Memory Manager

In the previous article we looked at how a user program crosses the ring 3 → ring 0 boundary to ask the kernel for help. The example we used was read() — a file descriptor, a buffer pointer, a byte count. But we glossed over something important: what is that buffer? Who decided it existed? Who owns the physical RAM behind it? Those questions are what the memory manager answers. And it answers them for every process on the machine, simultaneously, for every allocation that has ever happened since boot. It’s one of the most complex subsystems in the kernel, so I want to approach it the way you’d approach an unfamiliar library — start at the front desk with the catalog, then walk back through the stacks.