100 ;DEFINE VARIABLE LABELS
110 XPOS    .BYT 0      ;SAVE X POSITION
120 YPOS    .BYT 0      ;SAVE Y POSITION
130 BTNS    .BYT 0      ;SAVE BTN STATUS
140 ;
150 ;SUBROUTINE TO READ CONTROL PORT 2
160 RDPORT  SEI         ;LOCK OUT KEYBOARD
170 LDA #$C0    ;
180 STA $DC02   ;SET DDR TO READ
190 LDA #$80    ;
200 STA $DC00   ;READ CONTROL PORT 2
210 LDX #$00    ;ALLOW TIME FOR
220 INX:BNE *-1 ;LINES TO SETTLE
230 LDX $D419   ;READ POT X
240 LDY $D41A   ;READ POT Y
250 LDA #$FF    ;
260 STA $DC00   ;RESET PORT 2
270 RTS         ;
280 ;
290 ;SUBROUTINE FOR MOVING AVERAGE AL(null)RITHM
300 AVRG    BCS AVRGP   ;IF SIGN POSITIVE
310 AVRGN   EOR #-1     ;IF NEGATIVE, DO
320 ADC #1      ;REVERSE SUBT
330 LSR         ;ALLOW HALF-WEIGHT
340 EOR #-1     ;INVERT BYTE
350 CLC         ;TO PRESERVE SIGN
360 ADC #1      ;
370 CLC:RTS     ;
380 AVRGP   LSR         ;ALLOW HALF-WEIGHT
390 CLC:RTS     ;TO THE BYTE
400 ;
410 ;MAIN ROUTINE TO READ THE STICK
420 STICK   JSR RDPORT  ;
430 LDA $DC00   ;READ PORT 2
440 AND #$0C    ;FILTER BTN 1 & 2
450 EOR #$FF    ;INVERT LOGIC
460 STA BTNS    ;SAVE BTN1 & BTN2
470 TXA:SEC     ;STORE X IN XPOS
480 SBC XPOS    ;USING A SIMPLE
490 JSR AVRG    ;MOVING AVERAGE
500 ADC XPOS    ;AL(null)RITHM
510 STA XPOS    ;UPDATE XPOS
520 TYA:SEC     ;STORE Y IN YPOS
530 SBC YPOS    ;USING THE SAME
540 JSR AVRG    ;AL(null)RITHM
550 ADC YPOS    ;AND UPDATE YPOS
560 STA YPOS    ;
570 ;
580 ;NOW TEST THE BUTTONS AND EXIT
590 TEST    LDX #$FF    ;
600 STX $DC02   ;RESET DDR
610 CLI         ;FINISHED WITH PORT
620 LDA #4      ;TEST BIT 2
630 BIT BTNS    ;OF BTNS
640 BNE BTN1    ;IF BTN1 PRESSED
650 ASL         ;TEST BIT 3
660 BIT BTNS    ;OF BTNS
670 BNE BTN2    ;IF BTN2 PRESSED
680 RTS         ;EXIT Z=1 NO BTNS
690 BTN1    LDA #-1     ;FLAG FOR BTN1
700 .BYT $2C     ;SKIP OVER
710 BTN2    LDA #1      ;FLAG FOR BTN2
720 RTS         ;EXIT Z=0
730 ;
740 ;MAIN ROUTINE TO READ THE MOUSE
750 MOUSE   JSR RDPORT  ;
760 LDA $DC00   ;READ PORT 2
770 AND #$10    ;FILTER BTN1
780 STA BTNS    ;SAVE BIT 4
790 TXA         ;READ POT X
800 BMI LOW     ;DISCERN STATE
810 HIGH    LDA #$20    ;SET BIT 5
820 .BYT $2C     ;SKIP OVER
830 LOW     LDA #$00    ;CLR BIT 5
840 ORA BTNS    ;COMBINE BITS 4 & 5
850 LSR:LSR     ;SHIFT TO BITS 2 & 3
860 EOR #$FF    ;INVERT LOGIC
870 STA BTNS    ;SAVE BTN1 & BTN2
880 ;NOW WE HAVE OUR LEFT AND RIGHT BUTTONS!
890 LDA $DC00   ;READ PORT 2
900 AND #$0F    ;FILTER DIRECTIONS
910 CMP #$0F    ;"ANY MOVEMENT ?
920 BEQ EXIT    ;NO, FINISH UP
930 TAX         ;YES, MOUSE ROLLING
940 UP      [175] #1      ;CHECK UP
950 BNE DN      ;
960 INC Y[185]    ;
970 DN      TXA         ;
980 [175] #2      ;CHECK DOWN
990 BNE LFT     ;
1000 DEC Y[185]    ;
1010 LFT     TXA         ;
1020 [175] #4      ;CHECK LEFT
1030 BNE RHT     ;
1040 DEC X[185]    ;
1050 RHT     TXA         ;
1060 [175] #8      ;CHECK RIGHT
1070 BNE EXIT    ;
1080 INC X[185]    ;
1090 EXIT    JMP TEST    ;TEST BUT[164]N [175] EXIT