Other Miscellaneous Useful Tit-bits

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                     Assembly Language, in Devpac, Tutorial


                          John Cove (Tronic of Effect)



             Series 1, Part 2, Other Miscellaneous Useful Tit-bits

     In this part I will cover  very  simple  routines,  just so I can then
     move onto the more difficult routines.

     I will cover resolution, hertz changing, loops and picture displaying.

     Section 1.1 - Resolutions
     Right, first of all you  need  to  know  the  register that deals with
     resolution!! This register   is  the  $ffff8260  register.  There  are
     many different  ways  to alter the resolution, I will show you two.

     Section 1.11 - Using the $ffff8260 register
     First of all, it is best to  alter  the resolution after one vbl cycle
     is complete otherwise, you may experience  a distorted screen, so here
     is a simple routine to achieve this:-

                move.w  #37,-(sp)               ... line 1
                trap    #14                     ... line 2
                addq.l  #2,sp                   ... line 3
                move.b  #?,$ffff8260            ... line 4

     Lines 1 to 3  are  the  v_sync  routine,  otherwise  known  as the vbl
     synchronisation routine.

     Line 4 is altering the resolution value. Here is the table of what the
     question mark represents:-

                            0 - low resolution
                            1 - medium resolution
                            2 - high resolution

     So, if  you  wanted to alter  the   resolution  to low, then you would
     write the following routine:-

                move.w  #37,-(sp)               ... line 1
                trap    #14                     ... line 2
                addq.l  #2,sp                   ... line 3
                move.b  #0,$ffff8260            ... line 4

     Here is  another  way  of  altering  the  resolution,  just  for  your
     information and so I cover resolution completely.

     Before I continue, note well...Do  not  try  and set the resolution to
     high on a  colour  output!!!   It  won't  work,  unless  you are using
     overscan techniques!!!!!

     Here's that other routine:-

                move    #?,-(sp)                ... line 1
                pea     -1.w                    ... line 2
                pea     -1.w                    ... line 3
                move    #5,-(sp)                ... line 4
                trap    #14                     ... line 5
                lea     12(sp),sp               ... line 6

     Line 1 sets the  resolution  value,  so  see  the  above table for the
     value, and bear in mind that IT IS ALWAYS THE SAME!!!

     Lines 2 to 6 are the system bits to implement the resolution change!!

     Section 2.1 - Hertz
     This is another very easy routine.  Again  all you really need to know
     is the  register  you  are  going  to  play  with!!   The  register is

     Here is a table of values you have to use to alter the hertz:-

                            50 Hertz - 2
                            60 Hertz - 0

     and you implement it in the following way:-

                             move.b  #0,$ffff820a

     The above line will set the  hertz  to  60,  and  if you do not have a
     monitor or a 60 hertz output device,  the  screen will look like it is
     very ill, basically it will look and sound very bad!!!

     If you want to "flash"  the  hertz  to  make  a wicked effect, used by
     myself, STAX and most other coders, then  here is the source code, try
     it out!!!

                program start

                move.b  #0,$ffff820a.w
                jsr     v_sync
                move.b  #2,$ffff820a.w
                jsr     v_sync
                move.b  #0,$ffff820a.w
                jsr     v_sync
                move.b  #2,$ffff820a.w

                program end

        v_sync  movem.l d0-d7/a0-a7,-(sp)
                move.w  #$25,-(sp)
                trap    #14
                addq.l  #2,sp
                movem.l (sp)+,d0-d7/a0-a7

     There's a wicked effect if you use  it, please credit me!!!   The code
     will only work in supervisor mode,  so  if  you are not sure, refer to
     Series 1, Part 1 to  make  sure!!    Though,  you should not really be
     here if you have not got the basics perfected!!!

     N.B.  When compiling, the labels  (for  example, in the above program,
     the   label  is  "v_sync")   this   MUST   be   at  the  beginning  of
     the line, otherwise you WILL get an error!!!
     Section 3.1 - Loops
     Loops, a very important part of any program!!!

     Here is a key  loop  program.....remember  I  am  trying to make these
     articles as easy to read  as  possible,  so  do  not complain if I get
     straight   into   the   facts   instead    of   faffing   around   the
     topic........okay!!!!  (Rough justice, Rough LAW!)

        key_loop        jsr     v_sync                  ... line 1
                        cmp.b   #$39,$fffffc02.w        ... line 2
                        bne.w   key_loop                ... line 3

     Line 1 - This has a  few  components  to  it,  first of all you need a
     label to call when the condition,  in  Line  3,  is not met, and it is
     also a good idea to have a  v_sync  in THIS kind of loop, basically so
     you do not overload the  keyboard  buffer  ($114.w - keyboard vector).
     The v_sync routine is the same as in Section 2.1.

     Line 2 - This is the cOmpARE condition.  Basically it is comparing the
     keyboard buffer to  the  value  #$39,  which  is  the  space bar.  The
     $fffffc02.w is the keyboard handler.   So,  you are comparing $39 with
     the keyboard buffer which will  be  true  when  you have pressed space
     bar!!  The compare statement must  be  conditioned  as a "byte", ie. a
     ".b", not a ".w" etc, because this will never match the hex #$39!!!

     Line 3 - If the cOmpARE condition  is  not true then it will branch to
     the key_loop label.  BNE is  the  mnemonic  for Branch Not Equal, just
     like CMP means COMPARE!! Lovely jubbly!!!

     Here is another loop, using addition:-

                        program start

        add_loop        add.w   #1,counter              ... line 1
                        cmp.w   #1000,counter           ... line 2
                        bne.w   add_loop                ... line 3

                        program end

        counter         dc.w    0                       ... line 4

     Remember what I was saying that you have the same declaration!!!  I.e.
     the ".w", if you are using  an  "add.w"  to a declared store, then you
     must declare the store as a "dc.w"  and you must make all compares and
     mathematics use the ".w" declaration,  DO NOT MIX THE DECLARATIONS!!!!
     You program simply won't work!!!

     Line 1 - Again, you need a label to call if the condition is not met!!

     This line also adds 1 to the  value in counter, which at the beginning
     is 0.

     Line 2 - This compares the value in counter with 1000.

     Line 3 - This compares  whether  the  condition, in the previous line,
     was met and if it was  then  the  loop  has ended and the program will
     continue on its course, past the  loop.   If  the condition is not met
     then it will simply return to the label "add_loop".

     Section 4.1 - Displaying pictures of different formats
     Time for a break to watch  Eastenders,  I think.........I will be back
     on this wonderful tour  into  the  ST very soon....."Red alert....I've
     got mine!!!".....

     Section 4.11 - Displaying a Degas Elite picture.
     Here is a complete, and working, routine to display a Degas picture.

                        clr.l   -(sp)                   ... line  1
                        move    #$20,-(sp)              ... line  2
                        trap    #1                      ... line  3
                        addq.l  #6,sp                   ... line  4

                        jsr     v_sync                  ... line  5
                        move.b  #0,$ff8260              ... line  6

                        lea     pic+34,a0               ... line  7
                        move.l  $44e,a1                 ... line  8
                        move    #(32000/4)-1,d0         ... line  9
        copypic         move.l  (a0)+,(a1)+             ... line 10
                        dbra    d0,copypic              ... line 11

                        movem.l pic+2,d0-d7             ... line 12
                        movem.l d0-7,$ff8240            ... line 13

                        move    #7,-(sp)                ... line 14
                        trap    #1                      ... line 15
                        addq.l  #2,sp                   ... line 16

                        jsr     v_sync                  ... line 17
                        move.b  #1,$ff8260              ... line 18

                        clr.w   -(sp)                   ... line 19
                        trap    #1                      ... line 20

        pic             incbin  degas.pi1               ... line 21

     The rather LARGE explanation
     Lines 1 to 4 - This opens the supervisor mode.

     Line 5 - This waits until a vbl cycle is complete.

     Line 6 - This alters  the  screen  mode  to  low  resolution, as a PI1
     picture file will only  display   correctly  in  low resolution, a PI2
     picture file  will  only  display  correctly    in  medium  resolution
     and  a PI3   picture   file  will   only   display  correctly  in high

     Line 7 - This points to the  picture  data in the picture file & moves
     the contents into the data  register,  a0.  The  processor  knows when
     it has reached the end of  the  picture   data  because after the file
     is  loaded,  a  pointer  is   placed,  in  memory,  at  the end of the
     picture data and the data will   stop  being loaded into the  register
     a0.  The  actual  picture data  starts  at   the   offset  +34, or the
     seventeenth byte in the file.

     Line 8 - The logic screen address is stored at address $44e.   So  all
     that  is  happening is the screen  address, the  final  destination of
     the  picture data, is being copied into the register a1.

     Line 9 - The actual number  of  bytes  that  the  picture  data  takes
     up is 32000, which is  also  the  same  as  the standard sized  ST Low
     resolution  screen. So, you are  loading  that number of bytes, 32000,
     into the register d0.

     Line 10 - This serves   a   few   functions. First  a  label,  because
     you  will be performing a conditional  loop  on  Line 11, and  this is
     where  you  will jump to. The   second   is  the moving of the data in
     a0  and  literally  moving  it  onto  the  screen.   The plus   symbol
     after the (a0) register,  and  (a1),is  simply telling the computer to
     take the next part of picture data, and so on!!

     Line 11 - This is  a  condition,  and  when  all  the picture has been
     displayed, this is when the condition is met, and basically, when  you
     jump  back  to the  loop,  the  pointer  of  the  picture data will be
     implemented by one and in EFFECT will  jump  to the  next byte of  the
     picture  data, to be displayed.  God that was DIFFICULT to explain...

     Line 12 - This  is   implementing   the   palette,  from  the  picture
     data, on  the screen. The palette is also contained within the picture
     file, at the offset of +2. So,  the  data starts at the second byte in
     the file. The palette data uses 16 bytes and is picked up in registers

     Line 13 - This  puts   the  palette  values, from  the  picture  file,
     in d0-d7 and implements it onto the screen.

     Line 14 to 16 - This waits for the user to  press  any  key, before it

     Line 17 - This waits until a vbl cycle is complete.

     Line 18 - This sets  the  screen  mode to  medium  resolution,  really
     used  for the returning to GENst.

     Line 19 to 20 - This is  the  standard exit routine, out of supervisor

     Line 21 - This is  the  incLUDE  binARY  command,  that loads  a  file
     from the disk and is then present at that label.

     In the next part  we  will   look   at  how  to  display  a  Neochrome
     picture  and the different structures  of the picture formats....watch
     out man!!
     Tronic of Effect, aka John  Cove,  [C]opyright 1995 .. Started: 14-11-
     1995.  Finished: 14-11-1995

            "I reserve the right to publish these tutorial series
             wherever  i  choose...   Only,  with express written
             confirmation,  is  this  to  be  published by anyone
             other than myself.   These  series  were written for
             st world, but if i feel that the series is not being
             taken advantage of in the way that most st users are
             able  to read  the  series, then  i will publish the
             series in my own, and other  peoples, disk magazines
             and products."

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