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 directly following the MBR.
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.