Game Runs Too Fast in DOSBox? Fix the Cycles — In Plain Words

What “cycles” means

DOSBox emulates — that is, it copies in software — the CPU (the computer’s main chip), the sound and graphic cards, and other peripherals (add-on parts) of a PC, all at the same time.

The manual says: “The speed of an emulated DOS application depends on how many instructions can be emulated, which is adjustable (number of cycles).”

The wiki says it the same way in plain numbers: cycles is the “Amount of instructions DOSBox tries to emulate each millisecond.”

So: more cycles means a faster pretend computer. Fewer cycles means a slower one. A game that runs too fast usually just needs fewer cycles.

Why the game runs too fast

By default, the setting is cycles=auto. The manual explains what that means:

“By default (cycles=auto) DOSBox tries to detect whether a game needs to be run with as many instructions emulated per time interval as possible (cycles=max, sometimes this results in game working too fast or unstable), or whether to use fixed amount of cycles (cycles=3000, sometimes this results in game working too slow or too fast).”

In plain words: on auto, DOSBox guesses. Sometimes it guesses “as fast as possible,” and the manual itself says that can make a game work too fast or unstable. The fix is to stop the guessing and set a fixed number yourself.

The wiki adds the same warning for a fixed number that is set too high: “If this value is too high some games will run too fast or crash.”

Quick fix — slow it down while the game runs

The manual’s list of special keys says:

• CTRL-F11 — “Slow down emulation (Decrease DOSBox Cycles).”
• CTRL-F12 — “Speed up emulation (Increase DOSBox Cycles)”.

So if a game is racing, hold CTRL and tap F11 until it feels right. If you go too slow, CTRL-F12 raises the cycles again.

In a fixed-cycles setting, the manual says you can reduce the amount of cycles even more by hitting CTRL-F11 — “you can go as low as you want” — or raise it by hitting CTRL-F12 as much as you want, but you will be limited by the power of one core of your computer’s CPU.

Lasting fix — set a fixed number in the configuration file

The manual says: “you can always manually force a different setting in the DOSBox's configuration file.”

The configuration file is DOSBox’s settings file. The manual says it “is automatically created the first time you run DOSBox,” and you can find it here:

• Windows: “Start/WinLogo Menu”->“All Programs”->DOSBox-0.74-3->Options
• Linux: ~/.dosbox/dosbox-0.74-3.conf
• MAC OS X: “~/Library/Preferences/DOSBox 0.74-3 Preferences”

In that file, find the cycles line and set a fixed amount. The manual’s own example is:

cycles=10000

The manual explains what you will see: “If you for example set cycles=10000, then DOSBox window will display a line ‘Cpu Speed: fixed 10000 cycles’ at the top.”

The wiki gives a smaller example, too: cycles=fixed 5000 and cycles=5000 both mean “All games you start are run with a fixed speed of ~5 MIPS.” The wiki calls this “Useful for speed sensitive games or games that need a continuous CPU speed,” and notes: “You can change the actual value with Ctrl+F11 and Ctrl+F12 while DOSBox runs.”

There is no one right number for every game. Start with the manual’s example, test the game, then nudge the number down with CTRL-F11 or up with CTRL-F12 until the speed feels right, and write the number that worked into the file.

Another way — change cycles without editing the file

Inside DOSBox, the built-in CONFIG program can change settings while DOSBox runs. The manual’s example is:

config -set "cpu cycles=10000"

The manual describes that example as: “To set the cpu cycles to 10000”.

What cycles=max does — and why it can be too fast

You can also force the fast behavior by setting cycles=max in the DOSBox configuration file. The manual says the DOSBox window will then display a line “Cpu Speed: max 100% cycles” at the top, and that in this mode DOSBox “will always use 100% of your real CPU's one core.” In this mode you can reduce the amount of your real CPU’s core usage by CTRL-F11 or raise it with CTRL-F12.

For a game that already runs too fast, cycles=max is the setting to move away from, not toward — the manual ties cycles=max to the “game working too fast or unstable” result quoted above.

If the sound starts to skip

That can be a sign the cycles number is higher than your real computer can keep up with. The wiki says of a fixed value: “If the value is too high for your CPU the emulation will slow down and the sound starts to skip.”

The manual’s sound answer says the same thing from the other side: if “The sound stutters or sounds stretched/weird,” “You may be using too much CPU power to keep DOSBox running at the current speed. You can lower the cycles” — and it also lists skipping frames, reducing the sampling rate of the sound device, and increasing the prebuffer as other options in the configuration file.

For careful tinkerers — the advanced cycles line

The manual says the cycles=auto and cycles=max settings can be given extra parts, so real mode games (older-style games) and protected mode games (newer-style DOS games) can start at different speeds. Its example is:

cycles=auto 5000 80% limit 20000

The manual explains that line word for word: it “will use cycles=5000 for real mode games, 80% CPU throttling for protected mode games along with a hard cycle limit of 20000”.

Most people with one too-fast game do not need this line. The fixed number plus CTRL-F11 and CTRL-F12 covers the fix the manual points to first.