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In some games (Tekken 3 on emulator for example) I need to press multiple keys at the same time. But other that ctrl, alt and shift no other keys seem to work together.

For example Down arrow + num 1 + num 3 does not work.

Is there any software to solve that problem?

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Certain keys will not work when pressed together due to the way multi-key rollover is implemented.

There's no software solution as it's a hardware limitation - some keyboards support more keys simultaneously than others, but you can't change how your keyboard handles it.

Keys like Shift, Alt and Ctrl tend to be special cases - they are almost always used with other keys so keyboard manufacturers tend to ensure that they don't exhibit rollover problems.

As ayckoster points out, "gaming" keyboards exist, with higher-than-normal rollover counts. True "n-key-rollover" requires that the keyboard recognise all keys when pressed simultaneously, but a limitation in the USB interface prevents this over that interface requiring the use of a PS/2 connection in most cases.

Apparently, Cooler Master now have a keyboard that supports full N-key-rollover over USB, so possibly some sort of keyboard driver can work around the issue with USB's signalling.

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    This is correct. Either get a "gaming" keyboard, they tend to have a n-key rollover where n >= 5, or a gamepad. – ayckoster Jul 24 '12 at 0:27
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    I solved this problem by mapping the emulator keys to CTRL, SHIFT, CAPS LOCK etc. Well, ultimately I just got a gamepad... – Zommuter Aug 4 '12 at 7:29

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