sides of the screen are showing you the same information in
      different ways. On the right, the (hex) bytes are displayed as they
      were before they were written to the disk. On the left are the
      converted (GCR) bytes as they actually appear when reading, or
      writing to, the track directly.
      
      For every four hex bvtes there are five GCR bytes. Group Code
      Recording ensures that there are never more than eight consecutive
      "1" bits or two consecutive "0" bits written to the disk. This
      allows the drive to use ten consecutive "1" bits as a signal that a
      header or data block will be read starting with the first "0" bit
      read. This is referred to as a sync mark. A normal sync mark is
      forty consecutive "1" bits (five hex $FF bytes). This is a
      deliberate overkill to make the disk format as reliable as
      possible.
      
      Using < CURSOR UP/DOWN >, you can highlight either a header block,
      whose first byte is GCR $52 or hex $08; or a data block - 
      GCR $55/hex $07. Cursor down to the last data block. This is sector
      $14 (20) of track 1. Press [SPACE] to read the entire data block
      into memory. The GCRED will display an editing screen, again with
      GCR on the left and hex on the right. Pressing [S] (Side) will move
      the cursor from the left to the right side or visa-versa. We will
      onlv be working on the GCR side.
      
      Above the sector data, POS shows you the position in the data
      block of your cursor. Use the cursor keys to place the cursor at
      position $0144. This is the last byte of the data block. This is
      where every software-only nybbler stops reading the data block. Any
      GCR bytes written past this point are ignored by the copier. Many,
      MANY protection schemes depend on this fact when they create their
      physical disk protection. The logical protection involves a custom
      drive program to look past the end of the data block for the
      special bytes that have been placed there. A special routine is not
      needed to write the physical protection: the GCRED is fully capable
      of such chores.
      
      Move the cursor to position $0145. Press [SPACE] to enter EDIT
      mode and type the following:
      
         AA AB AC AD AE 55 55
      
      then press (RETURN) to exit EDIT mode, [W] to write the sector back
      to disk, and [R] to re-read the modified sector. Verify that the
      bytes $AA - $AE are present at positions $0145 - $0149 (ignore the
      two $55 bytes). If not, try entering and writing them again. We
      have just created the physical protection.
      
      The next thing we concern ourselves with is the logical
      
            K.J. REVEALED TRILOGY    PAGE [146]    (C)1990 K.J.P.B.