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Questions about ALT+F12 (turbo button / fast-forward) feature

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First post, by rfnagel

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I've noticed this (at least on my PC) that:

I can press ALT+F12 to unlock the speed (turbo button/fast forward), which of course, fast-forwards/speeds up the DOSBox emulation considerably (a "guess-stimate" of about 2x the normal speed).

What I discovered is, if I keep holding down the ALT key and quickly release and re-press the F12 key, the speedup is even MORESO (another "guess-stimate" of about 3x the normal speed).

Still holding down the ALT key, and for a third time quickly releasing and re-press the F12 key, results in the emulation skipping frames and really chugging down; I'm sure due to it reaching the maximum capibilites of my CPU (Intel P4 2.666Ghz).

Anyhoo, is this by design, possibly an undocumented Fast/FASTER/**EVENFASTER** <grin> sort of feature? If so, quite a cool thing indeed 😀

Lastly, in the DOSBox readme it says:

"NOTE: You need free CPU resources for this (the more you have, the faster it goes), so it won't work at all with cycles=max or a too high amount of fixed cycles."

I have "cycles=max" set, and the ALT+F12 (as described above) indeed does function 😀

P.S. This seems to work in both DOSBox version 0.74, as well as in version 0.73.

Rich ¥Weeds¥ Nagel
http://www.richnagel.net

Reply 1 of 5, by ykhwong

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a too high amount of fixed cycles.

Setting cycles to too high fixed values really won't work, but

with cycles=max

this statement is wrong. I created a patch for it long time ago and it seems to be already applied to current DOSBox source.
http://sourceforge.net/tracker/?func=detail&a … 551&atid=467234

If cycles value is set to max, DOSBox will try to decrease maximum cycles to 1/3 while pressing Alt-F12. This makes it function as expected.

static void DOSBOX_UnlockSpeed( bool pressed ) {
	static bool autoadjust = false;
	if (pressed) {
		LOG_MSG("Fast Forward ON");
		ticksLocked = true;
		if (CPU_CycleAutoAdjust) {
			autoadjust = true;
			CPU_CycleAutoAdjust = false;
			CPU_CycleMax /= 3;
			if (CPU_CycleMax<1000) CPU_CycleMax=1000;
		}
	} else {
		LOG_MSG("Fast Forward OFF");
		ticksLocked = false;
		if (autoadjust) {
			autoadjust = false;
			CPU_CycleAutoAdjust = true;
		}
	}
}
rfnagel wrote:

What I discovered is, if I keep holding down the ALT key and quickly release and re-press the F12 key, the speedup is even MORESO (another "guess-stimate" of about 3x the normal speed).

Was that with cycles=max or auto? Somewhat decreased cycles might speed up more at a moment.

Reply 2 of 5, by rfnagel

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ykhwong wrote:

Was that with cycles=max or auto? Somewhat decreased cycles might speed up more at a moment.

Yes, I had "cycles=max" in my DOSBox.conf.

Rich ¥Weeds¥ Nagel
http://www.richnagel.net

Reply 4 of 5, by igermi

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edit: I did notice that my dosbox ece works the same way as you describe above. If I release and re-hit f12 it goes much faster. After the 4th time it appears to freeze. I hit f11 and it "woke up" again, nearly 4 minutes ahead in game time waiting just a matter of seconds in real time.

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