c't 11/99, page 206 - Booting Windows From = CD-ROM

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Tobias Remberg, Hajo Schulz

Last Aid

Booting Windows From CD-ROM

Regular backups often only offer a false feeling of = security:=20 After a system crash Windows frequently doesn't even start up = anymore, and=20 this puts the restore program out of reach, too. Therefore, a = bootable and=20 virus-proof Windows installation on CD ROM should really be found = in every=20 well-stocked emergency kit.

'To start Windows 95 or 98 in GUI mode, it must be installed on = a=20 writeable medium. You cannot start Windows from CD.' Similar = statements=20 have been published in various PC magazines, and c't has said it, = too. The=20 time has come to revise this statement.=20

The reason for this assumption lies in the fact that Windows = spits out=20 masses of error messages when unable to access the registry during = startup. However, this is essentially only cosmetic: If you can = live with=20 it, Windows 9x can indeed be started from a CD ROM. This article = will deal=20 with how to eliminate the error messages and optimize CD ROM = booting.=20

The advantages of starting Windows from CD sound like a list of = things=20 administrators and teachers, power users and normal users have = always=20 wanted. For one thing would there are the long file names: When = there is=20 no Windows running - be it because of a system crash or because of = user=20 thoughtlessness (format c:) - it is often very problematic to save = long=20 file names. Windows normally also needs to be up for extracting a = file=20 from a backup archive or an encoded file system. According to = Murphy's=20 Law, however, it is exactly after a system crash when there is no = time for=20 re-installing the system. In addition, a re-installation would = possibly=20 overwrite the very data you want to rescue.=20

Another advantage is the option to run one's own system and = customized=20 programs on someone else's computer. This may prove the last = resort to=20 rescue things on business trips. A cryptographically secure system = which=20 can only be accessed by booting one's own system on someone else's = computer is a must in security sensitive areas.=20

In addition, bootable CDs could be advantageous for schools and = Internet caf=E9s, i.e. places where computers are subjected to = games and=20 hack-loving users. This way, computer loving students find their = popular=20 dice game of 'how to install a Trojan horse on the school computer = to log=20 password entries and then acquire admin rights' turns into a = Gordian knot.=20 It could be the silver lining stressed-out first generation = computer=20 science and maths teachers have been looking for in the land of = invincible=20 computer freaks.=20

Take...

To make a bootable Windows CD, you need a CD ROM = burner=20 and the respective software as well as at least 500 MBytes of hard = disk=20 space and a few freeware programs from the Internet we'll tell you = about=20 later. Also, you should already have installed Windows on a = computer and=20 know a few basic DOS commands - it does get a bit tricky, you see. = And=20 time-consuming: You'll probably need at least three hours because = of the=20 numerous reboots.=20

What's being described will so far only work with Windows 95b = upwards=20 and Windows 98. Windows 95a gets stuck during installation. = Windows NT or=20 2000 can - until further notice - not be put on CD due to their = completely=20 different structure.=20

In fact it's all down to registry write access. There are two = places=20 which make sense for starting Windows from CD. Number one would be = directly on CD. A system which has its registry on a CD will work. = It=20 proves wrong to assume that Windows 95 needs a writeable medium = during=20 startup. Although Windows will constantly moan about the hard disk = being=20 full things like backups can be restored without difficulties.=20

The second - and more elegant - place for the registry is a RAM = disk. A=20 RAM disk is a part of main memory pretending to be a normal hard = disk=20 which doesn't retain its data beyond a reboot. The trick is that = you don't=20 have to copy all of the Windows files into RAM disk, only the = registry=20 files. This takes the RAM disk space required for the 40MB of a = minimal=20 Windows installation down to less than 4MB. All the other files = which=20 won't change after startup anyway will remain on CD. Then, Windows = will=20 run on a combination of RAM disk and CD ROM. This way, the = registry has=20 write access without a hard disk being present.=20

Temporary Injunction

Before you set out to install Windows = on CD=20 ROM you should make a bootable startup disk from a running system = and copy=20 the \win95 or \win98 directory from you Windows CD onto your hard = disk.=20 This will for one thing speed up installation and will in addition = -=20 trivial as it may seem - prove advantageous because you don't need = any CD=20 ROM drivers. You may find yourself in trouble when you boot from a = disk=20 which includes CD ROM drivers and then try to reboot after running = the=20 installation procedure.=20

Then, you must create a few hard disk directories: c\w serves = for=20 preparing the CD ROM boot image, c:\cdrom will store everything = that will=20 afterwards be put on CD. The data which needs to go into RAM disk = will=20 initially be saved in c:\cdrom\ramdisk, the RAM disk's 'Windows = directory'=20 will be c:\cdrom\ramdisk\w. Also, you should create a c:\backup = directory=20 and copy msdos.sys, io.sys, config.sys and autoexec.bat into it = from c:\.=20

The c:\w directory should also hold dblbuff.sys, himen.sys, = ifshlp.sys=20 and setver.exe from your Windows directory as well as attrib.exe,=20 keyb.com, keyboard.sys, mscdex.exe, subst.exe, xcopy.exe, = xcopy32.exe and=20 - for Windows 98 - xcopy32.mod from \windows\command. Plus the DOS = driver(s) for your CD ROM drive and a RAM disk driver. = Ramdrive.sys, which=20 comes with Windows, is unsuitable because you cannot assign a = drive=20 letter. A well-tested alternative is xmsdsk.exe, a free utility = you can=20 among others download at [1]. If you want to use a different one, = change=20 the settings in autoexec.bat accordingly.=20

Before re-installing Windows you should delete c:\config.sys = and=20 c:\autoexec.bat (have you really made backups?) and then create a = new=20 autoexec.bat containing the following:=20

c:\w\subst.exe x: c:\cdrom
path = c:\;c:\w

Because the system will later run from CD and the CD ROM drive = can only=20 be assigned a drive letter which hasn't been assigned yet, the = system=20 should be installed on a drive with a letter from the back of the=20 alphabet. This is important to make all registry links and paths=20 partition-independent. Instead of setting up a number of dummy = partitions,=20 you can use the subst DOS command to assign a drive letter to a = hard disk=20 directory of your choice. The first line in autoexec.bat makes the = c:\cdrom drive accessible as drive X: the same way the CD will be = accessed=20 after booting the system.=20

To avoid overwriting existing installations with the following = Windows=20 setup you should rename all win.com and system.ini files in all = Windows=20 directories on all partitions, even in the current one. Similar = things=20 apply to files called system.dat. However, to get to them you must = leave=20 Windows and reboot the computer to its command line. Then, you = make the=20 system.dats accessible by typing attrib -r -h -s and giving them a = new=20 name. This precaution has a background: When Windows can't find = the=20 registry in the place it is looking for it during startup, it will = look=20 for a system.dat file - which contains the registry - on all the = other=20 partitions and will start it from there. This may cause the wrong=20 system.dat to be booted and might even influence other = installations.=20

New From The Start

Then proceed to re-install Windows by = starting=20 setup.exe from the hard disk directory containing the Win9x branch = you=20 copied from your Windows CD. Setup will complain that subst.exe is = loaded.=20 Ignore this message by pressing ESC against the program's = recommendation.=20 Use X:\W as the installation path.=20

The first installation reboot must be done from the Windows = startup=20 floppy you made previously. You should therefore ignore the = instruction to=20 remove all floppy disks from the drives. When installed on a = network drive=20 - and virtual drives created with subst belong in this category - = Windows=20 does not automatically choose the right paths for autoexec.bat and = config.sys. Therefore, the first reboot must be done from the = startup=20 floppy, enabling you to correct these paths and add ifshlp.sys, a = missing=20 file which supports VFAT, to your config.sys. Use edit to load=20 c:\config.sys from the command line and make sure it contains at = least the=20 following lines with correct path instructions:=20

devicehigh=3Dc:\w\himem.sys
devicehigh=3Dc:\w\ifshlp.sysdevicehigh=3Dc:\w\dblbuff.sys
devicehigh=3Dc:\w\setver.exe
Then check autoexec.bat in the same way. Here, the path must be = extented to include the Windows and Windows\Command directories on = our=20 future CD. Without this information, the system won't find win.com = when=20 booted from CD, a file which initializes the GUI mode startup = process.=20 Your minimal configuration looks like this:=20

c:\w\subst.exe x: c:\cdrom
path=20 c:\w;x:\w;x:\w\command;x:\w\system

Remove the startup floppy, restart your computer using = ctrl-alt-del and=20 finish your installation. Now, you should adapt this Windows setup = to=20 include your own preferences. Whatever you configure at a later = date will=20 be forgotten again as the registry will reside in a RAM disk. = Therefore,=20 you should also be sure to install all required drivers - for = example=20 sound and graphics boards drivers - and to prepare any other = programs=20 which are to be included on CD. To make the following steps a bit = easier,=20 do install the TweakUI utility. In Windows 98, you find it in the=20 \tools\reskit\powertoy directory on your Windows CD; the free = Windows 95=20 version is available on the Internet at [2].=20
Separate Ways
Preparing a RAM disk for the registry again = involves=20 the DOS command subst. Add the following line as the second one to = c:\autoexec.bat:=20

c:\w\subst.exe w: c:\cdrom\ramdisk

Windows expects to find the registry files in \msdos.sys on the = startup=20 volume. To edit it, you must first make it accessible with attrib = -s -h=20 -r. Then, you can adapt the path instructions in the first four = lines:=20

[Paths]=20 = WinDir=3Dw:\w
WinBootDir=3Dw:\w
HostWinBootDrv=3Dw

While you're editing msdos.sys, you should add a line at the = end of the=20 last text section with=20

DisableLog=3D1

If there's already a DisableLog=3D0, don't add another entry = for this,=20 but just change it to 1.=20
To prevent the system from using a hard disk system.dat when = booted=20 from CD, causing the side effects described, the registry should = be=20 renamed. Its name is noted in c:\io.sys. To patch it, you must = again=20 render it visible. Then, edit it in a hex editor - failing that = use edit=20 /70 c:\io.sys in a DOS window -, search for the character sequence = system.dat and change it to system.tat. This little operation = makes sure=20 only files named system.tat will be recognized as registry files, = any=20 system.dat files will be ignored.=20
This patch is very recommendable for Windows 95, but for = Windows 98=20 there are more things to consider: Here, the registry name is not = only=20 wired into the io.sys file but also in the program files = responsible for=20 automatically checking the registry during startup. If you make = the=20 change, you face a registry error message every time you boot. In=20 addition, scanregw.exe must be prevented form being loaded, for = example by=20 deactivating it with msconfig.exe in its autostart folder.=20
For the next Windows reboot to work smoothly, copy the start = menu=20 folder from c:\cdrom\w to c:\cdrom\ramdisk\w now.=20
To fill your temporary RAM disk substitute, close Windows and = start a=20 command prompt only. Copy system.dat, system.ini, user.dat and = win.ini=20 from c:\cdrom\w to c:\cdrom\ramdisk\w after having made them = accessible=20 with attrib. In case you included the io.sys patch, rename the = system.dat=20 file in the target directory to system.tat.=20
Restarting Windows will now make the program use the drive W: = registry.=20 However, the system needs write access not only to the registry = but also=20 to the Windows directory. Therefore, this directory should be put = into RAM=20 disk after booting from CD. Its position is noted in the registry = at the=20 HKLM\Software\Microsoft\Windows\CurrentVersion key. Use = regedit.exe to=20 change the value systemroot to 'w:\w'.=20
At present, the start menu resides in the RAM disk simulated = with=20 subst, but it only uses up unneccessary space there. You should = therefore=20 move it back to the CD: Start TweakUI from the system controls = folder,=20 choose 'General' and readjust the 'Special Folders' entries for=20 'Programs', 'Start Menu' and 'Startup' to read 'x:\w\startmenu or = the=20 respective subdirectories. For Windows 98, you should also = readjust the=20 'Desktop' entry to read 'x:\w\Desktop'. After rebooting, the=20 w:\w\Startmenu and w:\w\Desktop folders can be deleted.=20
Casual Aquaintance
Setting up a real RAM disk requires = rebooting=20 to DOS again. The command attrib -s -h -r c:\cdrom\ramdisk\*.* /s = removes=20 'bad' flags in the files which are to go into the RAM disk. Now, = use edit=20 in c:\autoexec.bat to delete or disable the line subst w: = c:\cdrom\ramdisk=20 per REM. In its place, add the following lines:=20

c:\w\xmsdsk 4000 w: /y
copy c:\command.com = w:\
set=20 COMSPEC=3Dw:\command.com
c:\w\xcopy c:\cdrom\ramdisk\*.* w:\=20 /s

During startup, this sets up a 4000KByte RAM disk instead of a = subst=20 drive. The copy commands fill it with a command line interpreter = which has=20 been designated current shell via COMSPEC, and with the contents = of the=20 directory containing the registry.=20
If everything runs smoothly after rebooting, you can delete all = files=20 in c:\cdrom\ramdisk\w except system.ini, user.dat, win.ini, = control.ini=20 and system.tat or system.dat respectively.=20
Burnt Out
To create a bootable CD, you need a startup disk = or -=20 varying with burner software - an image of it. Should your = software not be=20 capable of burning bootable CDs you can still make a normal CD and = start=20 it with this (write protected) disk. With computer BIOSs which = don't=20 support booting from CD, a disk is again the only way of breathing = life=20 into you CD.=20
Therefore, create a normal startup disk using format a:/s or = sys a:.=20 Copy the patched io.sys and msdos.sys files as well as the = config.sys and=20 autoexec.bat you just made from c:\, replacing existing files. In=20 addition, put the entire c:\w directory onto your disk.=20
Now, a:\config.sys must be amended to include the right paths = and any=20 CD ROM driver(s). The result should look something like this:=20
=
devicehigh=3Da:\w\himem.sys
devicehigh=3Da:\w\ifshlp.sysdevicehigh=3Da:\w\dblbuff.sys
devicehigh=3Da:\w\setver.exe
device= =3Da:\w\aspi8dos.sys
device=3Da:\w\aspicd.sys=20 /D:CD001

Again, paths must also be changed in a:\autoexec.bat; in = addition, the=20 subst command must be replaced with mscdex.exe. The finished file = should=20 read like this:=20

a:\w\mscdex.exe /D:CD001 /L:X /M:50
a:\w\xmsdsk = 4000 w:=20 /y
copy a:\command.com w:\
set=20 COMSPEC=3Dw:\command.com
a:\w\xcopy x:\ramdisk\*.* w:\ = /S
path=20 w:\;x:\w;x:\w\command; x:\w\system
x:

Make sure the mscdex.exe data buffer isn't too small. With the = usual=20 /M:12 and a fast drive, Windows might get stuck during startup = when the=20 drive doesn't provide the data fast enough. The parameter /L:X = states that=20 the CD ROM drive is to be given the drive letter X:. The reasoning = behind=20 the last line can be found in the box called 'bull fight'.=20
Before reactivating your old hard disk system it might be = advisable to=20 copy io.sys, msdos.sys, config.sys and autoexec.bat from c:\ to a = separate=20 directory so you don't have to start from scratch if the CD = doesn't work=20 or you want to create a different version. Then, copy the files = from=20 c:\backup back to c:\ and change the altered system.dat, = system.ini and=20 win.com back to their original names.=20
If your burner software can't read a bootable disk, use a free = command=20 line tool like getbtimg [3] or the Windows shareware program = WinImage [4]=20 to create an image of the disk you just made. What happens next = varies=20 strongly with the respective burner software. Make sure you use = attrib -s=20 -h c:\cdrom\*.* /s to remove unwanted flags from the directory = contents to=20 be copied before burning your CD. The CD is to have a Joliet file = system=20 and contain all of c:\cdrom in its root directory.=20
In [3], you will also find instructions on how to create a = bootable CD=20 without the burner software being equipped for it.=20
Should reading this article have left you with the impression = that=20 creating a bootable Windows CD is quite a strenuous task you can = put your=20 mind at ease: It may seem complicated, but actually doing it will = be more=20 of a stroll than anything else. (hos)

Bibliography
[1] Free Software For = DOS, h= ttp://www.geocities.com/SiliconValley/Lakes/1401/softlib1.htm
[2] = Windows 95 Power Toys Set, http://www.microsoft.com/windows95/downloads/contents/wutoys/= w95pwrtoysset/
[3]=20 How to make a PC-bootable CD, http://www.os.= rim.or.jp/~gigo/bootcd/index_e.html
[4]=20 WinImage, http://www.winimage.com/
[5] = Klaus=20 Ditze, Booten von CD, Neues Feature neue Fragen, c't 12/95, S. = 72
[6]=20 Curtis E. Stevens, Stan Merkin, `El Torito=B4 Bootable CD-ROM = Format=20 Specification, PDF-Dokument at http://www.ptltd.com/techs= /specs.html
[7]=20 Ahead Software GmbH, Nero Burning ROM, http://www.ahead.de/=20
Top=20 of page

Bull Fight
CD ROM booting was set out in IBM and Phoenix's = 'El=20 Torito' spec. This is an extension to the possibility of = furnishing a CD=20 with a boot sector, which is already included in the ISO-9660 = standard.=20 Among other things, El Torito controls the interactions between = bootable=20 CD and computer BIOS and is intended to enable CD booting both = through=20 ATAPI and SCSI interfaces.=20
Bootable CDs can present themselves to the computer in various=20 'emulation modes'. In this article, we used what's called a floppy = emulation. It involves giving the CD the image of a startup floppy = disk=20 which the BIOS will mount as drive A: during the startup = procedure. The=20 real drive A: can be accessed as drive B:. If you want to read the = data=20 within the CD's ISO standard sector, the startup sequence must = load a=20 normal CD ROM driver.=20
All in all, it is two entirely different programs which access = the CD.=20 In our experience, this frequently creates conflicts: Immediately = after=20 the first access via CD ROM driver and MSCDEX, any read access to = drive A:=20 should diligently be avoided. Erroneous reactions with different = drive and=20 BIOS combinations ranged from unfound files to total system = crashes=20 leaving only the reset button as a last resort. It's impossible to = say=20 whether this is essentially down to sloppy BIOS programming or to = unclean=20 drive firmware. But taking a look at the El Torito spec could = certainly do=20 the manufacturers of both products no harm.=20
Things are rounded off by a Windows peculiarity: When loading = win.com,=20 Windows memorizes the current drive. It will want to access this = drive=20 again and again during operation. This defect is at the root of = the=20 COMSPEC effort and the autoexec.bat current drive change.=20
Therefore, if a Windows CD ROM installation crashes time after = time=20 this may be due to one of the programs installed trying to access = drive=20 A:. You can easily find out about this by putting the CD in its = drive but=20 booting from the floppy. During operation, all you have to do is = make sure=20 that the floppy drive's LED doesn't flash.=20
Translation by Eva Wolfram=20

Additional = Translations from=20 c't are available
Last changed by = Raymond Gannon, October 27, = 1999

Seitenanfang=20