299 lines
11 KiB
Plaintext
299 lines
11 KiB
Plaintext
[This documentation is rather crufty at the moment.]
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MEMDISK is meant to allow booting legacy operating systems via PXE,
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and as a workaround for BIOSes where ISOLINUX image support doesn't
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work.
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MEMDISK simulates a disk by claiming a chunk of high memory for the
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disk and a (very small - 2K typical) chunk of low (DOS) memory for the
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driver itself, then hooking the INT 13h (disk driver) and INT 15h
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(memory query) BIOS interrupts.
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MEMDISK allows for an OS to detect the MEMDISK instance. (See the
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"Additional technical information" section below.)
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To use it, type on the Syslinux command line:
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memdisk initrd=diskimg.img
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... where diskimg.img is the disk image you want to boot from.
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[Obviously, the memdisk binary as well as your disk image file need to
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be present in the boot image directory.]
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... or add to your syslinux.cfg/pxelinux.cfg/isolinux.cfg something like:
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label dos
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kernel memdisk
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append initrd=dosboot.img
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Note the following:
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a) The disk image can be uncompressed or compressed with gzip or zip.
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b) If the disk image is less than 4,194,304 bytes (4096K, 4 MB) it is
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assumed to be a floppy image and MEMDISK will try to guess its
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geometry based on the size of the file. MEMDISK recognizes all the
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standard floppy sizes as well as common extended formats:
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163,840 bytes (160K) c=40 h=1 s=8 5.25" SSSD
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184,320 bytes (180K) c=40 h=1 s=9 5.25" SSSD
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327,680 bytes (320K) c=40 h=2 s=8 5.25" DSDD
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368,640 bytes (360K) c=40 h=2 s=9 5.25" DSDD
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655,360 bytes (640K) c=80 h=2 s=8 3.5" DSDD
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737,280 bytes (720K) c=80 h=2 s=9 3.5" DSDD
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1,222,800 bytes (1200K) c=80 h=2 s=15 5.25" DSHD
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1,474,560 bytes (1440K) c=80 h=2 s=18 3.5" DSHD
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1,638,400 bytes (1600K) c=80 h=2 s=20 3.5" DSHD (extended)
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1,720,320 bytes (1680K) c=80 h=2 s=21 3.5" DSHD (extended)
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1,763,328 bytes (1722K) c=82 h=2 s=21 3.5" DSHD (extended)
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1,784,832 bytes (1743K) c=83 h=2 s=21 3.5" DSHD (extended)
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1,802,240 bytes (1760K) c=80 h=2 s=22 3.5" DSHD (extended)
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1,884,160 bytes (1840K) c=80 h=2 s=23 3.5" DSHD (extended)
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1,966,080 bytes (1920K) c=80 h=2 s=24 3.5" DSHD (extended)
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2,949,120 bytes (2880K) c=80 h=2 s=36 3.5" DSED
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3,194,880 bytes (3120K) c=80 h=2 s=39 3.5" DSED (extended)
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3,276,800 bytes (3200K) c=80 h=2 s=40 3.5" DSED (extended)
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3,604,480 bytes (3520K) c=80 h=2 s=44 3.5" DSED (extended)
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3,932,160 bytes (3840K) c=80 h=2 s=48 3.5" DSED (extended)
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A small perl script is included in the MEMDISK directory which can
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determine the geometry that MEMDISK would select for other sizes;
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in general MEMDISK will correctly detect most physical extended
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formats used, with 80 cylinders or slightly more.
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If the image is 4 MB or larger, it is assumed to be a hard disk
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image, and should typically have an MBR and a partition table. It
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may optionally have a DOSEMU geometry header; in which case the
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header is used to determine the C/H/S geometry of the disk.
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Otherwise, the geometry is determined by examining the partition
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table, so the entire image should be partitioned for proper
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operation (it may be divided between multiple partitions, however.)
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You can also specify the geometry manually with the following command
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line options:
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c=# Specify number of cylinders (max 1024[*])
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h=# Specify number of heads (max 256[*])
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s=# Specify number of sectors (max 63)
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floppy[=#] The image is a floppy image[**]
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harddisk[=#] The image is a hard disk image[**]
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iso The image is an El Torito ISO9660 image (drive 0xE0)
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# represents a decimal number.
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[*] MS-DOS only allows max 255 heads, and only allows 255 cylinders
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on floppy disks.
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[**] Normally MEMDISK emulates the first floppy or hard disk. This
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can be overridden by specifying an index, e.g. floppy=1 will
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simulate fd1 (B:). This may not work on all operating systems
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or BIOSes.
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c) The disk is normally writable (although, of course, there is
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nothing backing it up, so it only lasts until reset.) If you want,
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you can mimic a write-protected disk by specifying the command line
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option:
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ro Disk is readonly
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d) MEMDISK normally uses the BIOS "INT 15h mover" API to access high
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memory. This is well-behaved with extended memory managers which load
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later. Unfortunately it appears that the "DOS boot disk" from
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WinME/XP *deliberately* crash the system when this API is invoked.
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The following command-line options tells MEMDISK to enter protected
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mode directly, whenever possible:
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raw Use raw access to protected mode memory.
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bigraw Use raw access to protected mode memory, and leave the
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CPU in "big real" mode afterwards.
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int Use plain INT 15h access to protected memory. This assumes
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that anything which hooks INT 15h knows what it is doing.
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safeint Use INT 15h access to protected memory, but invoke
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INT 15h the way it was *before* MEMDISK was loaded.
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This is the default since version 3.73.
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e) MEMDISK by default supports EDD/EBIOS on hard disks, but not on
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floppy disks. This can be controlled with the options:
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edd Enable EDD/EBIOS
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noedd Disable EDD/EBIOS
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f) The following option can be used to pause to view the messages:
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pause Wait for a keypress right before booting
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g) The following option can be used to set the real-mode stack size.
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The default is 512 bytes, but if there is a failure it might be
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interesting to set it to something larger:
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stack=size Set the stack to "size" bytes
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h) Some systems without a floppy drive have been known to have
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problems with floppy images. To avoid that those problems, first
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of all make sure you don't have a floppy drive configured on the
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BIOS screen. If there is no option to configure that, or that
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doesn't work, you can use the option:
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nopass Hide all real drives of the same type (floppy or hard disk)
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nopassany Hide all real drives (floppy and hard disk)
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i) The following standard Linux option will mark memory as reserved.
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Please note that the Syslinux core already loads MEMDISK and its
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initrd below this point:
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mem=size Mark available memory above this point as Reserved.
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Some interesting things to note:
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If you're using MEMDISK to boot DOS from a CD-ROM (using ISOLINUX),
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you might find the generic El Torito CD-ROM driver by Gary Tong and
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Bart Lagerweij useful. It is now included with the Syslinux
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distribution, in the dosutil directory. See the file
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dosutil/eltorito.txt for more information.
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Similarly, if you're booting DOS over the network using PXELINUX, you
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can use the "keeppxe" option and use the generic PXE (UNDI) NDIS
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network driver, which is part of the PROBOOT.EXE distribution from
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Intel:
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http://www.intel.com/support/network/adapter/1000/software.htm
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Additional technical information:
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Starting with version 2.08, MEMDISK now supports an installation check
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API. This works as follows:
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EAX = 454D08xxh ("ME") (08h = parameter query)
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ECX = 444Dxxxxh ("MD")
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EDX = 5349xxnnh ("IS") (nn = drive #)
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EBX = 3F4Bxxxxh ("K?")
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INT 13h
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If drive nn is a MEMDISK, the registers will contain:
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EAX = 4D21xxxxh ("!M")
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ECX = 4D45xxxxh ("EM")
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EDX = 4944xxxxh ("DI")
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EBX = 4B53xxxxh ("SK")
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ES:DI -> MEMDISK info structures
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The low parts of EAX/ECX/EDX/EBX have the normal return values for INT
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13h, AH=08h, i.e. information of the disk geometry etc.
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See Ralf Brown's interrupt list,
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http://www.cs.cmu.edu/afs/cs.cmu.edu/user/ralf/pub/WWW/files.html or
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http://www.ctyme.com/rbrown.htm, for a detailed description.
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The MEMDISK info structure currently contains:
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[ES:DI] word Total size of structure (currently 30 bytes)
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[ES:DI+2] byte MEMDISK minor version
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[ES:DI+3] byte MEMDISK major version
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[ES:DI+4] dword Pointer to MEMDISK data in high memory
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[ES:DI+8] dword Size of MEMDISK data in sectors
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[ES:DI+12] 16:16 Far pointer to command line
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[ES:DI+16] 16:16 Old INT 13h pointer
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[ES:DI+20] 16:16 Old INT 15h pointer
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[ES:DI+24] word Amount of DOS memory before MEMDISK loaded
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[ES:DI+26] byte Boot loader ID
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[ES:DI+27] byte Sector size as a power of 2
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(If zero, assume 512-byte sectors)
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[ES:DI+28] word If nonzero, offset (vs ES) to installed DPT
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This pointer+16 contains the original INT 1Eh
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Sizes of this structure:
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3.71+ 30 bytes Added DPT pointer
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3.00-3.70 27 bytes Added boot loader ID
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pre-3.00 26 bytes
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In addition, the following fields are available at [ES:0]:
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[ES:0] word Offset of INT 13h routine (segment == ES)
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[ES:2] word Offset of INT 15h routine (segment == ES)
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The program mdiskchk.c in the sample directory is an example on how
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this API can be used.
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The following code can be used to "disable" MEMDISK. Note that it
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does not free the handler in DOS memory, and that running this from
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DOS will probably crash your machine (DOS doesn't like drives suddenly
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disappearing from underneath.) This is also not necessarily the best
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method for this.
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mov eax, 454D0800h
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mov ecx, 444D0000h
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mov edx, 53490000h
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mov dl,drive_number
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mov ebx, 3F4B0000h
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int 13h
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shr eax, 16
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cmp ax, 4D21h
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jne not_memdisk
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shr ecx, 16
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cmp cx, 4D45h
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jne not_memdisk
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shr edx, 16
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cmp dx, 4944h
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jne not_memdisk
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shr ebx, 16
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cmp bx, 4B53h
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jne not_memdisk
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cli
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mov bx,[es:0] ; INT 13h handler offset
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mov eax,[es:di+16] ; Old INT 13h handler
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mov byte [es:bx], 0EAh ; FAR JMP
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mov [es:bx+1], eax
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mov bx,[es:2] ; INT 15h handler offset
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mov eax,[es:di+20] ; Old INT 15h handler
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mov byte [es:bx], 0EAh ; FAR JMP
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mov [es:bx+1], eax
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sti
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MEMDISK supports the Win9x "safe hook" structure for OS detection.
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(See "Safe Master Boot Record INT 13h Hook Routines," available at
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http://www.osronline.com/ddkx/w98ddk/storage_5l6g.htm as of
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December 7th, 2009.) An OS driver can take a look at the INTerrupt table
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and try to walk along the chain of those hooks that implement the "safe hook"
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structure. For each hook discovered, a vendor can be identified and the OS
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driver can take appropriate action. The OS driver can mark the "flags" field
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of the "safe hook" to indicate that the driver has reviewed it already. This
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prevents accidental re-detection, for example.
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MEMDISK adds one additional extension field to the "safe hook" structure, a
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pointer to a special MEMDISK structure called the "mBFT." The mBFT is the
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"MEMDISK Boot Firmware Table" (akin to the iSCSI iBFT and the AoE aBFT). An
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OS driver looking at MEMDISK's "safe hook" should know that this field will
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be present based on the fact that MEMDISK is the vendor identifier.
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The mBFT is little more than an ACPI table to prefix MEMDISK's traditional
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MEMDISK info structure (the "MDI"). The ACPI table's details are:
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OEM ID. . . .: MEMDSK
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OEM Table ID : Syslinux
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There is a 1-byte checksum field which covers the length of the mBFT all
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the way through to the end of the MEMDISK info structure.
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There is also a physical pointer to the "safe hook" structure associated
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with the MEMDISK instance. An OS driver might use the following logic:
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1. Walk INT 13h "safe hook" chain as far as possible, marking hooks as
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having been reviewed. For MEMDISK hooks, the driver then follows the
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pointer to the mBFT and gathers the RAM disk details from the included
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MDI.
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2. The OS driver scans low memory for valid mBFTs. MEMDISK instances that
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have been "disconnected" from the INT 13h "safe hook" chain can be thus
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discovered. Looking at their associated "safe hook" structure will
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reveal if they were indeed reviewed by the previous stage.
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