The Keyboard Controller

The i8042-style controller sits between the CPU and the keyboard device. In a real PC, the details are tangled with history and compatibility. For this lab, the useful view is that the controller exposes a small register interface and raises an event when a scancode byte is waiting.

Register	Role
status register	output-buffer state and error bits
output buffer	next scancode byte
command register	controller commands
IRQ line	keyboard event notification

The safe read sequence is to read status, confirm that the output buffer is full, check the error bits, read the output byte, and feed that byte to the scancode parser. Each step has a reason. The status read prevents empty-buffer reads. The error checks prevent corrupted bytes from becoming trusted input. The parser call keeps device I/O separate from scancode assembly.

It is tempting to collapse all of this into a single "get key" helper. Avoid that too early. At this layer you are not yet getting a key; you are getting one byte from a controller stream, and the byte may be a prefix, a command response, or part of a larger event.

IMPORTANT

Do not read output data just because your program wants input. Read only when the status register says data is available.