how to dual boot? (was Re: adding boostrap code to boot system)

Matthew Dillon dillon at
Sun Mar 5 10:47:47 PST 2006

:On Sun, 5 Mar 2006, Sascha Wildner wrote:
:> Sometimes packet mode helps, see boot0cfg(8) man page.
:> # boot0cfg -B -o packet ad0
:> for example.
:I had tried that -o packet a couple times before, but not with the -B 
:So I booted the LiveCD again and did that command.
:Yeah!! Thank you, Sascha, that worked.
:Can anyone quickly summarize the differences between fdisk -B, disklabel 
:-B, and boot0cfg -B? How do you know which ones to use?
:Now that I have lost two hours, I will try to find time so I can get back 
:to working on the modular on DragonFly ...
: Jeremy C. Reed

    Well, fdisk -B installs /boot/mbr, which I think is a standard MBR
    rather then a BSD MBR.  The standard MBR will always use
    cylinder/track/sector addressing and is severely limited in its
    ability to address the disk.  Sometimes this MBR is necessary to get
    around stupid BIOSes which assume that the MBR is hardwired a certain
    way or else it must be a virus.  Never use the standard MBR if you can
    help it.

    boot0cfg manipulates the /boot/boot0 MBR code.  This code is capable of
    operating in cylinder/track/sector mode or in a logical block number
    mode called 'packet mode'.  Most modern BIOSes support packet mode and
    DragonFly turns on packet mode by default if you install a system from
    scratch from the install CD.

    Packet mode *MUST* be used with most modern disks if you want to boot
    anything past cylinder 1024 on the physical disk.  Packet mode also gets
    around BIOS limitations where the BIOS tries to 'fake' the
    cylinder/track/sector parameters but does such a bad job that addressing
    past cylinder 1024 winds up being completely broken anyhow.

					Matthew Dillon 
					<dillon at xxxxxxxxxxxxx>

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