10 REM The Interactive 6502
20 REM By Jason H. Moore, Ph.D.
30 REM The goal of this demo is to allow
40 REM  users to interact directly with the
50 REM  6502 processor using point & click
60 REM  assembly language.
70 REM For use with Atari BASIC XL
80 REM Atari 8-bit Version 1.0
90 REM March 14, 2020

100 REM ***********************
101 REM *** INITIALIZATIONS ***
102 REM ***********************

110 FAST:REM BASIC XL command to index line numbers for speed
120 GOSUB 2000:REM Memory & graphics initialization
130 GOSUB 3000:REM Draw title screen
140 GOSUB 4000:REM Move & redefine character set
150 GOSUB 5000:REM Read in machine language routines
160 GOSUB 6000:REM Draw main screen

200 REM *****************
201 REM *** MAIN LOOP ***
202 REM *****************

210 CX=1:CY=3:RAM=0:X=0:Y=0:A=0:REM Initialize variables
220 POKE 203,RAM:POKE 204,X:REM     Initialize RAM & 6502 registers 
230 POKE 205,Y:POKE 206,A:REM        using page 0 of memory

240 POSITION CX,CY:PRINT #6; ">":REM Draw cursor
250 POSITION 3,15:PRINT #6; RAM
260 POSITION 11,15:PRINT #6; X
270 POSITION 13,15:PRINT #6; Y
280 POSITION 15,15:PRINT #6; A

290 JOY=STICK(0):REM                 Read joystick value
300 IF JOY=15 THEN GOTO 410:REM      No joystick movement?
310 POSITION CX,CY:PRINT #6; " ":REM Clear old cursor
320 IF JOY=11 THEN CX=CX-6:REM       Left? Decrease X
330 IF CX<1 THEN CX=13:REM           Wrap around
340 IF JOY=7 THEN CX=CX+6:REM        Right? Increase X
350 IF CX>13 THEN CX=1:REM           Wrap around
360 IF JOY=14 THEN CY=CY-1:REM       Up? Decrease Y
370 IF CY<3 THEN CY=8:REM            Wrap around
380 IF JOY=13 THEN CY=CY+1:REM       Down? Increase Y
390 IF CY>8 THEN CY=3:REM            Wrap around
400 FOR I=1 TO 100:NEXT I:REM        Delay for smooth moves

410 IF PEEK(53279)=6 THEN GOTO 160:REM Reset when start pressed
420 IF STRIG(0)=1 THEN GOTO 240:REM    Read joystick fire button

450 IF CX<>1 THEN GOTO 520:REM         Check X position
460 IF CY<>3 THEN GOTO 520:REM         Check Y position
470 ADC=USR(ADR(ADC$)):A=PEEK(206):REM Execute ADC op code
480 IF A=255 THEN A=0:POKE 206,A:REM   Min value 0
490 IF A>9 THEN A=9:POKE 206,A:REM     Max value 9
500 POSITION 15,15:PRINT #6; A:REM     Print new A
510 FOR I=1 TO 500:NEXT I:GOTO 240

520 IF CX<>1 THEN GOTO 590
530 IF CY<>4 THEN GOTO 590
540 DEC=USR(ADR(DEC$)):RAM=PEEK(203):REM Execute DEC op code
550 IF RAM=255 THEN RAM=0:POKE 203,RAM
560 IF RAM>9 THEN RAM=9:POKE 203,RAM
570 POSITION 3,15:PRINT #6; RAM
580 GOTO 240

590 IF CX<>1 THEN GOTO 660
600 IF CY<>5 THEN GOTO 660
610 DEX=USR(ADR(DEX$)):X=PEEK(204):REM Execute DEX op code
620 IF X=255 THEN X=0:POKE 204,X
630 IF X>9 THEN X=9:POKE 204,X
640 POSITION 11,15:PRINT #6; X
650 FOR I=1 TO 500:NEXT I:GOTO 240

660 IF CX<>1 THEN GOTO 730
670 IF CY<>6 THEN GOTO 730
680 DEY=USR(ADR(DEY$)):Y=PEEK(205):REM Execute DEY op code
690 IF Y=255 THEN Y=0:POKE 205,Y
700 IF Y>9 THEN Y=9:POKE 205,Y
710 POSITION 13,15:PRINT #6; Y
720 FOR I=1 TO 500:NEXT I:GOTO 240

730 IF CX<>1 THEN GOTO 800
740 IF CY<>7 THEN GOTO 800
750 INC=USR(ADR(INC$)):RAM=PEEK(203):REM Execute INC op code
760 IF RAM=255 THEN RAM=0:POKE 203,RAM
770 IF RAM>9 THEN RAM=9:POKE 203,RAM
780 POSITION 3,15:PRINT #6; RAM
790 FOR I=1 TO 500:NEXT I:GOTO 240

800 IF CX<>1 THEN GOTO 870
810 IF CY<>8 THEN GOTO 870
820 INX=USR(ADR(INX$)):X=PEEK(204):REM Execute INX op code
830 IF X=255 THEN X=0:POKE 204,X
840 IF X>9 THEN X=9:POKE 204,X
850 POSITION 11,15:PRINT #6; X
860 FOR I=1 TO 500:NEXT I:GOTO 240

870 IF CX<>7 THEN GOTO 940 
880 IF CY<>3 THEN GOTO 940 
890 INY=USR(ADR(INY$)):Y=PEEK(205):REM Execute INY op code
900 IF Y=255 THEN Y=0:POKE 205,Y
910 IF Y>9 THEN Y=9:POKE 205,Y
920 POSITION 13,15:PRINT #6; Y
930 FOR I=1 TO 500:NEXT I:GOTO 240

940 IF CX<>7 THEN GOTO 990 
950 IF CY<>4 THEN GOTO 990
960 LDA=USR(ADR(LDA$)):A=PEEK(206):REM Execute LDA op code
970 POSITION 15,15:PRINT #6; A
980 FOR I=1 TO 500:NEXT I:GOTO 240

990 IF CX<>7 THEN GOTO 1040 
1000 IF CY<>5 THEN GOTO 1040 
1010 LDX=USR(ADR(LDX$)):X=PEEK(204):REM Execute LDX op code
1020 POSITION 11,15:PRINT #6; X
1030 FOR I=1 TO 500:NEXT I:GOTO 240

1040 IF CX<>7 THEN GOTO 1090 
1050 IF CY<>6 THEN GOTO 1090 
1060 LDY=USR(ADR(LDY$)):Y=PEEK(205):REM Execute LDY op code
1070 POSITION 13,15:PRINT #6; Y
1080 FOR I=1 TO 500:NEXT I:GOTO 240

1090 IF CX<>7 THEN GOTO 1170 
1100 IF CY<>7 THEN GOTO 1170 
1110 IF RAM>A THEN GOTO 1170 
1120 SBC=USR(ADR(SBC$)):A=PEEK(206):REM Execute SBC op code
1130 IF A=255 THEN A=0:POKE 206,A
1140 IF A>9 THEN A=9:POKE 206,A
1150 POSITION 15,15:PRINT #6; A
1160 FOR I=1 TO 500:NEXT I:GOTO 240

1170 IF CX<>7 THEN GOTO 1220 
1180 IF CY<>8 THEN GOTO 1220 
1190 STA=USR(ADR(STA$)):RAM=PEEK(203):REM Execute STA op code
1200 POSITION 3,15:PRINT #6; RAM
1210 FOR I=1 TO 500:NEXT I:GOTO 240

1220 IF CX<>13 THEN GOTO 1270 
1230 IF CY<>3 THEN GOTO 1270 
1240 STX=USR(ADR(STX$)):RAM=PEEK(203):REM Execute STX op code
1250 POSITION 3,15:PRINT #6; RAM
1260 FOR I=1 TO 500:NEXT I:GOTO 240

1270 IF CX<>13 THEN GOTO 1320
1280 IF CY<>4 THEN GOTO 1320
1290 STY=USR(ADR(STY$)):RAM=PEEK(203):REM Execute STY op code
1300 POSITION 3,15:PRINT #6; RAM
1310 FOR I=1 TO 500:NEXT I:GOTO 240

1320 IF CX<>13 THEN GOTO 1370
1330 IF CY<>5 THEN GOTO 1370
1340 TAX=USR(ADR(TAX$)):X=PEEK(204):REM Execute TAX op code
1350 POSITION 11,15:PRINT #6; X
1360 FOR I=1 TO 500:NEXT I:GOTO 240

1370 IF CX<>13 THEN GOTO 1420 
1380 IF CY<>6 THEN GOTO 1420 
1390 TAY=USR(ADR(TAY$)):Y=PEEK(205):REM Execute TAY op code
1400 POSITION 13,15:PRINT #6; Y
1410 FOR I=1 TO 500:NEXT I:GOTO 240

1420 IF CX<>13 THEN GOTO 1470 
1430 IF CY<>7 THEN GOTO 1470 
1440 TXA=USR(ADR(TXA$)):A=PEEK(206):REM Execute TXA op code
1450 POSITION 15,15:PRINT #6; A
1460 FOR I=1 TO 500:NEXT I:GOTO 240

1470 IF CX<>13 THEN GOTO 1600 
1480 IF CY<>8 THEN GOTO 1600 
1490 TYA=USR(ADR(TYA$)):A=PEEK(206):REM Execute TYA op code
1500 POSITION 15,15:PRINT #6; A
1510 FOR I=1 TO 500:NEXT I:GOTO 240

1600 GOTO 240

2000 REM ******************************************
2001 REM *** MEMORY AND GRAPHICS INITIALIZATION ***
2002 REM ******************************************

2010 MEMTOP=PEEK(106):REM                 Find top of RAM
2020 GRTOP=MEMTOP-4:REM                   Make room for character set
2030 POKE 106,GRTOP:REM                   New top of RAM
2040 CHRAM=GRTOP*256:REM                  Address for character set

2050 DIM CSET$(50):REM                    Initialize strings
2060 DIM ADC$(10):DIM DEC$(6):DIM DEX$(9)
2070 DIM DEY$(9):DIM INC$(6):DIM INX$(9)
2080 DIM INY$(9):DIM LDA$(8):DIM LDX$(8)
2090 DIM LDY$(8):DIM SBC$(11):DIM STA$(8)
2100 DIM STX$(8):DIM STY$(8):DIM TAX$(9)
2110 DIM TAY$(9):DIM TXA$(9):DIM TYA$(9)

2120 GRAPHICS 1+16:POKE 752,1:REM  Set GR1 & clear the cursor

2300 RETURN

3000 REM *************************
3001 REM *** DRAW TITLE SCREEN ***
3002 REM *************************

3010 SETCOLOR 0,12,10:REM       Green text, upper case
3020 SETCOLOR 1,9,8:REM         Blue text, lower case
3030 SETCOLOR 4,0,0:REM         Black background
3040 PRINT #6;CHR$(125):REM     Clear the screen

3050 POSITION 0,0:PRINT #6; "THE INTERACTIVE 6502"
3060 POSITION 0,1:PRINT #6; "____________________"
3070 POSITION 2,3:PRINT #6; "ADC   INY   STX"
3080 POSITION 2,4:PRINT #6; "DEC   LDA   STY"
3090 POSITION 2,5:PRINT #6; "DEX   LDX   TAX"
3100 POSITION 2,6:PRINT #6; "DEY   LDY   TAY"
3110 POSITION 2,7:PRINT #6; "INC   SBC   TXA"
3120 POSITION 2,8:PRINT #6; "INX   STA   TYA"
3130 POSITION 0,9:PRINT #6; "____________________"
3140 POSITION 0,12:PRINT #6; "         by"
3150 POSITION 0,14:PRINT #6; "   jason h moore    "
3160 POSITION 0,18:PRINT #6; "____________________"
3170 POSITION 0,20:PRINT #6; "    please wait     "

3200 RETURN

4000 REM ***************************************
4001 REM *** MOVE AND REDEFINE CHARACTER SET ***
4002 REM ***************************************

4010 FOR I=1 TO 50:REM                Read in machine language commands
4020 READ DAT:CSET$(I)=CHR$(DAT):REM  Store code in string
4030 NEXT I:REM                       Loop
4040 GO=USR(ADR(CSET$),GRTOP,1,4):REM Execute machine code

4100 POUND=CHRAM+24:REM	             Address for # symbol to change
4110 FOR ADDR=POUND TO POUND+7:REM   Change # to filled square
4120 READ DAT:POKE ADDR,DAT:REM      Read new char data & put in memory
4130 NEXT ADDR:REM                   Loop

4140 POKE 756,GRTOP:REM              Tell ANTIC Location of character set

4200 RETURN

5000 REM ***************************************************
5001 REM *** READ IN MACHINE LANGUAGE ROUTINES (OPCODES) ***
5002 REM ***************************************************

5010 FOR I=1 TO 10:REM               ADC
5020 READ DAT:ADC$(I)=CHR$(DAT)
5030 NEXT I:REM

5040 FOR I=1 TO 6:REM                DEC         
5050 READ DAT:DEC$(I)=CHR$(DAT)
5060 NEXT I 

5070 FOR I=1 TO 9:REM                DEX                
5080 READ DAT:DEX$(I)=CHR$(DAT)
5090 NEXT I

5100 FOR I=1 TO 9:REM                DEY                
5110 READ DAT:DEY$(I)=CHR$(DAT)
5120 NEXT I

5130 FOR I=1 TO 6:REM                INC
5140 READ DAT:INC$(I)=CHR$(DAT)
5150 NEXT I
     
5160 FOR I=1 TO 9:REM                INX                
5170 READ DAT:INX$(I)=CHR$(DAT)
5180 NEXT I   

5190 FOR I=1 TO 9:REM                INY                
5200 READ DAT:INY$(I)=CHR$(DAT)
5210 NEXT I     

5220 FOR I=1 TO 8:REM                LDA
5230 READ DAT:LDA$(I)=CHR$(DAT)
5240 NEXT I  

5250 FOR I=1 TO 8:REM                LDX
5260 READ DAT:LDX$(I)=CHR$(DAT)
5270 NEXT I  

5280 FOR I=1 TO 8:REM                LDY
5290 READ DAT:LDY$(I)=CHR$(DAT)
5300 NEXT I   

5310 FOR I=1 TO 11:REM               SBC
5320 READ DAT:SBC$(I)=CHR$(DAT)
5330 NEXT I 

5340 FOR I=1 TO 8:REM                STA
5350 READ DAT:STA$(I)=CHR$(DAT)
5360 NEXT I  

5370 FOR I=1 TO 8:REM                STX
5380 READ DAT:STX$(I)=CHR$(DAT)
5390 NEXT I  

5400 FOR I=1 TO 8:REM                STY
5410 READ DAT:STY$(I)=CHR$(DAT)
5420 NEXT I  

5430 FOR I=1 TO 9:REM                TAX
5440 READ DAT:TAX$(I)=CHR$(DAT)
5450 NEXT I   

5460 FOR I=1 TO 9:REM                TAY
5470 READ DAT:TAY$(I)=CHR$(DAT)
5480 NEXT I  

5490 FOR I=1 TO 9:REM                TXA
5500 READ DAT:TXA$(I)=CHR$(DAT)
5510 NEXT I 

5520 FOR I=1 TO 9:REM                TYA
5530 READ DAT:TYA$(I)=CHR$(DAT)
5540 NEXT I  

5900 RETURN  

6000 REM ***********************
6001 REM *** DRAW THE SCREEN ***
6002 REM ***********************

6020 SETCOLOR 0,12,10:REM          Green text, black background
6030 PRINT #6;CHR$(125):REM        Clear the screen

6040 POSITION 0,0:PRINT #6; "THE INTERACTIVE 6502"
6050 POSITION 0,1:PRINT #6; "____________________"
6060 POSITION 2,3:PRINT #6; "ADC   INY   STX"
6070 POSITION 2,4:PRINT #6; "DEC   LDA   STY"
6080 POSITION 2,5:PRINT #6; "DEX   LDX   TAX"
6090 POSITION 2,6:PRINT #6; "DEY   LDY   TAY"
6100 POSITION 2,7:PRINT #6; "INC   SBC   TXA"
6110 POSITION 2,8:PRINT #6; "INX   STA   TYA"
6120 POSITION 0,9:PRINT #6; "____________________"
6130 POSITION 2,11:PRINT #6; "RAM    6502 CPU"
6140 POSITION 2,13:PRINT #6; "203      X Y A"
6150 POSITION 2,14:PRINT #6; "###     #######"
6160 POSITION 2,15:PRINT #6; "#0#######0#0#0#"
6170 POSITION 2,16:PRINT #6; "###     #######"
6180 POSITION 6,16:PRINT #6; "BUS"
6190 POSITION 0,18:PRINT #6; "____________________"
6200 POSITION 0,20:PRINT #6; "PRESS START TO RESET"

6300 RETURN

9000 REM ******************************************
9001 REM *** MACHINE LANGUAGE ROUTINES AND DATA ***
9002 REM ******************************************

9010 REM MOVE CHARACTER SET
9010 DATA 104,169,224,133,206,104,104,133,204,104,104,201,1
9020 DATA 240,4,230,206,230,204,104,104,141,176,6,169,0,133
9030 DATA 205,133,203,168,177,205,145,203,200,208,249,230
9040 DATA 206,230,204,206,176,6,208,240,96,0,0

9100 REM DATA FOR CUSTOM CHARACTER, # -> FILLED SQUARE
9110 DATA 255,255,255,255,255,255,255,255

9200 REM ASSEMBLY ROUTINE FOR ADC
9210 REM PLA,CLC,CLD,LDA page 0,206,ADC,203,STA page 0,206,RTS
9220 DATA 104,216,24,165,206,101,203,133,206,96

9230 REM ASSEMBLY ROUTINE FOR DEC
9240 REM PLA,CLC,CLD,DEC,203,RTS
9250 DATA 104,216,24,198,203,96

9260 REM ASSEMBLY ROUTINE FOR DEX
9270 REM PLA,CLC,CLD,LDX page 0,204,DEX,STX page 0,204,RTS
9280 DATA 104,216,24,166,204,202,134,204,96

9290 REM ASSEMBLY ROUTINE FOR DEY
9300 REM PLA,CLC,CLD,LDY page 0,205,DEY,STY page 0,205,RTS
9310 DATA 104,216,24,164,205,136,132,205,96

9320 REM ASSEMBLY ROUTINE FOR INC
9330 REM PLA,CLC,CLD,INC,203,RTS
9340 DATA 104,216,24,230,203,96

9350 REM ASSEMBLY ROUTINE FOR INX
9360 REM PLA,CLC,CLD,LDX page 0,204,INX,STX page 0,204,RTS
9370 DATA 104,216,24,166,204,232,134,204,96

9380 REM ASSEMBLY ROUTINE FOR INY
9390 REM PLA,CLC,CLD,LDY page 0,205,INY,STY page 0,205,RTS
9400 DATA 104,216,24,164,205,200,132,205,96

9410 REM ASSEMBLY ROUTINE FOR LDA
9420 REM PLA,CLC,CLD,LDA page 0,203,STA page 0,206,RTS
9430 DATA 104,216,24,165,203,133,206,96

9440 REM ASSEMBLY ROUTINE FOR LDX
9450 REM PLA,CLC,CLD,LDX page 0,203,STX page 0,204,RTS
9460 DATA 104,216,24,166,203,134,204,96

9470 REM ASSEMBLY ROUTINE FOR LDY
9480 REM PLA,CLC,CLD,LDY page 0,203,STY page 0,205,RTS
9490 DATA 104,216,24,164,203,132,205,96

9500 REM ASSEMBLY ROUTINE FOR SBC
9510 REM PLA,CLC,CLD,SEC,LDA page 0,206,SBC,203,
9520 REM STA page 0,206,RTS
9530 DATA 104,216,24,56,165,206,229,203,133,206,96

9540 REM ASSEMBLY ROUTINE FOR STA
9550 REM PLA,CLC,CLD,LDA page 0,206,STA page 0,203,RTS
9560 DATA 104,216,24,165,206,133,203,96

9570 REM ASSEMBLY ROUTINE FOR STX
9580 REM PLA,CLC,CLD,LDX page 0,204,STX page 0,203,RTS
9590 DATA 104,216,24,166,204,134,203,96

9600 REM ASSEMBLY ROUTINE FOR STY
9610 REM PLA,CLC,CLD,LDY page 0,205,STY page 0,203,RTS
9620 DATA 104,216,24,164,205,132,203,96

9630 REM ASSEMBLY ROUTINE FOR TAX
9640 REM PLA,CLC,CLD,LDA page 0,206,TAX,STX page 0,204,RTS
9650 DATA 104,216,24,165,206,170,134,204,96

9660 REM ASSEMBLY ROUTINE FOR TAY
9670 REM PLA,CLC,CLD,LDA page 0,206,TAY,STY page 0,205,RTS
9680 DATA 104,216,24,165,206,168,132,205,96

9690 REM ASSEMBLY ROUTINE FOR TXA
9700 REM PLA,CLC,CLD,LDX page 0,204,TXA,STA page 0,206,RTS
9710 DATA 104,216,24,166,204,138,133,206,96

9720 REM ASSEMBLY ROUTINE FOR TYA
9730 REM PLA,CLC,CLD,LDY page 0,205,TYA,STA page 0,206,RTS
9740 DATA 104,216,24,164,205,152,133,206,96