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 int Use plain INT 15h access to protected memory. This assumes
109 that anything which hooks INT 15h knows what it is doing.
111 safeint Use INT 15h access to protected memory, but invoke
112 INT 15h the way it was *before* MEMDISK was loaded.
113 This is the default since version 3.73.
115 e) MEMDISK by default supports EDD/EBIOS on hard disks, but not on
116 floppy disks. This can be controlled with the options:
119 noedd Disable EDD/EBIOS
121 f) The following option can be used to pause to view the messages:
123 pause Wait for a keypress right before booting
126 Some interesting things to note:
128 If you're using MEMDISK to boot DOS from a CD-ROM (using ISOLINUX),
129 you might find the generic El Torito CD-ROM driver by Gary Tong and
130 Bart Lagerweij useful:
132 http://www.nu2.nu/eltorito/
135 Similarly, if you're booting DOS over the network using PXELINUX, you
136 can use the "keeppxe" option and use the generic PXE (UNDI) NDIS
137 network driver, which is part of the PROBOOT.EXE distribution from
140 http://www.intel.com/support/network/adapter/1000/software.htm
143 Additional technical information:
145 Starting with version 2.08, MEMDISK now supports an installation check
146 API. This works as follows:
148 EAX = 454D08xxh ("ME") (08h = parameter query)
149 ECX = 444Dxxxxh ("MD")
150 EDX = 5349xxnnh ("IS") (nn = drive #)
151 EBX = 3F4Bxxxxh ("K?")
154 If drive nn is a MEMDISK, the registers will contain:
156 EAX = 4D21xxxxh ("!M")
157 ECX = 4D45xxxxh ("EM")
158 EDX = 4944xxxxh ("DI")
159 EBX = 4B53xxxxh ("SK")
161 ES:DI -> MEMDISK info structures
163 The low parts of EAX/ECX/EDX/EBX have the normal return values for INT
164 13h, AH=08h, i.e. information of the disk geometry etc.
166 See Ralf Brown's interrupt list,
167 http://www.cs.cmu.edu/afs/cs.cmu.edu/user/ralf/pub/WWW/files.html or
168 http://www.ctyme.com/rbrown.htm, for a detailed description.
170 The MEMDISK info structure currently contains:
172 [ES:DI] word Total size of structure (currently 30 bytes)
173 [ES:DI+2] byte MEMDISK minor version
174 [ES:DI+3] byte MEMDISK major version
175 [ES:DI+4] dword Pointer to MEMDISK data in high memory
176 [ES:DI+8] dword Size of MEMDISK data in 512-byte sectors
177 [ES:DI+12] 16:16 Far pointer to command line
178 [ES:DI+16] 16:16 Old INT 13h pointer
179 [ES:DI+20] 16:16 Old INT 15h pointer
180 [ES:DI+24] word Amount of DOS memory before MEMDISK loaded
181 [ES:DI+26] byte Boot loader ID
182 [ES:DI+27] byte Currently unused
183 [ES:DI+28] word If nonzero, offset (vs ES) to installed DPT
184 This pointer+16 contains the original INT 1Eh
186 Sizes of this structure:
188 3.71+ 30 bytes Added DPT pointer
189 3.00-3.70 27 bytes Added boot loader ID
192 In addition, the following fields are available at [ES:0]:
194 [ES:0] word Offset of INT 13h routine (segment == ES)
195 [ES:2] word Offset of INT 15h routine (segment == ES)
197 The program mdiskchk.c in the sample directory is an example on how
198 this API can be used.
200 The following code can be used to "disable" MEMDISK. Note that it
201 does not free the handler in DOS memory, and that running this from
202 DOS will probably crash your machine (DOS doesn't like drives suddenly
203 disappearing from underneath.) This is also not necessarily the best
227 mov bx,[es:0] ; INT 13h handler offset
228 mov eax,[es:di+16] ; Old INT 13h handler
229 mov byte [es:bx], 0EAh ; FAR JMP
232 mov bx,[es:2] ; INT 15h handler offset
233 mov eax,[es:di+20] ; Old INT 15h handler
234 mov byte [es:bx], 0EAh ; FAR JMP