion between the two memory operations. Some architectures support more com- plex versions of the test-and-set instruction.

Take from Introduction to Process Management.