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[This documentation is rather crufty at the moment.]
MEMDISK is meant to allow booting legacy operating systems via PXE,
and as a workaround for BIOSes where ISOLINUX image support doesn't
work.
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.
MEMDISK allows for an OS to detect the MEMDISK instance. (See the
"Additional technical information" section below.)
To use it, type on the Syslinux command line:
memdisk initrd=diskimg.img
... where diskimg.img is the disk image you want to boot from.
[Obviously, the memdisk binary as well as your disk image file need to
be present in the boot image directory.]
... or add to your syslinux.cfg/pxelinux.cfg/isolinux.cfg something like:
label dos
kernel memdisk
append initrd=dosboot.img
Note the following:
a) The disk image can be uncompressed or compressed with gzip or zip.
b) 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.
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.)
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.
c) 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
d) 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.
e) 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
f) The following option can be used to pause to view the messages:
pause Wait for a keypress right before booting
g) 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
h) 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 the option:
nopass Hide all real drives of the same type (floppy or hard disk)
nopassany Hide all real drives (floppy and hard disk)
Some interesting things to note:
If you're using MEMDISK to boot DOS from a CD-ROM (using ISOLINUX),
you might find the generic El Torito CD-ROM driver 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.
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:
http://www.intel.com/support/network/adapter/1000/software.htm
Additional technical information:
Starting with version 2.08, MEMDISK now supports an installation check
API. This works as follows:
EAX = 454D08xxh ("ME") (08h = parameter query)
ECX = 444Dxxxxh ("MD")
EDX = 5349xxnnh ("IS") (nn = drive #)
EBX = 3F4Bxxxxh ("K?")
INT 13h
If drive nn is a MEMDISK, the registers will contain:
EAX = 4D21xxxxh ("!M")
ECX = 4D45xxxxh ("EM")
EDX = 4944xxxxh ("DI")
EBX = 4B53xxxxh ("SK")
ES:DI -> MEMDISK info structures
The low parts of EAX/ECX/EDX/EBX have the normal return values for INT
13h, AH=08h, i.e. information of the disk geometry etc.
See Ralf Brown's interrupt list,
http://www.cs.cmu.edu/afs/cs.cmu.edu/user/ralf/pub/WWW/files.html or
http://www.ctyme.com/rbrown.htm, for a detailed description.
The MEMDISK info structure currently contains:
[ES:DI] word Total size of structure (currently 30 bytes)
[ES:DI+2] byte MEMDISK minor version
[ES:DI+3] byte MEMDISK major version
[ES:DI+4] dword Pointer to MEMDISK data in high memory
[ES:DI+8] dword Size of MEMDISK data in sectors
[ES:DI+12] 16:16 Far pointer to command line
[ES:DI+16] 16:16 Old INT 13h pointer
[ES:DI+20] 16:16 Old INT 15h pointer
[ES:DI+24] word Amount of DOS memory before MEMDISK loaded
[ES:DI+26] byte Boot loader ID
[ES:DI+27] byte Sector size as a power of 2
(If zero, assume 512-byte sectors)
[ES:DI+28] word If nonzero, offset (vs ES) to installed DPT
This pointer+16 contains the original INT 1Eh
Sizes of this structure:
3.71+ 30 bytes Added DPT pointer
3.00-3.70 27 bytes Added boot loader ID
pre-3.00 26 bytes
In addition, the following fields are available at [ES:0]:
[ES:0] word Offset of INT 13h routine (segment == ES)
[ES:2] word Offset of INT 15h routine (segment == ES)
The program mdiskchk.c in the sample directory is an example on how
this API can be used.
The following code can be used to "disable" MEMDISK. Note that it
does not free the handler in DOS memory, and that running this from
DOS will probably crash your machine (DOS doesn't like drives suddenly
disappearing from underneath.) This is also not necessarily the best
method for this.
mov eax, 454D0800h
mov ecx, 444D0000h
mov edx, 53490000h
mov dl,drive_number
mov ebx, 3F4B0000h
int 13h
shr eax, 16
cmp ax, 4D21h
jne not_memdisk
shr ecx, 16
cmp cx, 4D45h
jne not_memdisk
shr edx, 16
cmp dx, 4944h
jne not_memdisk
shr ebx, 16
cmp bx, 4B53h
jne not_memdisk
cli
mov bx,[es:0] ; INT 13h handler offset
mov eax,[es:di+16] ; Old INT 13h handler
mov byte [es:bx], 0EAh ; FAR JMP
mov [es:bx+1], eax
mov bx,[es:2] ; INT 15h handler offset
mov eax,[es:di+20] ; Old INT 15h handler
mov byte [es:bx], 0EAh ; FAR JMP
mov [es:bx+1], eax
sti
MEMDISK supports the Win9x "safe hook" structure for OS detection.
(See "Safe Master Boot Record INT 13h Hook Routines," available at
http://www.osronline.com/ddkx/w98ddk/storage_5l6g.htm as of
December 7th, 2009.) An OS driver can take a look at the INTerrupt table
and try to walk along the chain of those hooks that implement the "safe hook"
structure. For each hook discovered, a vendor can be identified and the OS
driver can take appropriate action. The OS driver can mark the "flags" field
of the "safe hook" to indicate that the driver has reviewed it already. This
prevents accidental re-detection, for example.
MEMDISK adds one additional extension field to the "safe hook" structure, a
pointer to a special MEMDISK structure called the "mBFT." The mBFT is the
"MEMDISK Boot Firmware Table" (akin to the iSCSI iBFT and the AoE aBFT). An
OS driver looking at MEMDISK's "safe hook" should know that this field will
be present based on the fact that MEMDISK is the vendor identifier.
The mBFT is little more than an ACPI table to prefix MEMDISK's traditional
MEMDISK info structure (the "MDI"). The ACPI table's details are:
OEM ID. . . .: MEMDSK
OEM Table ID : Syslinux
There is a 1-byte checksum field which covers the length of the mBFT all
the way through to the end of the MEMDISK info structure.
There is also a physical pointer to the "safe hook" structure associated
with the MEMDISK instance. An OS driver might use the following logic:
1. Walk INT 13h "safe hook" chain as far as possible, marking hooks as
having been reviewed. For MEMDISK hooks, the driver then follows the
pointer to the mBFT and gathers the RAM disk details from the included
MDI.
2. The OS driver scans low memory for valid mBFTs. MEMDISK instances that
have been "disconnected" from the INT 13h "safe hook" chain can be thus
discovered. Looking at their associated "safe hook" structure will
reveal if they were indeed reviewed by the previous stage.
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