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* We distractedly chose to populate the message from our protective MBR for GPT/UEFI-only boot media into the 4KB that directly followed the MBR, which of course is space that is being used by the primary GPT. * This resulted on systems having to fall back to using the secondary GPT, which not all appear to be designed to do. * Alter the code to ensure the protective message is written at LBA 34, after the primary GPT. * Closes #1507 |
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| bochsrc.bxrc | ||
| Makefile | ||
| mbr.ld | ||
| mbr.S | ||
| msg.bin | ||
| msg.S | ||
| msg.txt | ||
| readme.txt | ||
Rufus: The Reliable USB Formatting Utility - Custom MBR # Description 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 at 0x81. This aims at mimicking the Microsoft Windows optical installation media feature, which may be necessary on for WinPE 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. An additional MBR (msg.S) also exists in this directory, that can be used to display an ASCII message contained in the sectors following the primary GPT (LBA sectors 34 and later). This can be used, for instance, to display a notice for media that cannot be booted in BIOS/Legacy mode. # Compilation Any gcc suite (except possibly the X-Code one on OS-X) should be able to compile the MBR by invoking 'make'. A 'make dis', that produces a disassembly dump is also provided for your convenience. If you have the Bochs emulator installed, you can also invoke 'make test' to test the MBR with Bochs. # Primer The way this bootloader achieves the feature highlighted above is as follows: 1. An attempt to read the MBR of the second bootable drive (0x81) is made through INT_13h (in either CHS or LBA mode depending on the extensions detected) 2. If that attempts succeeds, then the partition table from the newly read MBR is checked for an active/bootable entry. 3. If such a partition is found, a prompt is displayed to the user and an RTC timer interrupt (INT_8h) override is added so that dots are displayed at regular interval. Then the keyboard is checked for entry. 4. If the user presses a key, the first partition boot record from the USB is read (according to the values found in the USB MBR partition table) and executed 5. If no key is pressed, then an INT_13h (disk access interrupt) override is added to masquerade the second bootable drive (0x81) as the first one (0x80) so that the Windows second stage installer, or any other program relying on 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. * 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.