(c)  Copyright 1989-1999 Amiga, Inc.   All rights reserved.
The information contained herein is subject to change without notice, and 
is provided "as is" without warranty of any kind, either expressed or implied.  
The entire risk as to the use of this information is assumed by the user.



                         The Lost Blitter Docs

                           Carolyn Scheppner

 
   The original pre-1.0 Amiga Hardware docs contained some important
information on blitter moves which never made it into any revision of the
Hardware manual.  These docs detail the correct use of modulo, shift,
and size when moving data with the blitter. 



COPYING DATA USING THE BLITTER
------------------------------

3.1  WHEN SOURCE AND DESTINATION OVERLAP


3.1.1  HOW TO DETERMINE BLITTER DIRECTION

   IF THERE IS A DATA OVERLAP,
   
      If you are moving data to a lower memory address,
      run the blitter in ASCENDING mode.

      If you are moving data to a higher memory address,
      run the blitter in DESCENDING mode.

      NOTE: If you are doing a fill operation, the blitter has been
            designed to FILL only in the DESCENDING mode.


   IF YOU ARE SHIFTING BY ONLY 1 TO 15 BITS (in either direction)
   
      You should run the blitter in the direction which is correct
      for the SHIFT operation (see "3.3 WHEN A DATA SHIFT IS REQUIRED").
      Even though the data move directions are the opposite of those
      described above, the source data will not be destroyed because
      the blitter buffers one word internally during the move.

   
   IF THERE IS NO OVERLAP BETWEEN SOURCE AND DESTINATION

      You may run the blitter in ASCENDING or DESCENDING mode.
      In this case, you will choose the direction based on the
      need for shifting the source data relative to the word
      alignment of the destination area.


3.1.2  ASCENDING OR DESCENDING MODE

  To set up the blitter for ASCENDING mode:
      1. Set the BLTCON1 bit called DESC to 0 (Not DESCending)
      2. Write source and destination address pointers to
         point to the LOWEST addresses of the blocks.
      3. Set the modulos correctly (see section 3.2)
      4. Start the blitter by writing the count of words to move
         into BLTSIZE (bits 15-6 vert. size, bits 5-0 horiz. size in words)

  To set up the blitter for DESCENDING mode:
      1. Set the BLTCON1 bit called DESC to 1
      2. Write source and destination address pointers to
         the HIGHEST addresses of the data blocks.
      3. Set the modulos correctly (see section 3.2)
      4. Start the blitter.



3.2  WHEN DATA STORAGE LAYOUT DIFFERS

   Each of the sources and the destination has a separate register
   called a modulo.  As the blitter reaches the end of each horizontal
   word count (from the BLTSIZE register), the value of the modulo 
   for each source or destination is added to the pointer for that
   item.

   Example of single source and destination:

      Source area data (packed):

               *line1*line2*line3*

         Modulo for this source area is 0 since all of the data is
         packed end to end in sequentially increasing addresses.
         

      Destination area data (after the move)

               ----------------------------
               |                          |
               |                          |
               |    *line1*<-----modulo---|
               |--->*line2*               |
               |    *line3*               |
               |                          |
               |__________________________|

         Modulo = words between end of line1 and start of line2


   In cases where there is a bit shift required between the source
   and the destination, you must specify a modulo that is 1 less
   than your calculated modulo. (-1 for packed data)
   This is because one extra word per horizontal line must be
   moved to account for the blitter bit-shift operation.


3.3  WHEN A DATA SHIFT IS REQUIRED

   The source data may be shifted by 0 to 15 bits before it is stored
   in the destination area.  This shift may be applied to either
   source A, or B, or both.

3.3.1  HOW THE SOURCE DATA SHIFT WORKS

   In the blitter, a shift always means a delay.
   When data is taken from the memory, there will be a 0 to 15 bit
   delay before that data is used along with the other source data
   to form the output.

   If the blitter is running in ASCENDING mode, any delay will cause
   the fetched memory bits to be combined with bits at a higher memory
   address. (if onscreen, a shift to the right)

   If the blitter is running in DESCENDING mode, any delay will cause
   the fetched memory bits to be combined with bits at a lower memory
   address. (if onscreen, a shift to the left)

3.3.2  SIZE OF MOVE IF DATA IS SHIFTED

   When an object is being moved within a bitplane, you must tell the
   blitter how many data words (16 bits) are needed to fully contain
   the object you wish to move.  This is specified as a horizontal
   and a vertical count.

   If you have defined an object that is 4 words wide and aligned on
   a word boundary, and no bit-shift is required, you can tell the
   blitter that it should move 4 words horizontally.

   If there IS a bit-shift required, you MUST specify that the blitter
   move ONE MORE WORD horizontally than the number in which the 
   object can be contained.