(2006-08-06) rescue-bootcd
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extra/syslinux-3.09/memdisk/memdisk.doc
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extra/syslinux-3.09/memdisk/memdisk.doc
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$Id: memdisk.doc,v 1.17 2005/04/29 06:04:45 hpa Exp $
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[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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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 one of the following sizes, it's assumed to be a
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floppy image:
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368,640 bytes - 360K floppy
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737,280 bytes - 720K floppy
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1,222,800 bytes - 1200K floppy
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1,474,560 bytes - 1440K floppy
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1,720,320 bytes - 1680K floppy (common extended format)
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1,763,328 bytes - 1722K floppy (common extended format)
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2,949,120 bytes - 2880K floppy
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3,932,160 bytes - 3840K floppy (extended format)
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For any other size, the image is assumed to be a hard disk image,
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and should typically have an MBR and a partition table. It may
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optionally have a DOSEMU geometry header; in which case the header
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is used to determine the C/H/S geometry of the disk. Otherwise,
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the geometry is determined by examining the partition table, so the
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entire image should be partitioned for proper operation (it may be
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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=<number> Specify number of cylinders (max 1024[*])
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h=<number> Specify number of heads (max 256[*])
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s=<number> 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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[*] MS-DOS only allows max 255 heads, and only allows 255 cylinders
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on floppy disks.
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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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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:
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http://www.nu2.nu/eltorito/
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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 27 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 512-byte 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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MEMDISK 3.00 and higher has the size of this structure as 27; earlier
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versions had size 26 and did not include the boot loader ID.
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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
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suddenly disappearing from underneath):
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mov eax, 454D0800h
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mov ecx, 444D0000h
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mov edx, 53490000h + drive #
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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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