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$Id$
[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.
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 one of the following sizes, it's assumed to be a
floppy image:
368,640 bytes - 360K floppy
737,280 bytes - 720K floppy
1,222,800 bytes - 1200K floppy
1,474,560 bytes - 1440K floppy
1,720,320 bytes - 1680K floppy (common extended format)
1,763,328 bytes - 1722K floppy (common extended format)
2,949,120 bytes - 2880K floppy
For any other size, the image 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=<number> Specify number of cylinders (max 1024[*])
h=<number> Specify number of heads (max 256[*])
s=<number> Specify number of sectors (max 63)
floppy The image is a floppy image
harddisk The image is a hard disk image
[*] MS-DOS only allows max 255 heads, and only allows 255 cylinders
on floppy disks.
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 option tells MEMDISK to enter protected
mode directly, whenever possible:
raw Use raw access to protected mode memory
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:
http://www.nu2.nu/eltorito/
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 26 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 512-byte 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
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):
mov eax, 454D0800h
mov ecx, 444D0000h
mov edx, 53490000h + drive #
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
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