dosminix, mkfile - Running Minix under DOS
C:\MINIX> boot disk0.mnx (Typical example)
C:\MINIX> mkfile size disk
This text describes running Minix under DOS. The DOS version of the Boot
Monitor, described in monitor(8), grabs as much memory as DOS is willing
to give, loads Minix into that memory from the active partition of a
"file as disk", and jumps to the Minix kernel to let Minix take control.
As far as DOS is concerned Minix is just a part of the boot.com program.
In the example above disk0.mnx is the "file as disk". It is a file of
many megabytes that is used by Minix as a disk of four partitions. These
partitions will normally be /dev/dosd1 through /dev/dosd4, with
/dev/dosd0 for the whole "disk". The Boot Monitor will set the dosd0
boot variable to the name of the disk (its first argument), the root file
system will be the active partition, usually dosd1. It is better to use
the special name bootdev to indicate this device, usually in the setting
Once Minix is running it will operate the same as if started from a
regular disk partition until it is shut down. On shutdown from protected
mode it will return to the Boot Monitor prompt, and with the exit command
you leave the Boot Monitor and return to DOS. Shutting down from real
mode will reboot the machine, just like when run from a disk partition.
(This more or less crashes DOS, but DOS is used to such abuse.)
Minix can't run in protected mode (286 or 386 mode) if DOS is using a
memory manager like EMM386. You can either temporarily comment out
EMM386 from CONFIG.SYS, or you can press F8 on startup to bypass
CONFIG.SYS. This is only possible with the later DOS versions.
Press F8 at startup to make the boot menu visible. Choose "Command
prompt", or "Safe mode command prompt" to run DOS. Use the "safe mode"
if EMM386 is started in CONFIG.SYS.
Typing F8 at the right moment isn't easy, so you may want to change the
way Windows boots by editing the MSDOS.SYS file found in the root
directory of your Windows system. This is alas not trivial. Open a
window on your main drive, click on "View" and choose "Options." In the
Options window choose "View" and enable "Show all files". The MSDOS.SYS
file should now be visible, among several other hidden files. Right-
click on the MSDOS.SYS icon, choose "Properties" and disable "Read-only".
Bring MSDOS.SYS into a simple text editor such as Notepad. In the
[Options] segment add the following lines (or change existing lines
The first setting makes the Windows boot menu always visible, and the
second line changes the delay before booting to 5 seconds. Take care not
to change anything else, or things will go horribly wrong. Save
MSDOS.SYS and exit. Don't forget to make MSDOS.SYS read-only again, and
also hide all the hidden files again, unless you like it this way.
DOS compatibility box
The 16-bit version of standard Minix can be run in real mode in a DOS
box. This is somewhat surprising, because it means Windows 95 simulates
devices like the keyboard, timer, and interrupt controller well enough to
fool Minix into thinking that all is well. Alas it doesn't work as well
under Windows NT. Keypresses get lost if you type to fast, and using the
floppy occasionally locks Minix up. This is a bit disappointing, because
it is the only way to run Minix under NT. Under Windows 95 one is better
off putting the system in DOS at boot and then to run Minix in protected
One thing that is better under NT is that the Boot Monitor is able to get
a so-called "Upper Memory Block", thereby raising useful memory to about
750K. Windows 95 however hogs leftover UMB memory in a process named
vmm32, whatever that may be. To get some of this memory you can put BOOT
/U at the start of autoexec.bat. The monitor will grab a 64K UMB if it
can get it, and keep that memory safe for use by Minix when it is later
started from Windows.
The easiest way to start Minix is to give all Minix disk files the suffix
MNX. Doubleclick on the disk you want to run to make the "Open With"
window appear. Click on "Other" and browse to the BOOT.COM program. Set
the name of the .mnx files to "Minix "disk" file" in the description box
if you want everything right. In the future you can just click on a
Minix disk file to run it, you don't have to start a DOS box first. (To
make it perfect use "View", "Options", "File Types", choose "Minix "disk"
file", "Edit", "Change Icon", "Browse", select MINIX.ICO.)
When Minix shuts down it will try to reboot what it thinks is a PC.
Windows seems to assume that the DOS session has exited. Right-click on
the BOOT.COM program, "Properties", "Program", and enable "Close on exit"
to make the DOS box disappear automatically when Minix thinks it reboots.
You may also want to lock the font to 7x12, or any other font that isn't
Minix disk files are opened in a write-exclusive mode. A second Minix
session can only open it read-only, which may lead to a "can't open root
Minix disk files can be created or resized with the mkfile utility. Its
two arguments are the size and name of the disk file. The size is a
number optionally followed by the letter k, m or g to specify kilobytes,
megabytes, or even gigabytes. So the call
mkfile 50m disk5.mnx
will create a 50 megabyte file named disk5.mnx. If the file already
exist then it is shrunk or grown to 50 megabytes. No data is lost if the
file is grown. If the file is shrunk then only the data that is cut off
is lost. These features allow one to inrease the size of a Minix /usr
partition with the following recipe:
copy disk0.mnx disk0.new Copy the disk to disk0.new
mkfile 100M disk0.new Enlarge to 100 megabytes
boot disk0.mnx Boot the old "disk"
[ESC] Get the attention of the monitor
dosd5=disk0.new /dev/dosd5 becomes disk0.new
part Choose dosd5, move to the Size field of
dosd7 partition, hit 'm' to fill it out to
the end of the "disk". Write and quit.
mkfs /dev/dosd7 Recreate the file system, but larger
mount /dev/dosd7 /mnt
cpdir -v /usr /mnt Copy /usr to the new disk's /usr to be
shutdown Back to the monitor
exit Back to DOS
ren disk0.mnx disk0.old
ren disk0.new disk0.mnx Replace old by new
boot disk0.mnx Run the larger system
Now Minix runs from a larger "disk". Don't worry if it claims to have
crashed, there wasn't a "shutdown" entry in /usr/adm/wtmp at the time it
The above recipe is for a ordinary standard Minix installation with /usr
on the second and last partition.
In the recipe above you saw how simple it is to create a new system, just
copy a disk file. It is equally simple to make a backup, you just copy
the disk file. To make a test system: copy the disk file. To make
another test system: copy the disk file. Let friends have their own
Minix: copy the disk file again. (Exciting, eh?)
You may want to save a Minix disk file in a ZIP file to save space. It
may look as a good idea to first run make clean in /usr/src to remove all
the binary junk, but alas that has no effect at all. The disk file is
compressed under DOS, and there it is unknown which blocks are in use and
which are free. With the following trick you can make those deleted
blocks compress really well:
while cat junk >>junk; do :; done
After these commands all free blocks contain newlines. Long runs of the
same byte happen to compress by a factor 1000, so the unused disk blocks
will almost disappear in the ZIP file.
The dos disk driver, described in dosd(4), has two identities. By
default you get the "file" driver, that uses DOS file I/O calls to access
a large DOS file as a disk. The other alternative is the "FAT" driver.
The FAT driver sits on top of an ordinary Minix disk driver, and
interprets a partition as a FAT (File Access Table) file system to find a
file to use as a Minix disk. The result has the same effect as the file
driver, except that no costly calls to DOS are made. To enable this
feature you have to use the following Boot environment settings:
dosd = fat
dosd0 = hd1:\minix\disk0.mnx
The dosd setting tells Minix to use the FAT driver, and the dosd0 setting
tells the Minix device and DOS file name to use. Disk I/O should be sped
up nicely by this change, although typical use of Minix doesn't require
fast disk I/O, so the difference won't be too noticable.
Support for FAT-32 (big file system support added in the later Windows 95
releases) has not been tested very well. The FAT-12 and FAT-16 code has
been used a lot, and seems safe. Note the risks inherent in these
drivers: The file driver uses simple DOS file I/O calls, leaving it to
DOS to know its own file system. The FAT driver interprets FAT file
system structures by itself. Minix booted from a real hard disk
partition can only use DOS disk files through the FAT driver.
dosd(4), monitor(8), usage(8).
Use at your own risk.
Hasn't been tried under Windows 98 yet.
Pray the deity of your choice will forgive you for running a UNIX-like
system as an ordinary DOS program. The author of this code is already
doomed. When his time comes the daemons wi*(&%*$%*&
Memory fault - core dumped
Kees J. Bot (email@example.com)