[BOOTDATA]: Diverse improvements for mkisofs support and ISO image configuration:
[reactos.git] / reactos / sdk / tools / mkisofs / schilytools / mkisofs / README.eltorito
1 /* @(#)README.eltorito 1.2 00/03/18 eric */
2
3 What is El Torito?
4 ------------------
5 Simply put, El Torito is a specification that says how a cdrom should
6 be formatted such that you can directly boot from it.
7
8 The "El Torito" spec says that ANY cdrom drive should work (scsi/eide)
9 as long as the BIOS supports El Torito. So far this has only been
10 tested with EIDE drives because none of the scsi controllers that has
11 been tested so far appears to support El Torito. The motherboard
12 definately has to support El Torito. The ones that do let you choose
13 booting from HD, Floppy, Network or CDROM.
14
15 How To Make Bootable CDs
16 ------------------------
17
18 For the x86 platform, many BIOS's have begun to support bootable CDs.
19 The standard my patches for mkisofs is based on is called "El Torito".
20
21 The "El Torito" standard works by making the CD drive appear, through BIOS
22 calls, to be a normal floppy drive. This way you simply put an floppy
23 size image (exactly 1440k for a 1.44 meg floppy) somewhere in the
24 iso fs. In the headers of the iso fs you place a pointer to this image.
25 The BIOS will then grab this image from the CD and for all purposes it
26 acts as if it were booting from the floppy drive. This allows a working
27 LILO boot disk, for example, to simply be used as is.
28
29 It is simple then to make a bootable CD. First create a file, say "boot.img"
30 which is an exact image of the boot floppu currently in use. There is
31 at least one HOWTO on making bootable floppies. If you have a bootable
32 floppy handy, you can make a boot image with the command
33
34 dd if=/dev/fd0 of=boot.img bs=10k count=144
35
36 assuming the floppy is in the A: drive.
37
38 Place this image somewhere in the hierarchy which will be the source
39 for the iso9660 filesystem. It is a good idea to put all boot related
40 files in their own directory ("boot/" under the root of the iso9660 fs,
41 for example), but this is not necessary.
42
43 One caveat - Your boot floppy MUST load any initial ramdisk via LILO,
44 not the kernel ramdisk driver! This is because once the linux kernel
45 starts up, the BIOS emulation of the CD as a floppy disk is circumvented
46 and will fail miserably. LILO will load the initial ramdisk using BIOS
47 disk calls, so the emulation works as designed.
48
49 The "El Torito" specification requires a "boot catalog" to be created as
50 ll.
51 This is a 2048 byte file which is of no interest except it is required.
52 My patches to mkisofs will cause it to automatically create the
53 boot catalog. You must specify where the boot catalog will go in the
54 iso9660 filesystem. Usually it is a good idea to put it the same place
55 as the boot image, and a name like "boot.catalog" seems appropriate.
56
57
58 So we have our boot image in the file "boot.image", and we are going to
59 put it in the directory "boot/" under the root of the iso9660 filesystem.
60 We will have the boot catalog go in the same directory with the name
61 "boot.catalog". The command to create the iso9660 fs in the file
62 bootcd.iso is then
63
64 mkisofs -b boot/boot.img -c boot/boot.catalog -o bootcd.iso .
65
66 The -b option specifies the boot image to be used (note the path is
67 relative to the root of the iso9660 disc), and the -c option is
68 for the boot catalog file.
69
70 Now burn the CD and its ready to boot!
71
72 CAVEATS
73 -------
74
75 I don't think this will work with multisession CDs.
76
77 If your bootable floppy image needs to access the boot floppy, it has
78 to do so through BIOS calls. This is because if your O/S tries to talk to
79 the floppy directly it will bypass the "floppy emulation" the El Torito spec
80 creates through BIOS. For example, under Linux it is possible to
81 have an initial RAM disk loaded when the kernel starts up. If you let the
82 kernel try to read in the initial RAM disk from floppy, it will fail
83 miserably because Linux is not using BIOS calls to access the floppy drive.
84 Instead of seeing the floppy image on the CD, Linux will be looking at
85 the actually floppy drive.
86
87 The solution is to have the initial boot loader, called LILO, load your
88 initial RAM disk for you. LILO uses BIOS calls entirely for these
89 operations, so it can grab it from the emulated floppy image.
90
91 I don't think making a CD bootable renders it unreadable by non-El Torito
92 machines. The El Torito spec uses parts of the iso9660 filesystem which
93 were reserved for future use, so no existing code should care what it does.
94
95 Mkisofs currently stores identification records in the iso9660 filesystem
96 saying that the system is a x86 system. The El Torito spec also allows
97 one to write PowerPC or Mac id's instead. If you look at the code in write.c
98 you could figure out how to change what is written.