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authorH. Peter Anvin <hpa@zytor.com>2008-02-28 16:04:08 -0800
committerH. Peter Anvin <hpa@zytor.com>2008-02-28 16:04:08 -0800
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Rename plaintext documentation from doc/*.doc to doc/*.txt, to avoid the chronic problem of web sites thinking they're in MS-Word format. Sigh. As if Microsoft somehow had a monopoly on the word "document".
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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.
+
+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
+ 3,932,160 bytes - 3840K floppy (extended format)
+
+ 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=# 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[**]
+
+ # 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.
+
+ safeint Use INT 15h access to protected memory, but invoke
+ INT 15h the way it was *before* MEMDISK was loaded.
+
+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
+
+
+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 28 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
+ [ES:DI+26] byte Boot loader ID
+
+MEMDISK 3.00 and higher has the size of this structure as 27; earlier
+versions had size 26 and did not include the boot loader ID.
+
+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
+ 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