Xbox Hacks: The A20 (2021) | Hacker News Reader

Xbox Hacks: The A20 (2021)

connortumbleson.com

106 pointsmattweinberg21 comments

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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/

It's always blown my mind that software would depend on that behavior. What possessed people to do that? It's very obviously a bad idea.

If the system behavior is defined to wrap around, using defined system behavior to reduce code complexity makes perfect sense.

This was in an era where new computer systems usually meant throwing away all the existing software and rewriting it (or doing a fresh port anyway). Why would you assume someone would extend the system and cause trouble.

Im blown away at how small a number of software depended on the wraparound and IBM still forced A20 hack on us all.

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).

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).

Linux kernel still has the option to try and use the keyboard to reset the system while rebooting. (Just an aside)

Similarly, on the original Macintosh one of the mouse axis encoders was connected to the carrier detect pin of the serial port chip. Thus, in early versions of the OS, if you closed the serial driver, the mouse would only move horizontally (or was it vertically?).

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.

Indeed, more than you ever wanted to know about the A20 line: https://aeb.win.tue.nl/linux/kbd/A20.html (from your article)

This is one hell of a rabbit hole...

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

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

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

Building an operating system currently. This isn't just Xbox, we still have to do this in bootloaders.

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.

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 is a bot, right? Same with all the new accounts commenting completely unrelated things?

Based on this post and others, it looks like a spammy account to me.

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