Concepts
The ideas that cut across many files, each explained once, with diagrams, and linked to the code and the tours that use them.
1The stacks of xv6Which stack a hart is running on, what lives on each one, and the four instructions that switch between them.2Mode, stack and page table: the master questionAt every instant each hart is in some privilege mode, on some stack, with some page table. The eight kinds of transition that change them, and every combination that occurs.3Locks and interrupt stateWhy three harts need locks, how a spinlock works down to the instruction, how xv6 keeps interrupts off while one is held, how a lock crosses a context switch, and every lock in the kernel, with its measured order and nesting.