system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: I hit the 'a' key on my keyboard, and a letter 'a' appears on my screen. What are the steps that link these two?

Let's assume I'm using a word processor like Microsoft Word. What then are the steps from 'pressing a key' to 'text appearing on screen', both in terms of the hardware and the software.
gpt
**ELI5**
The controller circuit in your keyboard detects the press of the key. It sends this information to the USB controller. That controller sends this information to the USB host controller inside your computer. 

The USB host controller sends that information to the processor. At this point it's a software thing.

The OS sees that a key has been pressed. It checks to see which applications are open and which one "has focus" (meaning the last program you clicked the mouse in).

In this case that program that has focus is MS-Word. The OS passes the info about the key press to MS-Word. 

MS-Word checks to see what the key was and decides what to do based on that. For this example let's assume a document was open and the cursor is visable. MS-Word sees where there cursor is and adds an "a" to the document at the cursor position. 

**ELI>5**

There are lots of levels of detail here. As an example, the keyboard actually sends two messages. One when the 'a' key is pressed and one when it is released.  Also the OS checks first to see if the 'a' has any system wide meaning. Since it's 'a' it doesn't, but if it was instead something like "volume up" or  CTRL-ALT-DEL then it would "trap" the signal and perform the system function instead of sending it all the way to MS-WORD.

Similarly word does the same thing. Many of the menu items in word have special keyboard shortcuts so word checks to see if the key is one of the shortcuts.