CHANGING YOUR ICONS by Richard Karsmakers and "Strike-a-Light"

People  who use MacIntosh or Amiga computers are  probably  quite 
familiar  with the fact that each program can be specified by  an 
icon  of its own.  A game program can actually be signified by  a 
joystick  or  even  a  small 'high-res'  screen  taken  from  the 
program.  The only limitation, most of the time, is that the icon 
can only use one color and the background color.

The ST's possibilities with regard to this changing of the  icons 
are  far  more  limited:  There are  only  five  possible  icons, 
excluding  the Atari sign in the "About" option of  the  desktop. 
These can, however, be changed.

The principle is extremely simple.  You simply make a bit pattern 
of  an  icon  and calculate that into  numbers.  You  then  start 
hunting for those numbers and that's all.  A new icon can  simply 
be loaded onto that space, somewhere in RAM. A program to achieve 
this,  written  for Data Becker's "Profimat",  is  the  following 
(written by Robert Heessels):


 TEXT

 clr.l -(sp)                       ; supervisor mode
 move #$20,-(sp)
 trap #1
 addq.l #6,sp
 move.l d0,d7
 move.l #0,$4d2                    ; disable "STARTGEM" interrupt
 move.l #$d000,a6                  ; hunt for Icon start address
 move.l #$20000,a1                 ; and the ending address
zoek:
 cmp.l #$80000263,(a6)             ; this is a bit pattern in the
                                   ; disk drive Icon
 beq zoek_einde                    ; found? Then stop search!
 add.l #2,a6
 cmp.l a1,a6                       ; end address reached?
 bne zoek                          ; no - continue
 bra einde                         ; jump to end
zoek_einde:
 sub.l #180,a6                     ; start of all icons


 move #0,-(sp)                     ; Open file for new icon data
 move.l #naam,-(sp)
 move #$3d,-(sp)                   ; Open file
 trap #1
 addq.l #8,sp
 tst d0                            ; Error?
 bmi einde
 move d0,d6

 move.l a6,-(sp)                   ; load file at icon data
 move.l #1280,-(sp)                ; file length (5*256)
 move d6,-(sp)
 move #$3f,-(sp)                   ; Read
 trap #1
 add.l #12,sp

einde:                             ; user mode
 move.l d7,-(sp)
 move #$20,-(sp)
 trap #1
 addq.l #6,sp

 clr.l -(sp)                       ; terminate
 trap #1

 DATA

naam: dc.b 'a:\new_icon.icn',0

 END


On the disk,  you'll find a sample Icon file as well as a program 
called GEMSTART that is needed in the AUTO folder if you want  to 
start it from on system bootup.  Simply copy the GEMSTART program 
in  the  auto folder and make sure the actual program is  in  the 
root directory.  If the program would be NEW_ICON.PRG, you'd have 
to create a file called GEMSTART.INF on the disk,  containing the 
following line:

A:NEW_ICON.PRG



Of course,  you can also use folders, etc., or other disk drives. 
Any other program using GEM can also be started up  automatically 
using this program.

But  now more about the icon format.  Each icon is made up  of  a 
mask and a 'sprite'.  Each one of these is represented 128 bytes: 
4 bytes wide and 32 bytes high.  Horizontally,  the bits are used 
to specify if bits are on or off.  You'll just have to look at it 
binary.  A  bit pattern like this would create a 'sprite' like  a 
chessboard:

 %11111111000000001111111100000000
 %11111111000000001111111100000000
 %11111111000000001111111100000000
 %11111111000000001111111100000000
 %11111111000000001111111100000000
 %11111111000000001111111100000000
 %11111111000000001111111100000000
 %11111111000000001111111100000000
 %00000000111111110000000011111111
 %00000000111111110000000011111111
 %00000000111111110000000011111111
 %00000000111111110000000011111111
 %00000000111111110000000011111111
 %00000000111111110000000011111111
 %00000000111111110000000011111111
 %00000000111111110000000011111111
 %11111111000000001111111100000000
 %11111111000000001111111100000000
 %11111111000000001111111100000000
 %11111111000000001111111100000000
 %11111111000000001111111100000000
 %11111111000000001111111100000000
 %11111111000000001111111100000000
 %11111111000000001111111100000000
 %00000000111111110000000011111111
 %00000000111111110000000011111111
 %00000000111111110000000011111111
 %00000000111111110000000011111111
 %00000000111111110000000011111111
 %00000000111111110000000011111111
 %00000000111111110000000011111111
 %00000000111111110000000011111111

Get it?  It's that easy!  The only problem is that the icons  are 
always located on a different location in memory (this changes if 
you use an AUTO folder or not).  That's why a search routine  has 
been included in the source file from a few pages back.

By  the  way,  if the program need not be started  from  an  AUTO 
folder, the GEMSTART program can be omitted.

Originally published in ST NEWS Volume 2 Issue 4.