CPU

The cpu_cycles setting controls the speed of the emulated CPU; it's the single most important performance setting in dosbox-automation. Getting it right can mean the difference between a game that plays perfectly and one that stutters, crawls, or races through its intro sequence.

Too many cycles and you'll get sound stuttering and input lag as your host CPU struggles to keep up. Too few and the game turns into a slideshow. Finding the sweet spot for each game is part of the charm of retro gaming.

As a rough guide: early 8088-era games (Alley Cat, Sopwith) need around 300 cycles. 386-era titles want roughly 3,000--6,000. 486-class games like Doom are playable at around 25,000 but might need up 50,000 cycles for a completely smooth experience. Late Pentium-era titles such as Quake need 50,000 to 100,000 cycles or even more. See the cpu_cycles table for further details, or the Tutorials guide for a practical walkthrough of finding the right speed.

The auto setting for core is the most sensible default - it uses the faster dynamic recompiler for protected-mode games and the more accurate normal interpreter for real-mode programs. You'll rarely need to override this.

Protected and real mode

DOS games released before about 1993 use real mode (the legacy 16-bit programming model of the Intel 386), while games from 1993 onwards almost exclusively use protected mode (full 32-bit operation). The important distinction is that most CPU-hungry games are protected mode games (e.g., FPS games and flight simulators), while older real mode games generally have much less demanding performance requirements.

dosbox-automation detects the current mode automatically and applies different default speeds:

• Real mode: 3000 cycles/ms (roughly a 386SX at 20 MHz)
• Protected mode: 60,000 cycles/ms (roughly a Pentium at 90 MHz)

The conservative real mode default prevents older speed-sensitive games from running too fast. The higher protected mode default gives demanding games the headroom they need.

You can easily tell which mode a game uses by watching the cycles value in the dosbox-automation window's title bar - it will show 3000 in real mode and 60,000 in protected mode (assuming default settings). You can also spot protected mode games by the presence of DOS extenders such as DOS4GW.EXE, PMODEW.EXE, or CWSDPMI.EXE in their game directories.

Finding the correct cycles setting for a game

While the defaults get most games running, manual tweaking is often needed to make a game run smoothly. Setting the cycles too high wastes host CPU power that could be used for glitch-free audio emulation - there's no benefit to emulating a faster CPU than the game actually needs.

Use the cpu_cycles table as a starting point: look up what CPU the game was designed for, set the corresponding cycles value, then fine-tune from there:

• Early DOS games from the 1980s usually run fine at 3,000 cycles (386SX/20 level)
• For 2D games from the 1990s, 25,000 cycles (486DX2/66 level) handles virtually everything
• 3D games typically need 50,000--100,000 cycles (Pentium range)
• 3D SVGA gaming at 640×480 or above may require 200,000+ cycles

You can adjust the cycles on the fly while playing with Ctrl+F11 to decrease by 20% and Ctrl+F12 to increase by 10% (Cmd+F11 and Cmd+F12 on macOS). Once you've found a good value, update your config to match.

Always aim for the lowest cycles value that gives adequate performance --- overdoing it only increases the chance of audio glitches and wastes host CPU resources.

For a hands-on walkthrough of finding the right cycles setting for specific games, the Tutorials guide walks you through a few a practical examples:

Configuration examples

Fixed speed globally - Roughly emulates an 486DX2/66 in both real and protected mode:

[cpu]
cpu_cycles = 25000
cpu_cycles_protected = auto

Different real and protected mode speeds - 20,000 cycles for real mode, 400,000 for protected mode, with throttling enabled so the host CPU doesn't get overloaded:

[cpu]
cpu_cycles = 20000
cpu_cycles_protected = 400000
cpu_throttle = on

Max speed - As fast as your host CPU can handle. Useful when compiling programs, rendering 3D images, or playing late-90s 3D games at high resolutions:

[cpu]
cpu_cycles = max
cpu_cycles_protected = max

CPU type

The cputype setting controls which CPU architecture is emulated. The default auto is the most compatible option - technically a 386 with 486 extensions and CPUID. Only change it if a specific game requires it.

• 386_prefetch is needed for games that use self-modifying code or anti-debugging tricks (requires core = normal). Known games: Contra, FIFA International Soccer, Terminator 1, X-Men: Madness in The Murderworld.
• pentium is recommended for Windows 3.1 games (e.g., Betrayal in Antara) as it adds RDTSC instruction support.
• pentium_mmx adds MMX instructions, mostly useful for demoscene productions.

CPU speed

The cycleup and cycledown settings control how much the CPU speed changes when you press the increase/decrease hotkeys. Values below 100 are treated as a percentage change; values of 100 or above are treated as a fixed number of cycles to add or subtract.

Idle CPU usage

The cpu_idle setting reduces host CPU usage when DOSBox is idle (e.g., waiting for input at the DOS prompt) by emulating the HLT instruction. It's enabled by default. Disable it if it conflicts with DOS power management tools like DOSidle or FDAPM.

Configuration settings

You can set the CPU emulation parameters in the [cpu] configuration section.

core

Type of CPU emulation core to use.

Possible values:

• auto (default) -- normal core for real mode programs, dynamic core for protected mode programs. Most programs will run correctly with this setting.

• normal -- The DOS program is interpreted instruction by instruction. This yields the most accurate timings, but puts 3--5 times more load on the host CPU compared to the dynamic core. Therefore, it's generally only recommended for real mode programs that don't need a fast emulated CPU or are timing-sensitive. The normal core is also necessary for programs that self-modify their code.

• simple -- The normal core optimized for old real mode programs; it might give you slightly better compatibility with older games. Auto-switches to the normal core in protected mode.

• dynamic -- The instructions of the DOS program are translated to host CPU instructions in blocks and are then executed directly. This puts 3--5 times less load on the host CPU compared to the normal core, but the timings might be less accurate. The dynamic core is a necessity for demanding DOS programs (e.g., 3D SVGA games). Programs that self-modify their code might misbehave or crash on the dynamic core; use the normal core for such programs.

cputype

CPU type to emulate. You should only change this if the program doesn't run correctly on auto.

Possible values:

cpu_cycles

Speed of the emulated CPU (3000 by default). If cpu_cycles_protected is on auto, this sets the cycles for both real and protected mode programs.

Possible values:

• <number> -- Emulate a fixed number of cycles per millisecond (roughly equivalent to MIPS). Valid range is from 50 to 2000000.
• max -- Emulate as many cycles as your host CPU can handle on a single core. The number of cycles per millisecond can vary; this might cause issues in some DOS programs.

Ballpark cycles values for common CPUs (treat as starting points, then fine-tune per game):

cpu_cycles_protected

Speed of the emulated CPU for protected mode programs only.

Possible values:

• auto -- Use the cpu_cycles setting.
• <number> -- Emulate a fixed number of cycles per millisecond (roughly equivalent to MIPS). Valid range is from 50 to 2000000.
• max -- Emulate as many cycles as your host CPU can handle on a single core.

The default is 60000.

cpu_throttle

Throttle down the number of emulated CPU cycles dynamically if your host CPU cannot keep up. Only affects fixed cycles settings. When enabled, the number of cycles per millisecond can vary; this might cause issues in some DOS programs.

Possible values: on, off (default)

cycledown

Number of cycles to subtract with the Dec Cycles hotkey (20 by default). Values lower than 100 are treated as a percentage decrease.

cycleup

Number of cycles to add with the Inc Cycles hotkey (10 by default). Values lower than 100 are treated as a percentage increase.

cpu_idle

Reduce host CPU usage when the DOS shell or an application is idle. With cpu_cycles set to max, this can reduce host CPU core usage from ~100% to around 25% when the shell is idle.

This also makes the shell more multitasking-friendly, which benefits Windows 3.1 DOS prompts.

The feature works by emulating the HLT CPU instruction, so it might interfere with third-party DOS power management tools such as DOSidle and FDAPM - disable it if you use those.

Possible values: on (default), off