The computer's housekeeper

The housekeeper between apps and hardware that shares the parts around.

A program is a file at rest; a process is it running, alive. Each gets its own memory.

Just one worker, but switching so fast that many seem to run at once.

Saving and restoring state when work switches, and what it costs

Setting the order of waiting tasks, and the trade between speed and fairness

Splitting a task into strands sharing one space, its upside and its care

Handling work truly at once with several cores, its speed and its limit

The accident from fixing the same thing at once, and preventing it one at a time

Each process gets its own pretend addresses, so it seems to use a large memory alone. Pretend maps to real spots; unused parts go to disk, frequent ones stay close.

Not a blocking wall, but the result of each process having a different map with no path between them. An out-of-map address is caught and stops only that program, so one crash doesn't spread.

The OS uses each device's driver (a translator) to turn its standard language into device signals. A new device just needs its driver; a slow one taps with an interrupt when done.

A disk is just flat, numbered cells. The operating system binds files with a name and a map, and raises a folder tree from files that hold files.

When many share one tree, the operating system sets an owner and rights (read, write, execute) on each file. The administrator is a master key above those rules.

A program cannot touch hardware directly inside its fence (user mode). To get work done it asks at the official counter (a system call); crossing it briefly flips to kernel mode, which checks permission and then does the work.

Behind the system-call counter, the kernel is the core program that actually does the work. It stays in memory from boot to shutdown, governing CPU, memory, devices, files and permissions; if the kernel stops, the whole system stops.

Taking a resource one at a time is safe, but if two each hold one and wait for what the other holds, both freeze forever. It clears when someone yields or an order is fixed for grabbing.

Press power and a small startup program (firmware) wakes, loads the kernel into memory, and the kernel starts the first process, on top of which apps stack up. In this one moment everything learned in this area walks on stage in order.