Bart's free JO.SYS
...Press any key to boot from CD-Rom...
Last updated on Mar 12, 2002
What is jo.sys?
Jo.sys is used to decide if the system should boot from the first harddisk
or from the first floppy drive. The default is boot
from harddisk, but when a key is pressed within the timeout (default 5 sec.) or
when the first harddisk has no valid partitions jo.sys boots from floppy drive.
Jo.sys also supports booting from floppy image on a bootable CD-Rom. Very usefull
for fully automated installation from floppy boot disks or bootable CD-Roms.
From v1.4 it can also be used to schedule floppy or bootable CD-Rom boots.
Did you notice when booting from a windows 98 CD, a bootmenu "boot from
harddrive/boot from CD" is displayed, and you can select the device to
boot from. This behaviour is driven by the file "jo.sys", placed in the root
directory of the bootable disk.
By looking how the windows 98 jo.sys behaved, I've written my own jo.sys
which enables you to boot from harddisk when a 5 second (adjustable) timeout
has passed or when the harddisk is not bootable or when you press the
"Escape" key. Pressing any other key will boot from the currently
booted media, that can be floppy or bootable CD-Rom.
When booting it displays something like:
|jo.sys - Screen output|
Bart's free jo.sys v1.4
(c) 2000-2002 Bart Lagerweij - http://www.nu2.nu/jo/
Press any key to boot from CD-Rom Drive (Escape to boot from Harddisk)
You now have 5 seconds (default, can be changed using freejo.exe) to hit the
spacebar, and boot from floppy/CD-Rom, if not the system will boot from
Download jo.sys v1.4
You don't install it.
Unzip the files from the download zip file into some directory on your
harddisk. Start "freejo.exe" to configure jo.sys behaviour. Then
copy the jo.sys file onto a Windows 98 or ME boot diskette and your
No need to add anything to your config.sys (like some users seem to think...)
- When formating a partition as "not bootable" using "format c: /u"
jo.sys cannot see that the harddisk is not bootable, because the partition
boot sectors are the same for a bootable and a non-bootable partition. This
is causing a "NO OS Found" when jo.sys tries to boot from harddisk.
- Jo.sys is not compatible with diskemu or diskem1x !
Fix Windows 98 bootsector
Some Windows 98 bootable diskettes don't work with jo.sys, they
have a different bootsector, download
(16KB) to fix this.
Do no use this fix on Ms-Dos 6.x, jo.sys does not work with Dos 6.x!
Special credit and thanks go out to:
- Karl Zürn, for helping me fix some bugs.
- Todd Truman, for the boot schedule by time idea,
for helping me write it and for testing it. Thanks Todd!
- You can schedule to boot from Floppy (or CD-Rom) using a time frame. See
- Within countdown you can press the Escape key to boot from HD. This used
to be the "H" key.
- Bugfix, there was a bug in v1.2 (and below) that would cause the timer
to function incorrect on some machines. The timer went off immediate or would
- Bugfix, when booting from floppy image on a bootable CD-Rom jo.sys said
"press...to boot from Floppy Drive". This has been fixed.
- Enhanced checking of the harddisk to see if it is bootable. Now checking:
1) Can the master boot record sector be read?
2) Is the master boot record signature valid?
3) Is there an active partition?
4) Can the active partition's boot record sector be read?
5) Is the partition boot record signature valid?
If anything fails, jo.sys will automatically boot from current media (can be
floppy or CD).
- Jo.sys now says "floppy drive" if physically booted from
floppy, and "cd-rom" if physically booted from CD-ROM.
- Checks harddisk, if can't boot from harddisk, autoboot from
currently booted media.