From jdpotter@worldnet.att.net  Mon Sep 29 18:32:15 1997
Date: Mon, 29 Sep 1997 21:30:16 -0400
From: Jeff Potter <jdpotter@worldnet.att.net>
Newsgroups: comp.sys.atari.8bit
Subject: Game Link II sample code

Okay.  It took a while to remember some of the little tricks in writing
SIO bus interrupt handlers (like remember your interrupt routine is
called *after* a PHA instruction has been performed).  Here is actual
Action source code for a really simple application.  It reads the
joystick and sends out a word which affects the color of the other's
screen.  As a check, it shows the color it sent as the background color
on its own screen.  If two Ataris aren't connected, the program loops
slowly, and speeds up when a connection occurs (using the already
mentioned GameLink II hardware available from your local authorized AGDA
distributor :^)

BTW: if you wait too long after you've launched the program, attract
mode sets in.  Even though you are resetting the color registers,
attract mode XORs your values with other ones.  Press any key to stop
this annoying behavior (actual fix is left to the student as an
exercise).

This source will compile to binary executable directly (no need to
include the Action library routines, I didn't use any).  So after
compiling, the program will run without the Action cartridge.  You must
press RESET after running this program to restore the default interrupt
vectors.

Warning: this program only does what I set out to do, so modifications
you make are likely to cause you a lot of grief.  I can be of some help.

Jeff Potter

=====================================================================
;
; GameLink II demo program
;  coded to *not* use Action library routines
;  created  9/21/97 J.D.Potter
;  modified 9/29/97 J.D.Potter
;
BYTE stick0=$0278,     ;joystick 0
     color0=$02C4,     ;playfield 0
     color2=$02C6,     ;background color
     color4=$02C8,     ;border color
     sskctl=$0232,     ;skctl shadow
     audf3=$D204,      ;Channel 3 frequency
     audf4=$D206,      ;Channel 4 frequency
     audctl=$D208,     ;pokey control
     skres=$D20A,      ;serial port reset
     serin=$D20D,      ;serial port input
     serout=$D20D,     ;serial port output
     irqen=$D20E,      ;irq interrupt enable
     skctl=$D20F       ;serial port control

BYTE pokmsk=$0010,     ;pokey irq mask
     rtclok=$0014,     ;jiffy clock LSB
     status=$0030,     ;sio status
     recvdn=$0039,     ;sio receive done
     xmtdon=$003A      ;sio xmit done

CARD vserin=$020A,     ;receive int vector
     vseror=$020C,     ;xmit ready int vector
     vseroc=$020E      ;xmit complete int vector

DEFINE PHA="$48"
DEFINE TXA="$8A"
DEFINE TYA="$98"
DEFINE PLA="$68"
DEFINE TAX="$AA"
DEFINE TAY="$A8"
DEFINE RTI="$40"
DEFINE SEI="$78"
DEFINE CLI="$58"
DEFINE pushregs="[$8A $48 $98 $48]"
DEFINE pullrti="[$68 $A8 $68 $AA $68 $40]"

;
; setup receive interrupt
;
PROC rxen()
    skres=$CF
    sskctl=(sskctl&$07)%$13   
    skctl=sskctl
    pokmsk=(pokmsk&$C7)%$20
    irqen=pokmsk
RETURN

;
; setup transmit interrupt
;
PROC txen()
    skres=$CF
    sskctl=(sskctl&$07)%$23
    skctl=sskctl
    pokmsk=(pokmsk&$C7)%$10
    irqen=pokmsk
RETURN

;
; setup output complete interrupt
;
PROC ocen()
    skres=$CF
    skctl=sskctl
    pokmsk=(pokmsk&$C7)%$08
    irqen=pokmsk
RETURN

;
; setup null interrupt
;
PROC nxen()
    skres=$FF
    sskctl=(sskctl&$07)%$23
    skctl=sskctl
    pokmsk=(pokmsk&$C7)
    irqen=pokmsk
RETURN


;
; Output complete handler; setup to receive
;
PROC ocr()
    pushregs
    rxen()
    xmtdon=1 
    recvdn=0
    status=1
    pullrti
RETURN

;
; Receive complete handler (one byte)
;
PROC rxr()
BYTE i
    pushregs
    i = skctl          ;read status
    skres=$FF          ;clear

    IF i&$80 THEN
      status = $8C     ;input frame error
    ELSEIF i&$20 THEN
      status = $8E     ;overrun error
    ELSE status = 1
    FI

    color2 = serin     ;set color

    recvdn = 1
    pullrti
RETURN

;
; Transmit ready handler (one byte)
; replace with real routines to handle things like
; multi-byte count-down, etc.
;
PROC txr()
    pushregs
    ocen()
    pullrti
RETURN

;
; sets up serial bus speed, interrupt vectors
; (press RESET to undo this)
;
PROC setup()
    audf4  = $00       ;19kbps
    audf3  = $28
    audctl = $28
    recvdn = 0
    xmtdon = 0
    [SEI]
    vserin = rxr       ;point the 3 int vectors
    vseror = txr       ;to the right routines
    vseroc = ocr
    [CLI]
RETURN

;
; Main Program
;
PROC main()
BYTE i, x, colorx

    setup()
    colorx = 0
    DO
       recvdn = 0

       ; wait for a jiffy           
       i = rtclok
       DO 
          x = stick0   ;read joystick
       UNTIL i # rtclok
       OD

       ; update internal colorx variable
       IF (x & $01)=0 THEN
          colorx ==+$10
       ELSEIF (x & $02)=0 THEN
          colorx ==-$10
       FI

       IF (x & $04)=0 THEN
          colorx ==-$01
       ELSEIF (x & $08)=0 THEN
          colorx ==+$01
       FI

       ; send colorx to other node
       txen()          ;enable transmit Int
       serout = colorx ;send color
       color4 = colorx ;set own border color

       ; wait for xmit done
       DO UNTIL xmtdon OD
       xmtdon = 0

       ; receiving is automatic
       ; (controlled by interrupt routine)
       ; other colorx gets written to us

       rtclok = $F0
       DO
       UNTIL (recvdn=1) OR (rtclok=0)
       OD 
    OD

RETURN