Orchestration

Why an OS exists — you want to run many programs on one machine, and they'd destroy each other without a referee.

Processes and threads — a process is an isolated running program, a thread is a worker within it. Why isolation matters (one crash shouldn't kill everything).

Virtual memory — every process thinks it has all the memory. The OS + hardware translate fake addresses to real ones. Paging as the mechanism. This is the single most important abstraction in computing.

Files and filesystems — files are named byte sequences. The filesystem is how the OS maps names to bytes on disk. Directories are just files that list other files.

I/O — how programs talk to the outside (keyboard, screen, disk, network). Blocking vs non-blocking. The key insight: I/O is slow, so the OS switches to another process while waiting.

The syscall boundary — programs can't touch hardware directly. They ask the OS through system calls. This is the security wall.

Containers — not a new OS, just clever use of two Linux features (namespaces for isolation, cgroups for limits). Why this matters: package the environment with the program.