The content of doc/memdisk.txt (release 3.72):
(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:
... 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[**] # 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
f) The following option can be used to pause to view the messages:
pause Wait for a keypress right before booting
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:
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:
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 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 [ES:DI+27] byte Currently unused [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