Memory barriers – an introduction

Last but not least, let's briefly address another concern – that of the memory barrier. What does it mean? Sometimes, a program flow becomes unknown to the human programmer as the microprocessor, the memory controllers, and the compiler can reorder memory reads and writes. In the majority of cases, these "tricks" remain benign and optimized. But there are cases – typically across hardware boundaries, such as CPU cores on multicore systems, CPU to peripheral device, and vice versa on UniProcessor (UP) – where this reordering should not occur; the original and intended memory load and store sequences must be honored. The memory barrier (typically machine-level instructions embedded within the *mb*() macros) is a means to suppress such reordering; it's a way to force both the CPU/memory controllers and the compiler to order instruction/data in a desired ...