18.2.1 OS Device Handlers
Let's turn our attention to how the operating system (OS) deals with input/output devices. There are actually two parts to the discussion. First, we'll talk about how the OS interacts with the devices themselves. This will involve a combination of interrupt handlers and kernel buffers. Then we'll discuss how supervisor calls access the kernel buffers in response to requests from user-mode processes. As we'll see, this can get a bit tricky when the OS cannot complete the request at the time the SVC was executed. Here's the plan! When the user types a key on the keyboard, the keyboard triggers an interrupt request to the CPU. The interrupt suspends execution of the currently-running process and executes the handler whose job it is to deal with this particular I/O event. In this case, the keyboard handler reads the character from the keyboard and saves it in a kernel buffer associated with the process that has been chosen to receive incoming keystrokes. I...