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).
st_goliath · 3h ago
The original IBM PC used an Intel 8048 microcontroller inside the keyboard and an 8255 I/O controller on the main board to communicate with the keyboard.
The PC AT (which had an 80286), later replaced the 8255 with an 8042 microcontroller too. It was running firmware, so re-purposing it for a Hodge-poge of other tasks became trivial. A single GPIO pin was used for masking the A20 line and another for handling CPU reset. Having a total of 24 programmable I/O lines, I guess this could have been done with the 8255 too, but the microcontroller probably allowed simpler interfacing with the CPU and bought them more flexibility for future expansion.
For the article, this is mostly irrelevant. As somebody else noted, the Xbox wasn't supposed to be able to toggle/mask the A20 line, but later x86 CPUs had already integrated the A20 masking feature into the CPU itself and exposed an A20 control line. The Xbox simply tied the A20 enable line to a fixed potential, the hack described in the article requires a simple hardware modification to change that (https://xboxdevwiki.net/File:Haxar-a20m.jpg).
userbinator · 6h ago
For those wondering what software depended on A20 wraparound, there is this interesting series of articles:
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 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.
What a machine. Would have liked to see PS2 games maxed out on XBOX if possible. Imagine RE4/FFXII versions of those games on xbox?
Still, Halo 2 legendary is unmatched.
LongjumpingCat · 5h ago
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.
endlessvoid94 · 1h ago
Same. I modded my original Xbox back in the day. It was a lot of fun and I learned a lot.
OkPin · 4h ago
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.
eddythompson80 · 1h ago
This is a bot, right? Same with all the new accounts commenting completely unrelated things?
cebert · 1h ago
Based on this post and others, it looks like a spammy account to me.
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).
The PC AT (which had an 80286), later replaced the 8255 with an 8042 microcontroller too. It was running firmware, so re-purposing it for a Hodge-poge of other tasks became trivial. A single GPIO pin was used for masking the A20 line and another for handling CPU reset. Having a total of 24 programmable I/O lines, I guess this could have been done with the 8255 too, but the microcontroller probably allowed simpler interfacing with the CPU and bought them more flexibility for future expansion.
For the article, this is mostly irrelevant. As somebody else noted, the Xbox wasn't supposed to be able to toggle/mask the A20 line, but later x86 CPUs had already integrated the A20 masking feature into the CPU itself and exposed an A20 control line. The Xbox simply tied the A20 enable line to a fixed potential, the hack described in the article requires a simple hardware modification to change that (https://xboxdevwiki.net/File:Haxar-a20m.jpg).
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/
This is one hell of a rabbit hole...
https://xboxdevwiki.net/Exploits#A20M.23_hack
https://m.youtube.com/watch?v=9NqLljaHc80
Still, Halo 2 legendary is unmatched.