How to detect multiple keys at the same time? I need it for a little game I'm doing.
But I'm using Japhet's DOS Extender. I'll probably need to replace the interrupt 0x08, is not it? But how do I replace and by what do I replace?

INT 8h is a timer.

Try Google for "hooking int 9h".
I never did it myself, however.
Also, check out some gaming sources - some of them are released.
Like, as an example Duke Nukem 3D source may be released and it should
contain the hook for INT 9H.

If you would of taken my advice, theres a built in gaming keyboard to take multi key press

the only cool and good way is to replace the int9 handler by one that sets bytes in a 128 bytes tables.
one key = one byte

just read the byte to know the state of the key.

but some multiple keys are just impossible due to hardware under the buttons on the keyboard.

you will have to try a lot.

you can try it also http://board.flatassembler.net/topic.php?p=66344#66344

IIRC, there was an old DOS TASM example for int9 from the demogroup VLA:

http://ftp.lanet.lv/ftp/mirror/x2ftp/msdos/programming/vla/int9.zip

Anyways, most people just use HLL libraries for such things, e.g. libkb or Allegro. You could check what those do.

Or just look at FD KEYB sources or similar (mKEYB).

Probably they just use the BIOS, e.g. int 16h or whatever.

IIRC, I also remember reading somewhere (Atari800 emulator?) that most PC keyboards can't handle more than five keys at once. Just FYI (as the architecture originally wasn't really meant for gaming).

Multiple key is unavailable! You only got maximum 5 keys at same time!. Today's keyboard technology is too obsolete!!! Because of stupid people like you! The Internationally United States of Apes greedily stealing your moneys! They selling old cheap keyboard technology on high prices!!!. And you morons purchasing high priced keyboards!. $8-$15 is quite expensive for these old standard Intel8030 keyboard technology!.
Today you can buy in $5 mp3 player!. That means Today's poorest technology keyboard can be at least every button (analogue 8bit) press sensitive and independent all of 104 buttons!. Actual price is $2 for such keyboard!. If such keyboard exist they sell over $100!!!.

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In mass production! US $2 is quite enough even exceeded!!!.

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Friend of mine worked for Logitech for several years. He said their cost is over $5 before distribution within the US at quantities of 100k+, for the cheapest keyboard. That doesn't include software, advertising, transport, nor storage within the US. Just adding an aluminum metal back-plate can bump that cost by a dollar.