diff options
Diffstat (limited to 'contrib/syslinux-4.02/doc/memdisk.txt')
| -rw-r--r-- | contrib/syslinux-4.02/doc/memdisk.txt | 292 |
1 files changed, 292 insertions, 0 deletions
diff --git a/contrib/syslinux-4.02/doc/memdisk.txt b/contrib/syslinux-4.02/doc/memdisk.txt new file mode 100644 index 0000000..fecf2dc --- /dev/null +++ b/contrib/syslinux-4.02/doc/memdisk.txt @@ -0,0 +1,292 @@ +[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. + +MEMDISK allows for an OS to detect the MEMDISK instance. (See the +"Additional technical information" section below.) + +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 less than 4,194,304 bytes (4096K, 4 MB) it is + assumed to be a floppy image and MEMDISK will try to guess its + geometry based on the size of the file. MEMDISK recognizes all the + standard floppy sizes as well as common extended formats: + + 163,840 bytes (160K) c=40 h=1 s=8 5.25" SSSD + 184,320 bytes (180K) c=40 h=1 s=9 5.25" SSSD + 327,680 bytes (320K) c=40 h=2 s=8 5.25" DSDD + 368,640 bytes (360K) c=40 h=2 s=9 5.25" DSDD + 655,360 bytes (640K) c=80 h=2 s=8 3.5" DSDD + 737,280 bytes (720K) c=80 h=2 s=9 3.5" DSDD + 1,222,800 bytes (1200K) c=80 h=2 s=15 5.25" DSHD + 1,474,560 bytes (1440K) c=80 h=2 s=18 3.5" DSHD + 1,638,400 bytes (1600K) c=80 h=2 s=20 3.5" DSHD (extended) + 1,720,320 bytes (1680K) c=80 h=2 s=21 3.5" DSHD (extended) + 1,763,328 bytes (1722K) c=82 h=2 s=21 3.5" DSHD (extended) + 1,784,832 bytes (1743K) c=83 h=2 s=21 3.5" DSHD (extended) + 1,802,240 bytes (1760K) c=80 h=2 s=22 3.5" DSHD (extended) + 1,884,160 bytes (1840K) c=80 h=2 s=23 3.5" DSHD (extended) + 1,966,080 bytes (1920K) c=80 h=2 s=24 3.5" DSHD (extended) + 2,949,120 bytes (2880K) c=80 h=2 s=36 3.5" DSED + 3,194,880 bytes (3120K) c=80 h=2 s=39 3.5" DSED (extended) + 3,276,800 bytes (3200K) c=80 h=2 s=40 3.5" DSED (extended) + 3,604,480 bytes (3520K) c=80 h=2 s=44 3.5" DSED (extended) + 3,932,160 bytes (3840K) c=80 h=2 s=48 3.5" DSED (extended) + + A small perl script is included in the MEMDISK directory which can + determine the geometry that MEMDISK would select for other sizes; + in general MEMDISK will correctly detect most physical extended + formats used, with 80 cylinders or slightly more. + + If the image is 4 MB or larger, it 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[**] + iso The image is an El Torito ISO9660 image (drive 0xE0) + + # 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. + + int Use plain INT 15h access to protected memory. This assumes + that anything which hooks INT 15h knows what it is doing. + + safeint Use INT 15h access to protected memory, but invoke + INT 15h the way it was *before* MEMDISK was loaded. + This is the default since version 3.73. + +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 + +f) The following option can be used to pause to view the messages: + + pause Wait for a keypress right before booting + +g) The following option can be used to set the real-mode stack size. + The default is 512 bytes, but if there is a failure it might be + interesting to set it to something larger: + + stack=size Set the stack to "size" bytes + +h) Some systems without a floppy drive have been known to have + problems with floppy images. To avoid that those problems, first + of all make sure you don't have a floppy drive configured on the + BIOS screen. If there is no option to configure that, or that + doesn't work, you can use the option: + + nopass Hide all real drives of the same type (floppy or hard disk) + nopassany Hide all real drives (floppy and hard disk) + + +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. It is now included with the Syslinux +distribution, in the dosutil directory. See the file +dosutil/eltorito.txt for more information. + +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 30 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 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 + [ES:DI+27] byte Sector size as a power of 2 + (If zero, assume 512-byte sectors) + [ES:DI+28] word If nonzero, offset (vs ES) to installed DPT + This pointer+16 contains the original INT 1Eh + +Sizes of this structure: + +3.71+ 30 bytes Added DPT pointer +3.00-3.70 27 bytes Added boot loader ID +pre-3.00 26 bytes + +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.) This is also not necessarily the best +method for this. + + 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 + +MEMDISK supports the Win9x "safe hook" structure for OS detection. +(See "Safe Master Boot Record INT 13h Hook Routines," available at +http://www.osronline.com/ddkx/w98ddk/storage_5l6g.htm as of +December 7th, 2009.) An OS driver can take a look at the INTerrupt table +and try to walk along the chain of those hooks that implement the "safe hook" +structure. For each hook discovered, a vendor can be identified and the OS +driver can take appropriate action. The OS driver can mark the "flags" field +of the "safe hook" to indicate that the driver has reviewed it already. This +prevents accidental re-detection, for example. + +MEMDISK adds one additional extension field to the "safe hook" structure, a +pointer to a special MEMDISK structure called the "mBFT." The mBFT is the +"MEMDISK Boot Firmware Table" (akin to the iSCSI iBFT and the AoE aBFT). An +OS driver looking at MEMDISK's "safe hook" should know that this field will +be present based on the fact that MEMDISK is the vendor identifier. + +The mBFT is little more than an ACPI table to prefix MEMDISK's traditional +MEMDISK info structure (the "MDI"). The ACPI table's details are: + + OEM ID. . . .: MEMDSK + OEM Table ID : Syslinux + +There is a 1-byte checksum field which covers the length of the mBFT all +the way through to the end of the MEMDISK info structure. + +There is also a physical pointer to the "safe hook" structure associated +with the MEMDISK instance. An OS driver might use the following logic: + + 1. Walk INT 13h "safe hook" chain as far as possible, marking hooks as + having been reviewed. For MEMDISK hooks, the driver then follows the + pointer to the mBFT and gathers the RAM disk details from the included + MDI. + 2. The OS driver scans low memory for valid mBFTs. MEMDISK instances that + have been "disconnected" from the INT 13h "safe hook" chain can be thus + discovered. Looking at their associated "safe hook" structure will + reveal if they were indeed reviewed by the previous stage. |