1 [This documentation is rather crufty at the moment.]
3 MEMDISK is meant to allow booting legacy operating systems via PXE,
4 and as a workaround for BIOSes where ISOLINUX image support doesn't
7 MEMDISK simulates a disk by claiming a chunk of high memory for the
8 disk and a (very small - 2K typical) chunk of low (DOS) memory for the
9 driver itself, then hooking the INT 13h (disk driver) and INT 15h
10 (memory query) BIOS interrupts.
12 To use it, type on the SYSLINUX command line:
14 memdisk initrd=diskimg.img
16 ... where diskimg.img is the disk image you want to boot from.
18 [Obviously, the memdisk binary as well as your disk image file need to
19 be present in the boot image directory.]
21 ... or add to your syslinux.cfg/pxelinux.cfg/isolinux.cfg something like:
25 append initrd=dosboot.img
29 a) The disk image can be uncompressed or compressed with gzip or zip.
31 b) If the disk image is less than 4,194,304 bytes (4096K, 4 MB) it is
32 assumed to be a floppy image and MEMDISK will try to guess its
33 geometry based on the size of the file. MEMDISK recognizes all the
34 standard floppy sizes as well as common extended formats:
36 163,840 bytes (160K) c=40 h=1 s=8 5.25" SSSD
37 184,320 bytes (180K) c=40 h=1 s=9 5.25" SSSD
38 327,680 bytes (320K) c=40 h=2 s=8 5.25" DSDD
39 368,640 bytes (360K) c=40 h=2 s=9 5.25" DSDD
40 655,360 bytes (640K) c=80 h=2 s=8 3.5" DSDD
41 737,280 bytes (720K) c=80 h=2 s=9 3.5" DSDD
42 1,222,800 bytes (1200K) c=80 h=2 s=15 5.25" DSHD
43 1,474,560 bytes (1440K) c=80 h=2 s=18 3.5" DSHD
44 1,638,400 bytes (1600K) c=80 h=2 s=20 3.5" DSHD (extended)
45 1,720,320 bytes (1680K) c=80 h=2 s=21 3.5" DSHD (extended)
46 1,763,328 bytes (1722K) c=82 h=2 s=21 3.5" DSHD (extended)
47 1,784,832 bytes (1743K) c=83 h=2 s=21 3.5" DSHD (extended)
48 1,802,240 bytes (1760K) c=80 h=2 s=22 3.5" DSHD (extended)
49 1,884,160 bytes (1840K) c=80 h=2 s=23 3.5" DSHD (extended)
50 1,966,080 bytes (1920K) c=80 h=2 s=24 3.5" DSHD (extended)
51 2,949,120 bytes (2880K) c=80 h=2 s=36 3.5" DSED
52 3,194,880 bytes (3120K) c=80 h=2 s=39 3.5" DSED (extended)
53 3,276,800 bytes (3200K) c=80 h=2 s=40 3.5" DSED (extended)
54 3,604,480 bytes (3520K) c=80 h=2 s=44 3.5" DSED (extended)
55 3,932,160 bytes (3840K) c=80 h=2 s=48 3.5" DSED (extended)
57 A small perl script is included in the MEMDISK directory which can
58 determine the geometry that MEMDISK would select for other sizes;
59 in general MEMDISK will correctly detect most physical extended
60 formats used, with 80 cylinders or slightly more.
62 If the image is 4 MB or larger, it is assumed to be a hard disk
63 image, and should typically have an MBR and a partition table. It
64 may optionally have a DOSEMU geometry header; in which case the
65 header is used to determine the C/H/S geometry of the disk.
66 Otherwise, the geometry is determined by examining the partition
67 table, so the entire image should be partitioned for proper
68 operation (it may be divided between multiple partitions, however.)
70 You can also specify the geometry manually with the following command
73 c=# Specify number of cylinders (max 1024[*])
74 h=# Specify number of heads (max 256[*])
75 s=# Specify number of sectors (max 63)
76 floppy[=#] The image is a floppy image[**]
77 harddisk[=#] The image is a hard disk image[**]
79 # represents a decimal number.
81 [*] MS-DOS only allows max 255 heads, and only allows 255 cylinders
84 [**] Normally MEMDISK emulates the first floppy or hard disk. This
85 can be overridden by specifying an index, e.g. floppy=1 will
86 simulate fd1 (B:). This may not work on all operating systems
89 c) The disk is normally writable (although, of course, there is
90 nothing backing it up, so it only lasts until reset.) If you want,
91 you can mimic a write-protected disk by specifying the command line
96 d) MEMDISK normally uses the BIOS "INT 15h mover" API to access high
97 memory. This is well-behaved with extended memory managers which load
98 later. Unfortunately it appears that the "DOS boot disk" from
99 WinME/XP *deliberately* crash the system when this API is invoked.
100 The following command-line options tells MEMDISK to enter protected
101 mode directly, whenever possible:
103 raw Use raw access to protected mode memory.
105 bigraw Use raw access to protected mode memory, and leave the
106 CPU in "big real" mode afterwards.
108 safeint Use INT 15h access to protected memory, but invoke
109 INT 15h the way it was *before* MEMDISK was loaded.
111 e) MEMDISK by default supports EDD/EBIOS on hard disks, but not on
112 floppy disks. This can be controlled with the options:
115 noedd Disable EDD/EBIOS
117 f) The following option can be used to pause to view the messages:
119 pause Wait for a keypress right before booting
122 Some interesting things to note:
124 If you're using MEMDISK to boot DOS from a CD-ROM (using ISOLINUX),
125 you might find the generic El Torito CD-ROM driver by Gary Tong and
126 Bart Lagerweij useful:
128 http://www.nu2.nu/eltorito/
131 Similarly, if you're booting DOS over the network using PXELINUX, you
132 can use the "keeppxe" option and use the generic PXE (UNDI) NDIS
133 network driver, which is part of the PROBOOT.EXE distribution from
136 http://www.intel.com/support/network/adapter/1000/software.htm
139 Additional technical information:
141 Starting with version 2.08, MEMDISK now supports an installation check
142 API. This works as follows:
144 EAX = 454D08xxh ("ME") (08h = parameter query)
145 ECX = 444Dxxxxh ("MD")
146 EDX = 5349xxnnh ("IS") (nn = drive #)
147 EBX = 3F4Bxxxxh ("K?")
150 If drive nn is a MEMDISK, the registers will contain:
152 EAX = 4D21xxxxh ("!M")
153 ECX = 4D45xxxxh ("EM")
154 EDX = 4944xxxxh ("DI")
155 EBX = 4B53xxxxh ("SK")
157 ES:DI -> MEMDISK info structures
159 The low parts of EAX/ECX/EDX/EBX have the normal return values for INT
160 13h, AH=08h, i.e. information of the disk geometry etc.
162 See Ralf Brown's interrupt list,
163 http://www.cs.cmu.edu/afs/cs.cmu.edu/user/ralf/pub/WWW/files.html or
164 http://www.ctyme.com/rbrown.htm, for a detailed description.
166 The MEMDISK info structure currently contains:
168 [ES:DI] word Total size of structure (currently 30 bytes)
169 [ES:DI+2] byte MEMDISK minor version
170 [ES:DI+3] byte MEMDISK major version
171 [ES:DI+4] dword Pointer to MEMDISK data in high memory
172 [ES:DI+8] dword Size of MEMDISK data in 512-byte sectors
173 [ES:DI+12] 16:16 Far pointer to command line
174 [ES:DI+16] 16:16 Old INT 13h pointer
175 [ES:DI+20] 16:16 Old INT 15h pointer
176 [ES:DI+24] word Amount of DOS memory before MEMDISK loaded
177 [ES:DI+26] byte Boot loader ID
178 [ES:DI+27] byte Currently unused
179 [ES:DI+28] word If nonzero, offset (vs ES) to installed DPT
180 This pointer+16 contains the original INT 1Eh
182 Sizes of this structure:
184 3.71+ 30 bytes Added DPT pointer
185 3.00-3.70 27 bytes Added boot loader ID
188 In addition, the following fields are available at [ES:0]:
190 [ES:0] word Offset of INT 13h routine (segment == ES)
191 [ES:2] word Offset of INT 15h routine (segment == ES)
193 The program mdiskchk.c in the sample directory is an example on how
194 this API can be used.
196 The following code can be used to "disable" MEMDISK. Note that it
197 does not free the handler in DOS memory, and that running this from
198 DOS will probably crash your machine (DOS doesn't like drives suddenly
199 disappearing from underneath.) This is also not necessarily the best
223 mov bx,[es:0] ; INT 13h handler offset
224 mov eax,[es:di+16] ; Old INT 13h handler
225 mov byte [es:bx], 0EAh ; FAR JMP
228 mov bx,[es:2] ; INT 15h handler offset
229 mov eax,[es:di+20] ; Old INT 15h handler
230 mov byte [es:bx], 0EAh ; FAR JMP