Structured I/O software with diagram

I/O software is divided into four layers so that each layer handles one task and hides hardware details from the layer above.

 +--------------------------------+
 | User-level I/O software        |
 +--------------------------------+
 | Device-independent OS software |
 +--------------------------------+
 | Device drivers                 |
 +--------------------------------+
 | Interrupt handlers             |
 +--------------------------------+
 | Hardware (controllers, device) |
 +--------------------------------+

1. Interrupt handlers: run when a device finishes its work. They save the context, run the handler, unblock the driver that was waiting and return. They are hidden deep in the OS.
2. Device drivers: device-specific code that accepts requests from the layer above, writes commands to the controller registers and blocks until the interrupt arrives. A driver is needed for each device type.
3. Device-independent I/O software: common functions such as uniform driver interface, naming, protection, buffering, error reporting, allocating dedicated devices and device-independent block size.
4. User-level I/O software: library procedures such as printf and read, and spooling (e.g. print spooler daemon).

A request travels down the layers (read → system call → device-independent → driver → controller) and the reply returns up through the interrupt handler.