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2.9 KiB
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58 lines
No EOL
2.9 KiB
Text
Rufus: The Reliable USB Formatting Utility - Custom MBR
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# Description
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This directory contains all the resources required to create an MBR that prompts
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the user for boot selection, when a second bootable device (typically bootable
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fixed HDD) is reported by the BIOS at 0x81.
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This aims at mimicking the Microsoft Windows optical installation media feature,
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which may be necessary on for WinPE based installations.
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This MBR will also masquerade a bootable USB drive booted as 0x80 by the BIOS to
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a different ID according to the one found in its partition table entry. Eg. if
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the partition table lists the disk ID for the first partition as 0x81, then it
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will be swapped for 0x80.
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# Compilation
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Any gcc suite (except possibly the X-Code one on OS-X) should be able to compile
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the MBR by invoking 'make'. A 'make dis', that produces a disassembly dump is
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also provided for your convenience.
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# Primer
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The way this bootloader achieves the feature highlighted above is as follows:
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1. An attempt to read the MBR of the second bootable drive (0x81) is made
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through INT_13h (in either CHS or LBA mode depending on the extensions
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detected)
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2. If that attempts succeeds, then the partition table from the newly read MBR
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is checked for an active/bootable entry.
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3. If such a partition is found, a prompt is displayed to the user and an RTC
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timer interrupt (INT_8h) override is added so that dots are displayed at
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regular interval. Then the keyboard is checked for entry.
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4. If the user presses a key, the first partition boot record from the USB is
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read (according to the values found in the USB MBR partition table) and
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executed
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5. If no key is pressed, then an INT_13h (disk access interrupt) override is
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added to masquerade the second bootable drive (0x81) as the first one (0x80)
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so that the Windows second stage installer, or any other program relying on
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BIOS disk access, behave as if there was no USB drive inserted.
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6. In case there was a failure to read the second bootable drive's MBR, or no
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active partition was detected there, the USB is booted without prompts.
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7. In case USB is booted, and the drive ID of first partition of the USB (which
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is always assumed bootable) is read and if different from 0x80, then it is
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also swapped with 0x80 in the INT_13h override.
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# Limitations
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* If you are using software RAID or a non-conventional setup, the second
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bootable disk may not be accessible through the BIOS and therefore the USB
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will always be booted.
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* Some processes (notably XP's ntdetect.com) do not seem to like gaps in the
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bootable drive sequence, which means that if you set your bootable USB
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partition as 0x82 or higher, and it leaves any of 0x80/0x81 free as a result
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then these processes may report an error.
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* DOS also does not allow anything but 0x80 to be used as bootable disk. Thus
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it is not possible to run MS-DOS or FreeDOS off an USB drive unless the disk
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ID is 0x80 and not masqueraded. |