Some like it hot

DOS guest in VirtualBox is heating up the host machine

Have you ever wondered why the fan starts running when you run a DOS or FreeDOS guest in VirtualBox for a few minutes? Just see for yourself:

Windows XP with VirtualBox 3.2.10

Ubuntu 10.04 with VirtualBox 4.0.12

Mac OS X 10.6.8 with VirtualBox 4.1.4

In all screenshots you see a VirtualBox FreeDOS guest running FreeDOS Edit with no open document. The appropriate instruments (Task Manager, System Monitor, Activity monitor) of the operating systems show that one processor of the host machine is completely occupied by the VirtualBox guest.

The host machines are a Thinkpad X60T (Windows, Ubuntu) and a MacBook Pro (2011).

MS-DOS has the same problem

Of course it is kind of absurd to think a program like FreeDOS Edit, which has a size of 64kB, requires the full power of a Core2Duo or an i5. I doesn't need it. It is just a common problem of the DOS operating system and DOS programs, that becomes obvious if you run them in VirtualBox. MS-DOS has the same problem:

MS-DOS 6.22 running as guest in VirtualBox 4.1.4

As I have learned, the problem has something to do with the HLT instruction and the IDLE mode of the CPU. As far as I understand it, DOS programs seem to forget to signal the CPU that they don't need it any more. VirtualBox on the other hand gives these programs unlimited access to the CPU and reserves it for them. This seems a bit careless to me.

Warning!

I seriously doubt that it is healthy for any computer to run for a longer time with 100% CPU usage.

If I have a DOS guest running for some time in VirtualBox, with full CPU usage, OS X eventually will pop up a nice half transparent window. It tells me that my computer needs now to be shut down and that I should push the power button long enough to do so. It's the Mac version of the BSOD.

If the Mac automatically shuts down to prevent damage,  I think it is correct to assume that running a DOS/FreeDOS guest in VirtualBox can cause damages.

Possible Solutions: 1. Settings in VirtualBox

First of all limit the processor usage for your DOS/FreeDOS guest. This can be easily done in VirtualBox.

I can recommend a setting of 40%. I can not feel any difference in the look and feel of the DOS guest. The computer will not get as hot as before and the fan won't run as much as with a CPU usage of 100%.

2. Use DOS Power Management 

Using power management solves the problem as long as you use the shell / work at the DOS prompt.

To use power management in MS-DOS, edit your CONFIG.SYS to have a line:

C:\DOS\POWER.EXE

To use power management in FreeDOS, edit your AUTOEXEC.BAT to have a line:

LH FDAPM APMDOS

Unfortunately the CPU usage may go up again if you start other programs.

3. Use better DOS software

As Marcos Favero Florence de Barros writes on the FreeDOS user mailing list, there are several DOS programs that work correctly in VirtualBox and do not heat up your machine. He mentions these examples:

Norton Commander clones: DOSZIP, DOS Navigator, NDN and CONNECT.

As you see in the picture above, you have to activate  in DOSZIP a setting to prevent it from using 100% of your CPU in VirtualBox: Got to Setup, System Options and check the "Use DOS Idle Int 2Fh" checkbox.

Editors: He suggests Aurora, FTE, MinEd, PEdit, SetEdit, TDE and WordPerfect 6.

Viewers: For instance: "List".

4. Try to use a DOS TSR that controls the CPU usage of applications

Another tip of Marcos is the use of a DOS TSR (Terminate and Stay Ready) program, that runs in the background and limits the CPU usage of other applications.

One of them is the shareware TAME, which is still continued and can be tried for 30 days.

The other one is called DPAKBD which seems to be freeware (but not Open Source).

I just looked into DPAKBD, but found that the applications get too sluggish. But this was just a short experiment.

Conclusion

Running a DOS or FreeDOS guest in VirtualBox can get a hot experience. Take care to protect your host machine by limiting the processor usage for the guest. It is also a good idea to enable Power management. It may be a bit inconvenient, but try to use other programs than those you are used to, if you want to avoid loud fan noises and possible problems with your hardware.

The good news: Other than MS-DOS, FreeDOS is still developed and its developers are aware of the problem and are trying to solve it.