Creating and Using MS DOS Virtual Machine in QEMU

Create a disk image of desired size. I chose 50 MB

$ dd if=/dev/zero of=msdos622.img bs=1k count=0 seek=50000

Next boot from the DOS boot media. I had a bootable CD image.

$ qemu -hda msdos622.img -cdrom DOS6.22_bootdisk.iso -boot d

From the DOS prompt, create a partition on drive C using fdisk

A:\> fdisk

After the partition is created the machine will be rebooted. Once you get the DOS prompt back format the drive C and make it bootable:

A:\> format c: /s

The /s option of format command transfers the system to the drive.

Next create a directory DOS in C:\ and copy all the utilities into it:

A:\> mkdir c:\dos
A:\> copy *.* c:\dos

DOS is ready to be used. Now in order to transfer your old games and programs, you need to mount it as loopback device in linux. There is one problem with this. Since the image file has a partition table in it, when I tried to mount it, I got the following errors:

$ sudo mount msdos622.img /mnt -o loop -t msdos
mount: wrong fs type, bad option, bad superblock on /dev/loop0,
       missing codepage or helper program, or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

To work this around we need to calculate the offset size of the partition using the fidisk -ul command:

$ fdisk -ul msdos622.img
You must set cylinders.
You can do this from the extra functions menu.

Disk msdos622.img: 0 MB, 0 bytes
16 heads, 63 sectors/track, 0 cylinders, total 0 sectors
Units = sectors of 1 * 512 = 512 bytes
Disk identifier: 0x00000000

       Device Boot      Start         End      Blocks   Id  System
msdos622.img1   *          63       98783       49360+   6  FAT16

To calculate the offset we need to multiply the starting block (63 in our case) by 512. If the image has multiple partitions then the same approach can be used and each partition can be individually mounted on a separate mount point.

$ sudo mount -o loop,offset=32256 msdos622.img /mnt
$ ls /mnt  dos  drvspace.bin    io.sys    msdos.sys

Now you can copy your old dos games and other files to /mnt.

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