NAME
        TD_RAWREAD/ETD_RAWREAD -- read raw data from the disk.

   FUNCTION
        These commands read a track of raw data from disk and deposits it in
        the provided buffer. The data is taken straight from the disk with
        no processing done on it. It will appear exactly as the bits come out
        off the disk, hopefully in some legal MFM format.

        This interface is intended for sophisticated programmers only.
        Amiga reserves the right to make enhancements to the disk
        format in the future. We will provide compatibility via the
        cmd_read and etd_read commands, anyone using td_rawread is
        bypassing this upwards compatibility, and may thus stop working.

   IO REQUEST INPUT
        io_Device       preset by the call to opendevice()
        io_Unit         preset by the call to opendevice()
        io_Command      TD_RAWREAD or ETD_RAWREAD.
        io_Flags        if the IOTDB_INDEXSYNC bit is set then the driver
                        will make a best effort attempt to start reading
                        from the index mark.  Note that there will be at
                        least some delay, and perhaps a great deal of delay
                        (for example if interrupts have been disabled).
        io_Length       Length of buffer in bytes, with a maximum of 32768
                        bytes.
        io_Data         Pointer to CHIP memory buffer where raw track data is
                        to be deposited.
        io_Offset       The number of the track to read in.
        iotd_Count      (ETD_RAWREAD only) maximum allowable change counter
                        value.

   IO REQUEST RESULT
        io_Error - 0 for success, or an error code as defined in
                   <devices/trackdisk.h>

   NOTES
        The track buffer provided MUST be in CHIP memory

        There is a delay between the index pulse and the start of bits
        coming in from the drive (e.g. dma started). This delay
        is in the range of 135-200 microseconds. This delay breaks
        down as follows: 55 microsecs is software interrupt overhead
        (this is the time from interrupt to the write of the DSKLEN
        register). 66 microsecs is one horizontal line delay (remember
        that disk IO is synchronized with agnus' display fetches).
        The last variable (0-65 microsecs) is an additional scan line
        since DSKLEN is poked anywhere in the horizontal line. This leaves
        15 microsecs unaccounted for...  Sigh.

        In short, You will almost never get bits within the first 135
        microseconds of the index pulse, and may not get it until 200
        microseconds. At 4 microsecs/bit, this works out to be between
        4 and 7 bytes of user data of delay.

   BUGS
        This command does not work reliably under versions of Kickstart
        earlier than V36, especially on systems with 1 floppy drive.

   SEE ALSO
        td_rawwrite