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50 lines
2.4 KiB
Text
50 lines
2.4 KiB
Text
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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.
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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 2.x or earlier based installations.
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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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# 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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* If the bootable HDD uses LILO, a "LILO - Keytable read/checksum error" will
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be displayed when trying to boot it.
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* This MBR currently does not masquerade the bootable USB drive as secondary
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(0x81) therefore an installation program ran from USB to install an OS on
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an HDD may still configure that disk as the second drive, and prevent it to
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properly boot later on.
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