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Comment by mavidser | Hacker News Reader

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mavidser·

AFAIK, it was mostly because a lot more PS/2 keyboards support N-key rollovers than USB keyboards.

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That's because USB has an actual limitation so can't support n-key, just 5 or 6 I think.

PS/2 does, but of course not all keyboards can deliver n-key even over PS/2.

USB n-key rollover keyboards avoid that issue by looking like several keyboards to the OS, I think.

this is not true, USB HID has a 'boot protocol' mode for BIOS etc. which limits HID devices to 6KRO, but USB keyboards can send a report descriptor to an OS with as many keys at once as they want.

see: https://deskthority.net/wiki/USB

Well, the reality is that all USB mechanical and gaming keyboards have been limited to 6KRO until very recently. For instance: https://www.keyboardco.com/blog/index.php/2016/03/what-is-nk...

The very same keyboards provide n-key if you use the USB->PS/2 adaptor in the box with the keyboard.

If it were straightforward there wouldn't be hacks such as pretending there's 2 keyboards in the system.

Implementing it is straightforward, trivial even. As makomk hints, the problem is that you'd be doing something non-standard and thus have to rely on your own driver (or someone else's) rather than the system's built-in and ready-to-go hid drivers. Or you show up as multiple keyboards simultaneously, but that confuses some software.

The trouble is, if I recall correctly, that there's an awful lot of buggy driver code out there which cannot handle this correctly. So keyboards can't safely use this mode by default.

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