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 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.

  Windows 95
     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
     device" error.

     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  Copy the disk to
          mkfile 100M     Enlarge to 100 megabytes
          boot disk0.mnx            Boot the old "disk"
          [ESC]                     Get the attention of the monitor
           /dev/dosd5 becomes
          login: root
          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.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
     was copied.

     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:

          cd /usr/tmp
          echo >junk
          while cat junk >>junk; do :; done
          rm junk

     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.

  FAT driver
     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 (