It is an operation that is not interruptible. So, a single machine code read or write. Why is it important? SO that if you are reading a chunk of memory bigger that this you make sure you don't get interrupted. In the Linux kernel, the term 'atomic' (for which there is a func, in_atomic()) means 'is the code running at a privileged level' which is either at soft or hard interrupt level. Thus timer expiry is soft interrupt. A card triggering an interrupt is hard. So in the windows kernel DPC or DIRQL are the 'atomic' modes. Passive, is non atomic. Harolds answer is spot on, just wanted to add a bit more colour. One other feature of running 'in atomic', ie, at soft or hard interrupt, is you cant access paged out memory or functions. Else you get a page fault exception.
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