Are x86 Loads and Stores Atomic, and How Does the CPU Ensure This?

Atomicity of Loads and Stores on x86

The x86 architecture provides atomicity for aligned loads and stores of data up to 64 bits. This means that these operations are guaranteed to occur as a single, indivisible action, regardless of any other operations that may be occurring concurrently in the system.

How the CPU Implements Atomicity Internally

The CPU handles atomic operations internally using a combination of hardware and software mechanisms. For aligned loads and stores, the CPU simply ensures that the operation is executed as a single, indivisible operation within the cache hierarchy. This is possible because the cache is coherent, which means that all copies of data in the cache are kept synchronized.

For unaligned loads and stores, or for read-modify-write operations, the CPU may need to use additional mechanisms to ensure atomicity. These mechanisms can include:

• Cache locking: The CPU can lock the cache line that contains the data being accessed, preventing other cores from modifying the data while the atomic operation is in progress.
• Bus locking: The CPU can assert the LOCK# signal, which prevents other buses from accessing the memory bus while the atomic operation is in progress.

Implications for Software

The atomicity provided by the x86 architecture has important implications for software design. For example, it means that programmers can use atomic variables to protect shared data from concurrent access, without having to resort to locks or other synchronization mechanisms.

It's important to note, however, that atomicity does not guarantee that an operation will occur immediately. The CPU may delay the operation for performance reasons, or it may need to wait for other operations to complete before it can execute the atomic operation.

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