Back in the days of MS-DOS, when I needed to type a special character that wasn't part of the keyboard layout, I would press down the left Alt key and type out the character code on the numeric keypad (Alt+130 for "é", Alt+150 for "û") and the character would appear on screen. This was very convenient when the machine I was working with didn't have the proper
KEYB command active to support accented characters. This behaviour was so universal that I figured this was implemented by firmware in the keyboard itself.
With Windows NT, this was still possible (using the LEFT Alt key) but there was also a possibility of typing a character via its Windows "ANSI" character code (code page 1252 in my case) by prefixing with 0 : Alt+0233 for "é", Alt+0251 for "û". Coupled with the use of Ctrl+Alt+Del as an Attention key, it was clear that Windows itself had intimate involvement with the keyboard.
Now I'm using Linux Ubuntu and I find that these Alt+num shortcuts don't work at all.
So, has the Alt + numeric keypad feature always been implemented by the OS since MS-DOS 1.0? Or was it implemented in the keyboard firmware or BIOS at one point, and modern operating systems just circumvent it somehow?