The first two sectors on each floppy disk contain special boot
information. These sectors are read into the system at an arbitrary
position; therefore, the code must be position independent.  The first
three longwords come from the include file devices/bootblock.h. The type
must be BBID_DOS; the checksum must be correct (an additive carry
wraparound sum of 0xffffffff). Execution starts at location 12 of the
first sector read in.

The code is called with an open trackdisk.device I/O request pointer in A1
(see the ``Trackdisk'' chapter for more information). The boot code is
free to use the IO request as it wishes (the code may trash A1, but must
not trash the I/O request itself).

The boot code must return values in two registers:  D0 and A0. D0 is a
failure code - if it is non-zero then a system alert will be called, and
the system will reboot.

If D0 is zero then A0 must contain the start address to jump to.  The
strap module will free the boot sector memory, free the boot picture
memory, close the trackdisk.device I/O request, do any other cleanup that
is required, then jump to the location pointed to by A0.

Boot code may allocate memory, use trackdisk.device to load relocatable
information into the memory, then return with D0=0 and A0 pointing to
code. The system will clean up, then call the code.

 Commodore-Amiga Disk Format 
 MFM Track Encoding 

The following are details about how the bits on the Commodore-Amiga
disk are actually written.

    Gross Data Organization:

        3 1/2 inch (90mm) disk
        double-sided
        80 cylinders/160 tracks

    Per-track Organization:

        Nulls written as a gap, then 11 or 22 sectors of data.
        No gaps written between sectors.

    Per-sector Organization:

        All data is MFM encoded.  This is the pre-encoded contents
        of each sector:

            two bytes of 00 data    (MFM = $AAAA each)
            two bytes of A1*        ("standard sync byte" -- MFM
                                     encoded A1 without a clock pulse)
                                    (MFM = $4489 each)
            one byte of format byte (Amiga 1.0 format = $FF)
            one byte of track number
            one byte of sector number
            one byte of sectors until end of write (NOTE 1)
                [above 4 bytes treated as one longword
                 for purposes of MFM encoding]
            16 bytes of OS recovery info (NOTE 2)
                [treated as a block of 16 bytes for encoding]
            four bytes of header checksum
                [treated as a longword for encoding]
            four bytes of data-area checksum
                [treated as a longword for encoding]
            512 bytes of data
                [treated as a block of 512 bytes for encoding]


    NOTE:
    -----
    The track number and sector number are constant for each particular
    sector. However, the sector offset byte changes each time we rewrite
    the track.

    The Amiga does a full track read starting at a random position on the
    track and going for slightly more than a full track read to assure
    that all data gets into the buffer. The data buffer is examined to
    determine where the first sector of data begins as compared to the
    start of the buffer. The track data is block moved to the beginning
    of the buffer so as to align some sector with the first location in
    the buffer.

    Because we start reading at a random spot, the read data may be
    divided into three chunks: a series of sectors, the track gap, and
    another series of sectors. The sector offset value tells the disk
    software how many more sectors remain before the gap. From this the
    software can figure out the buffer memory location of the last byte
    of legal data in the buffer. It can then search past the gap for the
    next sync byte and, having found it, can block move the rest of the
    disk data so that all 11 sectors of data are contiguous.

        Example:

            The first-ever write of the track from a buffer looks
            like this:

             |sector0|sector1|sector2|......|sector10|

            sector offset values:

                      11      10      9     .....     1

            (If I find this one at the start of my read buffer, then I
             know there are this many more sectors with no intervening
             gaps before I hit a gap).  Here is a sample read of this
             track:

            |sector9|sector10||sector0|...|sector8|

            value of 'sectors till end of write':

                      2        1     ....    11   ...    3

            result of track re-aligning:

            |sector9|sector10|sector0|...|sector8|

            new sectors till end of write:

                     11       10      9    ...     1

            so that when the track is rewritten, the sector offsets
            are adjusted to match the way the data was written.

    Sector Label Area
    -----------------
    This is operating system dependent data and relates to how AmigaDOS
    assigns sectors to files.  Reserved for future use.

When data is MFM encoded, the encoding is performed on the basis of a data
block-size. In the sector encoding described above, there are bytes
individually encoded; three segments of 4 bytes of data each, treated as
longwords; one segment of 16 bytes treated as a block; two segments of
longwords for the header and data checksums; and the data area of 512
bytes treated as a block.

When the data is encoded, the odd bits are encoded first, then the even
bits of the block.

The procedure is: Make a block of bytes formed from all odd bits of the
block, encode as MFM.  Make a block of bytes formed from all even bits of
the block, encode as MFM.  Even bits are shifted left one bit position
before being encoded.

The raw MFM data that must be presented to the disk controller will be
twice as large as the unencoded data.  The relationship is:

 1 -> 01
 0 -> 10    ;if following a 0
 0 -> 00    ;if following a 1

With clever manipulation, the blitter can be used to encode and decode the
MFM.