MEMDISK
Contents
- 1 What is MEMDISK?
- 2 How can I use MEMDISK?
- 3 Supported image types
- 4 Parameters and options
- 4.1 Specifying geometry and image type manually
- 4.2 Set memory access method (raw, bigraw, int , safeint)
- 4.3 Write protected floppy and hard disk images
- 4.4 Hide real floppy or hard drive
- 4.5 Enable/Disable BIOS Enhanced Disk Drive Services
- 4.6 MEMDISK and PXE booting
- 4.7 Pause MEMDISK before booting
- 4.8 Set stack size
- 5 MEMDISK and generic El Torito CD-ROM driver for DOS
- 6 Accessing MEMDISK arguments from DOS
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 the 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. For example, some Windows ISO's need the 'raw' option on some PCs.
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 13h - any disk emulation will remain accessible from an OS that uses compatible mode disk access, e.g. DOS and Windows 9x. The emulation via INT 13h 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. If the OS contains drivers for accessing this mapped ISO, or knows how to find the ISO on the disk, there is no booting problem of course (see next section).
ISOHYBRID images
If the image is a so called ISOHYBRID image, you can also treat it as a harddisk image. This is possible because such an image also contains a MBR (Master Boot Record).
To check if your image is an ISOHYBRID, run:
fdisk -l your_image.iso
For a normal ISO image, the output will look like this:
Disk your_image.iso: 140 MB, 140509184 bytes 64 heads, 32 sectors/track, 134 cylinders, total 274432 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 Disk identifier: 0xcbacee9f Disk your_image.iso doesn't contain a valid partition table
For an ISOHYBRID, the output will look like this:
Disk your_image.iso: 140 MB, 140509184 bytes
64 heads, 32 sectors/track, 134 cylinders, total 274432 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
Disk identifier: 0xcbacee9f
Device Boot Start End Blocks Id System
your_image.iso1 * 0 274431 137216 83 Linux
INT 13h access: Not all images will complete the boot process!
Real mode operating systems and boot loaders that use INT 13h BIOS calls
DOS (MS-DOS, FreeDOS, DR-DOS, ...), Windows 95/98/ME and boot loaders (Syslinux, grub, grub4dos, gujin, gag, mbldr, ...) that use INT 13h to read from disks will successfully complete the boot process 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 INT 13h 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 complete the boot process.
Solutions:
- Drivers that detect the MEMDISK mapped floppy/disk/ISO image
- 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.
For more info about WinVBlock (it can do more than detecting MEMDISK mapped drives) and how to build your Windows images: http://reboot.pro/8168/
Windows images may need the 'raw' parameter on some PCs:
LABEL windows_winvblock LINUX memdisk INITRD windows_harddisk.img APPEND raw
LABEL windows_winvblock LINUX memdisk INITRD windows.iso APPEND iso raw
- Firadisk driver
Firadisk is a Windows driver comparable with WinVBlock. It detects the MEMDISK mapped drives (CD/DVD, hard disk and floppy disk images) so Windows can read and write to those disks.
For more info about Firadisk (it can do more than detecting MEMDISK mapped drives) and how to build your Windows images: http://www.boot-land.net/forums/index.php?showtopic=8804
Windows images may need the 'raw' parameter on some PCs:
LABEL windows_firadisk LINUX memdisk INITRD windows_harddisk.img APPEND raw
LABEL windows_firadisk LINUX memdisk INITRD windows.iso APPEND iso raw
- Windows PE based
You can also build a RAM disk based Windows PE. RAM disk based discs use ramdisk.sys and setupldr.bin files from windows 2003 server SP1 source, see:
- http://www.911cd.net/forums/index.php?showtopic=10482
- http://www.911cd.net/forums/index.php?showtopic=19333
- Windows Vista/2008/7 with WIM images
Windows Vista/2008/7 can be booted from a WIM image (loaded from disk via INT 13h).
If you get the following message while booting the ISO:
A required CD/DVD drive device driver is missing. If you have a floppy disk, CD, DVD, or USB flash drive, please insert it now.
use the ImDisk solution, described at [1].
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 13h, before booting the kernel.
There are ways to get around the problem of not finding the required files:
- Passing ISO parameter to the kernel
Some distributions allow you to pass an extra parameter to the kernel append line, which tells the init scripts to look for an ISO file on a disk. Some distro's require that the drive and partition number where the ISO is stored is explicitly specified, while others will search each partition for the specified filename.
This parameter is distro specific, so look at the docs of your distro. Some popular ones:
- findiso=
- iso-scan/filename=
- memdiskfind in combination with phram and mtdblock
There is also another solution, which requires the phram and mtdblock kernel module and memdiskfind utility of the Syslinux package (utils/memdiskfind). memdiskfind will detect the MEMDISK mapped image and will print the start and length of the found MEMDISK mapped image in a format phram understands:
modprobe phram phram=memdisk,$(memdiskfind) modprobe mtdblock
This will create a /dev/mtdblock0 device, which should be the .ISO image, and should be mountable.
If your image is bigger than 128MiB and you have a 32-bit OS, then you have to increase the maximum memory usage of vmalloc, by adding:
vmalloc=<at_least_size_of_your_image_in_MiB>Mi Example: vmalloc=256Mi
to your kernel parameters.
memdiskfind can be compiled with the klibc instead of with the glibc C library to get a much smaller binary for use in the initramfs:
cd ./syslinux-4.04/utils/ make spotless make CC=klcc memdiskfind
More info + links to the Arch Linux implementation: http://reboot.pro/11690/
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: [2]
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 of the Syslinux package (dosutil/mdiskchk.com)
- 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
mdiskchk.com
Usage: mdiskchk [--safe-hooks] [--mbfts] [--batch-output]
Action: --safe-hooks Will scan INT 13h "safe hook" chain
--mbfts Will scan memory for MEMDISK mBFTs
--batch-output Will output SET command output based
on MEMDISK kernel arguments
--no-sequential Suppresses probing all drive numbers
mdiskchk.com --mbfts (-m) This mode will scan between free base memory and 0xA00000 looking for MEMDISK mBFTs. For each one found, its detail will be reported. mdiskchk.com --safe-hooks (-s) This mode will attempt to walk the chain of INT 13h "safe hooks". For each MEMDISK "safe hook" found, its detail will be reported. mdiskchk.com --batch-output (-b) This mode suppresses the normal detail report and instead outputs a list of DOS SET commands. This is useful if one wishes to populate a batch file and call that batch file to set DOS environment variables, so that programs have access to those MEMDISK kernel arguments passed at boot-time. mdiskchk.com --no-sequential mode (-n) It might be useful to suppress MDISKCHK.COM's classic behaviour of probing all BIOS drive numbers in search of MEMDISKs. Some BIOSes might not enjoy being probed.
These command-line options may be combined.
For example, your AUTOEXEC.BAT might go:
@echo off echo @echo off>setenv.bat mdiskchk.com -m -s -b -n >>setenv.bat call setenv.bat>nul
When running mdiskchk.com without arguments, you will see something like this:
Q:\>mdiskchk.com
Drive 00 is MEMDISK 3.86:
Address = 0x06ff8000, len = 5760 sectors, chs = 80/2/36,
loader = 0x33 (ISOLINUX),
cmdline = raw initrd=fdubcd.img BOOT_IMAGE=memdisk
When scanning for MEMDISK mBFTs, mdiskchk will find previous MEMDISK instances. This can be useful to detect parameters passed to the first MEMDISK (e.g. an ISO) and use them in the MEMDISK mapped floppy image.
Here is an example of the UBCD Freedos floppy image packed in an ISO. The floppy image inside the ISO is booted with MEMDISK. The ISO, which uses ISOLINUX as bootloader, is also booted with MEMDISK.
ubcdcmd= is passed to the MEMDISK mapped ISO image and allows us to choose with specific tool we want to run once we are booted into FreeDOS from the bootloader (QEMU in this case, but normally ISOLINUX, SYSLINUX, ...) itself, instead of from a menu inside the FreeDOS image which is the normal way:
qemu -kernel ./memdisk -initrd fdubcd.iso -append 'iso ubcdcmd=xbios'
Running the mdiskchk.com --mbfts command from within FreeDOS:
Q:\>mdiskchk.com --mbfts
Drive 00 is MEMDISK 3.86:
Address = 0x06ff8000, len = 5760 sectors, chs = 80/2/36,
loader = 0x33 (ISOLINUX),
cmdline = raw initrd=fdubcd.img BOOT_IMAGE=memdisk
Drive E0 is MEMDISK 3.86:
Address = 0x072e9000, len = 6668 sectors, chs = 65535/255/15,
loader = 0xb0 (Qemu),
cmdline = iso ubcdcmd=xbios
Drive 00 is MEMDISK 3.86:
Address = 0x06ff8000, len = 5760 sectors, chs = 80/2/36,
loader = 0x33 (ISOLINUX),
cmdline = raw initrd=fdubcd.img BOOT_IMAGE=memdisk
Like you can see, the ubcdcmd=xbios parameter is passed to the memdisk instance that booted the ISO.
To use this ubcdcmd= parameter, we can better use the --batch-output method:
Q:\>mdiskchk.com --mbfts --batch-output set raw=1 set initrd=fdubcd.img set BOOT_IMAGE=memdisk set iso=1 set ubcdcmd=xbios set raw=1 set initrd=fdubcd.img set BOOT_IMAGE=memdisk
If we redirected the output to a file (.bat) and run it (like shown above), we now can use the %ubcdcmd% variable in our batch files to get assess to the 'xbios' value.
getargs.com
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.
