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Xbox Hacks: The A20 (2021)

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Re: Xbox Hacks: The A20 (2021)

#21

Post by toast0 »

bigstrat2003 wrote: post #16
userbinator wrote: post #4

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.

Re: Xbox Hacks: The A20 (2021)

#23

Post by rasz »

bigstrat2003 wrote: post #16
userbinator wrote: post #4

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.

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

Re: Xbox Hacks: The A20 (2021)

#24

Post by ljf »

glimshe wrote: post #20
ljf wrote: post #19

Earlier quoted context omitted.

As an FYI to others, it appears this account (and others in this thread) might be a bot.

I didn't notice it at first but once you said it, it became obvious. There is a cheesiness to LLM comments that is hard for humans to emulate... Although it's still plausible that a person would say something like that.

I'll be honest, I upvoted it at first, and it was only when I saw the 2nd very similar post here and then went into their cookie cutter post histories.

Re: Xbox Hacks: The A20 (2021)

#25

Post by privatelypublic »

st_goliath wrote: post #10
messe wrote: post #8

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…

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 res…

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