[Abyssal]

I can't push more then 3 keys at a time on keyboard(you know, it just doesn't work on ps/2 keyboards)
Is there a plugin or something for ePSXe to make possible assigning key combinations like R1+R2+L1+L2 to a single button?
Or the only way is to buy a joypad?

 

[KrossX]

You may be able to even press 6 keys at once (or more), if done in the
right combination... and might be keyboard dependant of course.

Here's a nice article about (just a quick search on google), and has
a little program for you to test the combinations for your keyboard.

I know there should be some programs to assign certain combinations
of keys to one, but none came to my mind. =S

#EDIT

You could try with the following program: AutoHotkey
And check this thread, to try and create the macro you want.

 

[Abyssal]

Thanks for your reply!
I've already solved my current problem, but this autohotkey program will be very useful in future
Thanks a lot