Compare-And-Swap
Let's look at compare-and-swap hardware primitive for supporting locks, in this lesson.
We'll cover the following...
Another hardware primitive that some systems provide is known as the compare-and-swap instruction (as it is called on SPARC, for example), or compare-and-exchange (as it called on x86). The C pseudocode for this single instruction is given in the code excerpt below.
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int CompareAndSwap(int *ptr, int expected, int new) {int original = *ptr;if (original == expected)*ptr = new;return original;}
The basic idea is for compare-and-swap to test whether the value at the address specified by ptr is equal to expected; if so, update the memory location pointed to by ptr with the ...