MEMDISK

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What is MEMDISK?

MEMDISK is meant to allow booting legacy operating systems. MEMDISK can boot floppy images, hard disk images and some ISO images.

MEMDISK simulates a disk by claiming a chunk of high memory for the disk and a (very small - 2K typical) chunk of low (DOS) memory for the driver itself, then hooking the INT 13h (disk driver) and INT 15h (memory query) BIOS interrupts.


How can I use MEMDISK?

MEMDISK is an auxiliary module used in conjunction with a boot loader that can load linux kernels (EXTLINUX/ISOLINUX/PXELINUX/SYSLINUX, grub, grub4dos grub2, ...). You need a disk image as well as the memdisk file itself. As far as the bootloader is concerned, MEMDISK is the "kernel" and disk image is the initial ramdisk (initrd).

The disk image, passed as initrd, can be a compressed zip or gzip file.

In the next examples hdt.img is a floppy image (hdt.iso is a ISO image) containing Hardware Detection Tool (runs directly on SYSLINUX) and dosboot.img is a floppy image that contains some version of DOS.


EXTLINUX/ISOLINUX/PXELINUX/SYSLINUX

You can use MEMDISK straight off the boot loader command line like the following: 

memdisk initrd=hdt.img

... where, of course, hdt.img is the name of the disk image file. The memdisk file and the disk image need to be present in the appropriate location (for PXELINUX, on your TFTP server, for ISOLINUX, in the /isolinux directory on your CD, etc.)

Normally, however, you would put something like following in the configuration file: 

 # Boot Hardware Detection Tool from floppy image
 LABEL hdt_floppy
  LINUX memdisk
  INITRD hdt.img

 # Boot Hardware Detection Tool from iso image (with 'iso' parameter)
 LABEL hdt_iso
  LINUX memdisk
  INITRD hdt.iso
  APPEND iso

 # Boot DOS from floppy image (with 'raw' parameter)
 LABEL dos_floppy_with_raw
  LINUX memdisk
  INITRD dosboot.img
  APPEND raw

GRUB and GRUB4DOS

In your menu.lst file, use something like: 

 title Boot Hardware Detection Tool from floppy
   kernel /memdisk
   initrd /hdt.img

 title Boot Hardware Detection Tool from iso image (with 'iso' parameter)
   kernel /memdisk iso
   initrd /hdt.iso

 title Boot DOS from floppy image (with 'raw' parameter)
   kernel /memdisk raw
   initrd /dosboot.img

GRUB2

Add the following in your config scripts for grub2: 

 menuentry "Boot Hardware Detection Tool from floppy" {
   linux16 /memdisk
   initrd16 /hdt.img
 }

 menuentry "Boot Hardware Detection Tool from iso" {
   linux16 /memdisk iso
   initrd16 /hdt.iso
 }

 menuentry "Boot DOS from floppy image (with 'raw' parameter)" {
   linux16 /memdisk raw
   initrd16 /dosboot.img
 }

Supported image types

The image file should contain a disk image, either a floppy disk or hard disk image, or an iso image.

The disk image can be compressed with zip or gzip.


Floppy images

If the disk image is less than 4,194,304 bytes (4096K, 4 MB) it is assumed to be a floppy image and MEMDISK will try to guess its geometry based on the size of the file. MEMDISK recognizes all the standard floppy sizes as well as common extended formats: 

     163,840 bytes  (160K) c=40 h=1 s=8		5.25" SSSD
     184,320 bytes  (180K) c=40 h=1 s=9		5.25" SSSD
     327,680 bytes  (320K) c=40 h=2 s=8		5.25" DSDD
     368,640 bytes  (360K) c=40 h=2 s=9		5.25" DSDD
     655,360 bytes  (640K) c=80 h=2 s=8		3.5"  DSDD
     737,280 bytes  (720K) c=80 h=2 s=9		3.5"  DSDD
   1,222,800 bytes (1200K) c=80 h=2 s=15	5.25" DSHD
   1,474,560 bytes (1440K) c=80 h=2 s=18	3.5"  DSHD
   1,638,400 bytes (1600K) c=80 h=2 s=20	3.5"  DSHD (extended)
   1,720,320 bytes (1680K) c=80 h=2 s=21	3.5"  DSHD (extended)
   1,763,328 bytes (1722K) c=82 h=2 s=21	3.5"  DSHD (extended)
   1,784,832 bytes (1743K) c=83 h=2 s=21	3.5"  DSHD (extended)
   1,802,240 bytes (1760K) c=80 h=2 s=22	3.5"  DSHD (extended)
   1,884,160 bytes (1840K) c=80 h=2 s=23	3.5"  DSHD (extended)
   1,966,080 bytes (1920K) c=80 h=2 s=24	3.5"  DSHD (extended)
   2,949,120 bytes (2880K) c=80 h=2 s=36	3.5"  DSED
   3,194,880 bytes (3120K) c=80 h=2 s=39	3.5"  DSED (extended)
   3,276,800 bytes (3200K) c=80 h=2 s=40	3.5"  DSED (extended)
   3,604,480 bytes (3520K) c=80 h=2 s=44	3.5"  DSED (extended)
   3,932,160 bytes (3840K) c=80 h=2 s=48	3.5"  DSED (extended)

A small perl script is included in the MEMDISK directory which can determine the geometry that MEMDISK would select for other sizes; in general MEMDISK will correctly detect most physical extended formats used, with 80 cylinders or slightly more. 

 LABEL floppy_image
  LINUX memdisk
  INITRD floppy.img

If your image is larger than 4 MB and a floppy image, you can force MEMDISK to treat it as a floppy image: 

 LABEL floppy_image
  LINUX memdisk
  INITRD floppy.img
  APPEND floppy

Hard disk images

If the image is 4 MB or larger, it is assumed to be a hard disk image, and should typically have an MBR and a partition table. It may optionally have a DOSEMU geometry header; in which case the header is used to determine the C/H/S geometry of the disk. Otherwise, the geometry is determined by examining the partition table, so the entire image should be partitioned for proper operation (it may be divided between multiple partitions, however.) 

 LABEL harddisk_image
  LINUX memdisk
  INITRD harddisk.img

If your image is smaller than 4 MB and it is a hard disk image, you can force MEMDISK to treat it as a hard disk image: 

 LABEL harddisk_image
  LINUX memdisk
  INITRD harddisk.img
  APPEND harddisk

ISO images

For ISO images, the parameter 'iso' must be passed to MEMDISK. 

 LABEL hdt_iso
   LINUX memdisk
   INITRD hdt.iso
   APPEND iso

Sometimes 

  APPEND iso raw

is the better "APPEND" option.

It is possible to map and boot from some CD/DVD images using MEMDISK. No-emulation, floppy emulation and hard disk emulation ISO's are supported. "The "map" process is implemented using INT 13 - any disk emulation will remain accessible from an OS that uses compatible mode disk access, e.g. DOS and Windows 9x. The emulation via INT13 can't however, be accessed from an OS which uses protected mode drivers (Windows NT/2000/XP/2003/Vista/2008/7, Linux, FreeBSD) once the protected mode kernel drivers take control.


INT13 access: Not all images will boot completely!

Real mode operating systems and boot loaders that use INT13 BIOS calls

DOS (MS-DOS, FreeDOS, DR-DOS, ...), Windows 95/98/ME and boot loaders (Syslinux, grub, grub4dos, gujin, gag, mbldr, ...) that use INT13 to read from disks will boot completely with MEMDISK (assuming that there are no BIOS bugs, or software bugs).

Windows NT/2000/XP/2003/Vista/2008/7 (NT based)

These Windows versions use INT13 access only in the start of the booting process (loading only the necessary drivers). Once the protected mode drivers are functional to access the disks, Windows can't see the memory mapped drives created by MEMDISK (CD/DVD, hard disk and floppy disk images) and it will fail to boot completely.


Solutions:

- WinVBlock driver

WinVBlock is a Windows driver that overcomes this problem. It detects the MEMDISK mapped drives (CD/DVD, hard disk and floppy disk images) so Windows can read and write to those disks.

http://www.boot-land.net/forums/index.php?showtopic=8168

It will require the 'raw' parameter on some PCs. 

 LABEL windows_winvblock
   LINUX memdisk
   INITRD windows_harddisk.img
   APPEND raw

- Windows PE based

You can also build a RAM disk based Windows PE. RAM disk based discs include Windows PE 2/2.1 and builds that utilize ramdisk.sys and setupldr.bin files from windows 2003 server SP1 source; see

- Windows Vista/2008/7 with WIM images

Windows Vista/2008/7 can be booted from a WIM image (loaded from disk via INT13).

Linux

The majority of Linux based CD images will also fail to work with MEMDISK ISO emulation. Linux distributions require kernel and initrd files to be specified, as soon as these files are loaded the protected mode kernel driver(s) take control and the virtual CD will no longer be accessible. If any other files are required from the CD/DVD they will be missing, resulting in boot error(s).

Linux distributions that only require kernel and initrd files function fully via ISO emulation, as no other data needs accessing from the virtual CD/DVD drive once they have been loaded. The boot loader has read all necessary files to memory by using INT 13, before booting the kernel.


Parameters and options

Specifying geometry and image type manually

You can also specify the geometry manually with the following command line options: 

   c=#		Specify number of cylinders (max 1024[*])
   h=#		Specify number of heads (max 256[*])
   s=#		Specify number of sectors (max 63)
   floppy[=#]	The image is a floppy image[**]
   harddisk[=#]	The image is a hard disk image[**]
   iso		The image is an El Torito ISO9660 image (drive 0xE0)

   # represents a decimal number.

    [*] MS-DOS only allows max 255 heads, and only allows 255 cylinders
        on floppy disks.

   [**] Normally MEMDISK emulates the first floppy or hard disk.  This
        can be overridden by specifying an index, e.g. floppy=1 will
        simulate fd1 (B:). This may not work on all operating systems
        or BIOSes.

Example: 

 # Boot harddisk image as second hard disk and specify C/H/S geometry
 LABEL hdt_floppy
   LINUX memdisk
   INITRD harddisk.img
   APPEND harddisk=1 c=255 h=64 s=22

Set memory access method (raw, bigraw, int , safeint)

MEMDISK normally uses the BIOS "INT 15h mover" API to access high memory. This is well-behaved with extended memory managers which load later. Unfortunately it appears that the "DOS boot disk" from WinME/XP *deliberately* crash the system when this API is invoked. The following command-line options tells MEMDISK to enter protected mode directly, whenever possible: 

   raw		Use raw access to protected mode memory.

   bigraw	Use raw access to protected mode memory, and leave the
		CPU in "big real" mode afterwards.

   int		Use plain INT 15h access to protected memory.  This assumes
   		that anything which hooks INT 15h knows what it is doing.

   safeint	Use INT 15h access to protected memory, but invoke
		INT 15h the way it was *before* MEMDISK was loaded.
		This is the default since version 3.73.

It is possible that your image works fine without those parameters on some PCs or virtual machines, but fail to boot on other PCs. In this situation, adding 'raw' will normally solve your problem.

Example: 

 LABEL dos_with_extended_memory_manager
   LINUX memdisk
   INITRD dos_emm.img
   APPEND raw

Write protected floppy and hard disk images

The disk is normally writable (although, of course, there is nothing backing it up, so it only lasts until reset.) If you want, you can mimic a write-protected disk by specifying the command line option: 

  ro           Disk is readonly

Example: 

 # Simulate write protected floppy
 LABEL hdt_write_protected_floppy
   LINUX memdisk
   INITRD hdt.img
   APPEND ro

Hide real floppy or hard drive

Some systems without a floppy drive have been known to have problems with floppy images. To avoid that those problems, first of all make sure you don't have a floppy drive configured on the BIOS screen. If there is no option to configure that, or that doesn't work, you can use: 

   nopass       Hide all real drives of the same type (floppy or hard disk)
   nopassany    Hide all real drives (floppy and hard disk)

Example: 

 # Hide real floppy drive (when no real floppy drive is attached)
 LABEL hide_floppy
   LINUX memdisk
   INITRD hdt.img
   APPEND nopass

Enable/Disable BIOS Enhanced Disk Drive Services

MEMDISK by default supports EDD/EBIOS on hard disks, but not on floppy disks. This can be controlled with the options: 

  edd          Enable EDD/EBIOS
  noedd        Disable EDD/EBIOS

Example: 

 # Enable EDD for the HDT floppy
 LABEL EDD_on_floppy
   LINUX memdisk
   INITRD hdt.img
   APPEND edd

MEMDISK and PXE booting

Similarly, if you're booting DOS over the network using PXELINUX, you can use the "keeppxe" option and use the generic PXE (UNDI) NDIS network driver, which is part of the PROBOOT.EXE distribution from Intel: [1] 

   keeppxe      Keep PXE capabilities when booted from PXELINUX

Pause MEMDISK before booting

To view the messages produced by MEMDISK, use: 

  pause        Wait for a keypress right before booting

Example: 

 # Pause MEMDISK before booting
 LABEL pause_memdisk
   LINUX memdisk
   INITRD hdt.img
   APPEND pause

Set stack size

The following option can be used to set the real-mode stack size. The default is 512 bytes, but if there is a failure it might be interesting to set it to something larger: 

  stack=size   Set the stack to "size" bytes

Example: 

 # Change stack size to 2kiB (2048 bytes)
 LABEL change_stack_size
   LINUX memdisk
   INITRD hdt.img
   APPEND stack=2048

MEMDISK and generic El Torito CD-ROM driver for DOS

If you're using MEMDISK to boot DOS from a CD-ROM (using ISOLINUX), you might find the generic El Torito CD-ROM driver (eltorito.sys) by Gary Tong and Bart Lagerweij useful. It is now included with the Syslinux distribution, in the dosutil directory. See the file dosutil/eltorito.txt for more information.

Example usage of eltorito.sys in CONFIG.SYS: 

device=eltorito.sys /X:MSCD0001

Corresponding MSCDEX command which can be placed in AUTOEXEC.BAT: 

MSCDEX /X:MSCD0001 /L:X

Where X is the drive letter.


Accessing MEMDISK arguments from DOS

Many times it is very useful to be able to access the command line arguments given to MEMDISK from within the DOS image. This would allow one to combine several DOS images into one, by executing different scripts depending on the arguments.

You can use 3 different programs to get the those arguments:

  • mdiskchk.com (not the Syslinux one), enhanced by Shao Miller: 
 http://git.etherboot.org/?p=people/sha0/syslinux.git;a=commit;h=44fb149957ce0b0848f6a51ea900842b8f06519f
  • getargs.com written by Eric Auer (also can check for the existence of drives): 
 http://ericauer.cosmodata.virtuaserver.com.br/soft/specials/combined-drivecheck-memdisk-getargs.zip
  • getargs.com written by Murali krishnan Ganapathy: 
 http://people.cs.uchicago.edu/~gmurali/gui/downloads.html

Add one of the getargs.com versions to your DOS image and use it as follows inside any of your .bat files: 

REM Retrieve and save MEMDISK arguments in settings.bat
getargs.com > settings.bat

REM Run setting.bat so the retreived MEMDISK arguments are
REM set as environment variables for further usage.
settings.bat

If the call to MEMDISK was: 

# Boot DOS and pass arguments
LABEL dos_with_args
LINUX memdisk
INITRD my_dos.img
APPEND ACTION=install NETSHARE=sharename

then after the call to settings.bat the environment variables INITRD, ACTION and NETSHARE will be set to my_dos.img, install and sharename respectively.

The tool mdiskchk.com (dosutil/mdiskchk.com) can display more info about MEMDISK from DOS.

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