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diff --git a/doc/memdisk.txt b/doc/memdisk.txt new file mode 100644 index 00000000..759a7b27 --- /dev/null +++ b/doc/memdisk.txt @@ -0,0 +1,202 @@ +[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 |