ms-dos-on-dosbox.md

MS-DOS on DOSBox

Reviving the operating system that won out through skullduggery

As I chose to go about doing this, I first created a directory dedicated to the task in question.

Then, therein, I created an image that serves as a virtual hard drive. 256 MB is a sensible default. This can be done accordingly:

dd if=/dev/zero of=ms-dos.img bs=1M count=256

The next thing I did was to prepare a DOSBox configuration file for MS-DOS. I wanted to keep it separate from the main DOSBox configuration file so that I can launch MS-DOS programs without having MS-DOS launch. As of this writing, the location of the main DOSBox configuration file was ~/.dosbox/dosbox-0.74.conf. I copied it into my MS-DOS directory with the name dosbox-0.74-ms-dos.conf.

Before making any changes to this configuration file, I needed to procure images of the MS-DOS 6.22 installation disks. There is a great website called WinWorld that has the images for MS-DOS 6.22 Plus Enhanced Tools. This will be downloaded as a 7zip archive. I used p7zip -d to extract it and then moved the following files from the directory that was created by extracting the archive into the parent MS-DOS directory:

• Disk1.img
• Disk2.img
• Disk3.img
• Suppdisk.img

The disk images now being immediately available, the next step was to alter the section all the way at the bottom of the configuration file, [autoexec]. Lines after [autoexec] will be treated as though they were DOSBox commands in an actual AUTOEXEC.BAT. This is what I added there:

mount Z .
Z:
imgmount 2 ms-dos.img -size 512,63,16,520 -t hdd -fs none
boot Disk1.img Disk2.img Disk3.img -l a

It's not clear what the purpose of 2 after imgmount is. The only information I can seem to find without digging through the DOSBox source code is that it signifies master (as opposed to slave) drive. As far as I understand things, the first three comma-delimited numbers following -size never change, but the fourth does if one opts for a different image size than 256 MB. If a different size is desired, multiply the desired number of megabytes by 2.03125 and (I guess) round to the nearest integer.

In any case, what these lines do, essentially is:

• Make the current directory the Z: drive
• Switch to the Z:
• Prepare the image as a blank hard drive
• Turn on the virtual machine and make the MS-DOS 6.22 installation disks available to boot from the A: drive.

With these preparations in place, I started the virtual machine like so:

dosbox -conf dosbox-0.74-ms-dos.conf &

From here the installation was fairly straightforward. I just did what made sense and got the message "Reboot requested, quitting now". It didn't look like the installation had completed so I thought an error occurred. It wasn't an error though; all I had to do was repeat the above launch of DOSBox. From there, installation was a breeze. The only issue is that, because a physical floppy drive and floppy disks aren't present here, DOSBox needs to be told which image to use. When I was asked for disk 2, I pressed Ctrl-F4 to load the second image and again when disk 3 was asked for.

But wait! The supplemental disk still hasn't been installed. Now that MS-DOS has been installed, the [autoexec] section needs to be changed to this:

mount Z .
Z:
imgmount C ms-dos.img -size 512,63,16,520 -t hdd -fs fat
boot Suppdisk.img -l c

The system will now boot from the C: drive rather than the A: drive. Apparently, DOSBox will assign Suppdisk.img to the A: drive by default. To install the supplemental disk, I typed A: to switch to the A: drive (because this has to be done; can't be done from the C: drive) and then SETUP C:\DOS. VGA was the appropriate graphics setting and I overwrote all the files. After this step is done, Suppdisk.img after boot can be removed from the configuration file as it is no longer needed.

Having followed all these steps, I finally attained a full installation of the last official standalone release of one of the mostly widely used operating systems ever released. But there was one glaring issue remaining: how can I install other software? If I could find installation disk images, then it would be no harder than what has already been done. But the software I would most like to use in an MS-DOS environment (i.e. games) is usually not distributed in this format. Because getting Internet to work on MS-DOS appears to be a very sketchy prospect, I would need to access the contents of the image directly. After doing a little poking around, I found a straightforward solution. I created a directory called mnt in the main MS-DOS directory and mounted the image, like so:

sudo mount -t msdos -o loop,offset=32256 ms-dos.img mnt

Why offset=32256? Here is the output of running fdisk -l ms-dos.img:

Disk ms-dos.img: 256 MiB, 268435456 bytes, 524288 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x00000000

Device      Boot Start    End Sectors   Size Id Type
ms-dos.img1 *       63 524159  524097 255.9M  6 FAT16

The start sector, numbered 63, has to be multiplied by the sector size, 512, so that mount knows where to look for the partition.

And why sudo? Only root users can mount with options. Additionally, all of the files in the image will have root ownership and this can't be changed, so sudo will be required for any installations or removals to be done. When direct access to the MS-DOS image is no longer desired, use sudo umount mnt to release that access.

Having followed all these steps, I finally attained a full MS-DOS installation to which new software can be added. I'm still having an issue with installing new software from multiple disk images but I may be able to clear that hurdle as well at some point in the future.