-------------- An introduction to demo coding on the Amiga -----------------

                --------------- By Kreator ----------------

   Well  due  to   requests  from  our
readers,  I have  split  this  article
into two:  one  part  backtracks  from
last  months  issue,  and  covers  the
copper  the  other covers  what  I had
originally  intended to  do this issue
but  not  in  as  much  depth,  ie. 3D
graphics.
   One defence  an ST owner  makes for
his    plastic    breeze-block    when
challenged by an  AMIGA owner  is that
the ST  has a  slightly  faster  68000
processor  ( 8 Mhz  compared  with the
miggys 7.14 Mhz) Unfortunately for him
however  the Amiga  has  an  array  of
very  powerful  custom chips to create
wonderful   graphics,   crisp   sounds
multiplicities of sprites and bobs and
dynamically alter the structure of the
display quickly and easily. The latter
is what I hope to cover this issue ie.
I will be talking about the Copper.
   The Copper is in fact a very simple
processor,   it    has    only   three
instructions namely WAIT , MOVE , SKIP





WAIT X,Y - This  instruction tells the
copper  to wait  until  the  specified
position is reached.  X is measured in
Bus cycles, or 2 low res. pixels, each
command  has two  words which means it
takes  two bus cycles  to  process the
command  ie.  we can only specify X to
an  accuracy  of  4  low  res  pixels.
Unfortunately the  Vertical Y position
is  specified in 8 bits  ie. from 0 to
255,  but there  are 313  ( 0 to 312 )
raster   lines  in   a  non-interlaced
screen.How do we specify these last 57
lines?  This  is  achieved by  Waiting
until  the   last   possible  position
recognised by the copper ie. 222,255 ,
then execute another WAIT instruction.
eg.

 You want  WAIT 50,270

 instead use:
     WAIT 222,255
     WAIT 50,15  (ie. 270 and 255)

MOVE A,REG - Moves  the  value  A into
the  specified  Register.  The  Copper
assumes the  REG is  an offset  to the
start  of  the  Custom  register  area
HEX DFF000.
  eg.
  Color00 is standard label for the
background colour and is at $DFF180

To set the screen to black at position
(0,0) use :
           WAIT 0,0
           MOVE 0,$180

SKIP X,Y - Very similar to WAIT but if
the  position  specified  has  already
been  passed by the  raster beam, then
the  copper carries on  but skips  the
following  instruction.  I  have never
needed to  use this command so I won't
go into anymore detail.
   As it  happens,  no assembler  will
generate  these commands  for you they
must be encoded yourself.  Some of the
tedium can be removed however with the
use of  macros,  which take  a  little
longer to  assemble but are infinitely
easier to  debug and  follow.  See the
source on  this disk (  in the  Custom
Registers file ) for examples of this.
For obvious reasons I have renamed the
MOVE command MOV.
 The Structure of the commands are as
follows ;

MOVE 
Command word 1
bits   0       0-8        9-15
       0     Register     Unused
Command word 2 holds the data word
WAIT
Command word 1
bits   0       0-7        8-15
       1    X position  Y position
Command word 2
bits   0       1-7     8-14     15
       0     X mask   Y mask   BFD
  A  lot of the  second word will look
unfamiliar. BFD means Blitter Finished
Disable,  if this  bit  is  clear  the
copper   will  always   wait  for  the
blitter to stop before continuing,this
is only  of any  use if you  start the
blitter  from  within  the copper. The
masks  allow  you  to   only  consider
certain   bits  of  the  (X,Y)  raster
position.
  Thats all for this months intro. the
source on the disk also covers setting
up screens. Next month I will tell you
how to write a scroll routine.