A Few Big OS Concepts

What does the OS do?
Provides the interface between the user/application and the hardware.
This is done through System calls, or the library functions that call them. There are system calls for pretty much everything you (or a program) can ask the hardware to do (process instantiation, memory allocation, file I/O, device I/O, network communication, …).
A complementary view is that the OS manages limited hardware resources, given all the various running apps and their demands.
User mode vs Kernel mode
No user program has direct access to the hardware. For example the user program cannot just decide to keep the CPU for its own purposes. This is one reason why there is kernel mode.
Device Driver vs Controller
The driver is software whereas the controller is usually circuitry that is part of the hardware the driver is ‘driving.’ Think if you’re driving a car, you’re the driver. You want to accelerate (or someone else tells you to). Whatever magic is connected to the other side of the accelerator pedal is the controller.
Mechanism vs Policy : How things get done, versus deciding which things can be done/to do and when. An important distinction in operating system design. Read more.

Be sure to read about booting for Friday’s class!