Original Xbox Hacks: The A20 CPU Gate (2021)

Something this article doesn't mention is how the A20 gate was toggled: by writing to registers on the keyboard controller.

I was always thought this was a completely inexplicable design choice, until I started working in embedded, working with hardware engineers, and having to go through schematics myself. I now entirely understand the choice of wanting to minimize the redesign work and going with the one free pin available (our product has made similar choices too at this stage).

For those wondering what software depended on A20 wraparound, there is this interesting series of articles:

https://www.os2museum.com/wp/who-needs-the-address-wraparoun...

https://www.os2museum.com/wp/the-a20-gate-it-wasnt-wordstar/

https://www.os2museum.com/wp/exepack-and-the-a20-gate/

https://www.os2museum.com/wp/the-a20-gate-fallout/

A20 bugs were still with us until at least 2009, when I tripped over one: https://mjg59.livejournal.com/118098.html . I love the visualisations in this post, it makes it much clearer what's actually going on.

I highly recommend watching the deconstructing xbox talk. It gets very funny.

https://m.youtube.com/watch?v=9NqLljaHc80

I was confused as to why Xbox would ever enable a feature intended for backwards compatibility with systems it does not need to be compatible with. Especially at boot time. Turns out it did not; this apparently required a hardware modification to pull off.

https://xboxdevwiki.net/Exploits#A20M.23_hack

What really caught my attention is how this marketing snippet highlights the tension between authenticity and polish in gaming culture. Xbox was trying to hit that sweet spot, it wanted to feel edgy and gamer-friendly but the copy ended up sounding like corporate speak.

This really hit home. Those old Xbox ads are such a time capsule, a mix of “we’re edgy” and “we want to be cool” but not taking themselves too seriously.