Nobody (including me) uses Xebra/Arbex because there is no proper English translation. I decided to finally give it a shot after reading your posts.
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First, you
MUST select a controller (As highlighted in the pic). In order to change the key used, click on the button you want to change and then hold down the key you would rather use to select and confirm it.
While we're at at, I noticed that all of my games ran like **** in this emu. I translated the included PDF document and found that the numbers in the video settings control skip rates. Setting all to '0' basically turns frame skipping completely off.
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Finally, under the 'Run' menu there's 'Run1', 'Run2', and 'Run3'. You execute a run command after you've loaded your CD or your disc image (from the CD drive via File > Open > CD-ROM via STPI or File > Open > CD-ROM Image respectively).
Run1 is interpreter mode. This is the most accurate mode and the most CPU intensive. Generally used for debugging. I can run it fullspeed on my system.
Run2 is single pass dynarec mode. Most people should try to use this mode as it is faster than interpreter mode and almost as accurate (emulation-wise).
Run3 is 2-pass dynarec mode. Even faster than single pass dynarec, but considerably less accurate. May cause bugs.