I feel rather ashamed to ask this question (but that hasn't stopped me asking it!). I am constructing a program where I'd like to allow 2 keys to be pressed at the same time, and for the program to recognise that with two different responses.
I don't suppose that that would be possible (either by INKEY$ or _KEYHIT). How would the hardware be behaving? How would the software recognise such? If, by some remote possibility, this could be done in some way I shan't feel quite so dumb! But pardon me for cluttering up the site with ridiculous questions (I'm trying to get the keyboard to have the property of a game controller left&right hand). If I were Odin, I'd create a Special Section for Qwerkey's Stupid Questions with an automatic flushing facility.
None of us is perfect and none of us knows everything.
Just to add to this:
The number of simultaneous key-downs that a system can handle is determined by the actual keyboard hardware. The most advanced USB keyboards allow detection of key up/down states for EVERY key simultaneously. With cheap keyboards, sometimes even 3 simultaneous keypresses can fail! All keyboards can handle Ctrl, Alt, Shift combinations because OSes so commonly use them as modifiers, but once you get into uncommon combinations like A+B+C, it's much less likely to register the keypresses.
Also to note, PS/2 and the original keyboard connectors physically limit how many keypresses can be sent simultaneously. USB keyboards, in theory, have no such limits, but in practise most cheap USB keyboards emulate PS/2 under-the-hood, along with their crappy limitations. Gaming keyboards often advertise how many keys can be held down simultaneously, though some lie about this! My current piece-of-shit Amazon Basics keyboard is terrible for this, it often fails with three-key combos, despite saying it can handle 128 simultaneous keys or some rubbish.
Back in the day I can remember trying tons of key combos to figure out which could be handled for multiplayer gaming on the same keyboard. It was tricky to find keys that wouldn't block each other.
PS/2 vs USB Technical Limitations
Keep the following in mind if you have an n-key rollover keyboard that can be hooked up to your computer through either USB or a PS/2 port:
USB protocol limitation - A max of 10 simultaneous key presses are recognized, 6 non-modifier keys ('w', 'a', 's', 'd', etc) + 4 modifier keys (Shift, Caps, Ctrl, etc). Although you are limited to 6 regular keys you are still guaranteed that any combination of keys will be recognized properly if you have an n-key rollover keyboard. I would guess that most people would not need support for more keys than this. I would also guess that the 6 key limit may have had something to do with braille input requirements rather than someone choosing an arbitrary limit (although that doesn't explain why the limit exists in the first place).
PS/2 - There are no limitations when using a PS/2 connection with your keyboard. You will truly get full n-key rollover support.
When given the choice between using PS/2 or USB, it is generally recommended to choose PS/2 since it doesn't have the rollover limitations. However, if you enjoy hotplug support which PS/2 doesn't have, USB may very well be the better choice for you.
From my personal experience, most USB keyboards top out at 6KRO, while you need a PS/2 keyboard for NKRO usage.