How I run virtual machines with QEMU

Use this lightweight open source emulator to run classic DOS programs on today’s Linux systems.

I don’t run Linux all the time. Sometimes, I like to run other operating systems too, like FreeDOS, an open source implementation of the classic DOS operating system. My quad-core Intel i3-8100T CPU (3.10GHz) with 32GB and 256GB Solid State Drive from December 2019 is probably just a tad overpowered to run a DOS operating system designed for the 1980s and early 1990s when computers had just one CPU and counted memory and disk space in mere megabytes instead of gigabytes. That’s why I don’t run FreeDOS on real hardware; instead, I boot FreeDOS in a virtual machine on my Linux system, so I can run FreeDOS in a “window.”

I like the QEMU virtual machine emulator. It’s open source, and runs great on Linux. Here’s how I use QEMU to boot FreeDOS on Linux:

Booting the LiveCD

If I want to boot the FreeDOS LiveCD without installing it, I could just run QEMU with the LiveCD. Let’s do that, to test that we can actually run QEMU as a virtual machine.

Where other virtual machine applications like VirtualBox use a graphical interface to provide settings, QEMU does everything on the command line. Fortunate, we only need a few command line options to get started:

• qemu-system-i386 emulates a 32-bit system.
• Use -cdrom to specify a path to the FreeDOS LiveCD.
• Add -enable-kvm to use the Linux kernel’s Virtual Machine subsystem. Technically, you don’t need to provide this, but QEMU will run really slow without it.

$ qemu-system-i386 -enable-kvm -cdrom FD14LIVE.iso

And that’s all we need to start. The virtual machine will look for a bootable device, which is usually the first floppy drive (which isn’t defined in this command line) and then the first hard drive (also not listed on the command line). As a fallback, the system will boot from the first bootable CD-ROM drive, which we’ve provided as the LiveCD.

After FreeDOS boots from the LiveCD, you’ll find yourself at a DOS command line where you can use FreeDOS from the CD-ROM filesystem.

Read more: 5 FreeDOS editors I love

Build a virtual machine at the command line

I like using QEMU to boot my virtual machine because the command line makes it possible to create any configuration I want. For example, after I installed FreeDOS 1.4 on a virtual disk in QEMU, I added other command line options to emulate an old-style PC speaker, using the Linux PulseAudio system to make the monaural beeps. I also added other options to emulate a classic Sound Blaster 16 sound card, and Ad Lib digital music card, also using Linux PulseAudio for the output:

-audiodev pa,id=snd -machine pcspk-audiodev=snd -device sb16,audiodev=snd -device adlib,audiodev=snd

I found that QEMU works great for almost every classic DOS application. The only exception was the legacy Borland compilers; using the arrow keys on my keyboard is like a “double tap” in DOS. I was able to fix this using a QEMU keyboard throttle:

-global i8042.kbd-throttle=on

I have other command line options to do other things. My full command line also defines the memory with -m, uses a real time clock with -rtc, and adds two hard disks with -hda and -hdb:

$ qemu-system-i386 -enable-kvm -m 32 -rtc base=localtime -audiodev pa,id=snd -machine pcspk-audiodev=snd -device sb16,audiodev=snd -device adlib,audiodev=snd -global i8042.kbd-throttle=on -hda freedos.qcow2 -hdb files.qcow2 -cdrom T2506BNS.iso

That’s quite a long command line, so I saved it in a Bash script called freedos. Now, whenever I want to run FreeDOS in a “window” on Linux, I just run the freedos command, and I can run my favorite DOS programs, or write new ones.

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