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[mbr] various fixes and improvements

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* Adds USB masquerading according to disk ID in USB part table
* Adds debug feature
* Closes #74 - was due to flags not being properly saved/restored
* Closes #71
* Fixes reentrant INT_13h for some platforms (eg. IBM t43p reenters
  INT_13h to issue an SCSI passthrough, AH=0x50)
* Embed MBR as a resource rather than an ms-sys header
This commit is contained in:
Pete Batard 2012-03-25 21:35:38 +01:00
parent 7874f5ea5d
commit d87f069963
9 changed files with 80 additions and 112 deletions
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res/mbr/readme.txt
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@ -4,11 +4,16 @@ Rufus: The Reliable USB Formatting Utility - Custom MBR
This directory contains all the resources required to create an MBR that prompts
the user for boot selection, when a second bootable device (typically bootable
fixed HDD) is reported by the BIOS.
fixed HDD) is reported by the BIOS at 0x81.
This aims at mimicking the Microsoft Windows optical installation media feature,
which may be necessary on for WinPE 2.x or earlier based installations.
This MBR will also masquerade a bootable USB drive booted as 0x80 by the BIOS to
a different ID according to the one found in its partition table entry. Eg. if
the partition table lists the disk ID for the first partition as 0x81, then it
will be swapped for 0x80.
# Compilation
Any gcc suite (except possibly the X-Code one on OS-X) should be able to compile
@ -35,15 +40,19 @@ The way this bootloader achieves the feature highlighted above is as follows:
BIOS disk access, behave as if there was no USB drive inserted.
6. In case there was a failure to read the second bootable drive's MBR, or no
active partition was detected there, the USB is booted without prompts.
7. In case USB is booted, and the drive ID of first partition of the USB (which
is always assumed bootable) is read and if different from 0x80, then it is
also swapped with 0x80 in the INT_13h override.
# Limitations
* If you are using software RAID or a non-conventional setup, the second
bootable disk may not be accessible through the BIOS and therefore the USB
will always be booted
* If the bootable HDD uses LILO, a "LILO - Keytable read/checksum error" will
be displayed when trying to boot it.
* This MBR currently does not masquerade the bootable USB drive as secondary
(0x81) therefore an installation program ran from USB to install an OS on
an HDD may still configure that disk as the second drive, and prevent it to
properly boot later on.
will always be booted.
* Some processes (notably XP's ntdetect.com) do not seem to like gaps in the
bootable drive sequence, which means that if you set your bootable USB
partition as 0x82 or higher, and it leaves any of 0x80/0x81 free as a result
then these processes may report an error.
* DOS also does not allow anything but 0x80 to be used as bootable disk. Thus
it is not possible to run MS-DOS or FreeDOS off an USB drive unless the disk
ID is 0x80 and not masqueraded.