[stella] Paddle demo

[Joe Grand <jgrand@xxxxxxxxxxxxxx>]

Hi everyone-

Attached is a demo of the paddle controls I've been playing with today.. 
I'm moving a sprite up and down the screen and using the score display to 
show the value of pot0 (the counter calculated every frame which should be 
the approximate position of the paddle).. All paddle-related code is 
denoted by ** PADDLE ** in the source to make it easy for people to 
see..  I'm testing this using an Atari 200Jr. and Supercharger.

My problem is this: if pot0 < $80, the program goes totally wacky.

Can anyone help me figure out why the program crashes at $80 when it should 
really be able to draw the sprite all the way to the bottom of the screen? 
Because of this, to get the program to start, the paddle needs to be turned 
counter-clockwise.

Thanks! Hopefully when this demo code works, it will be useful to other 
people who want to design paddles into their game. I haven't come across 
any other source (other than the code snippets on the list)..

Joe

Attachment: paddle.bin
Description: Binary data

;
; Paddle test code for the Atari 2600 VCS
;
; By Joe Grand, jgrand@xxxxxxxxxxxxxx
;
; To be used in SCSIcide (http://www.mindspring.com/~jgrand/atari)
;
; All paddle specific code denoted by ** PADDLE **
;

        processor 6502
        include vcs.h

        SEG.U defines

YTOP            = 20    ; Top-most line of playfield
YBOTTOM         = 160   ; Bottom-most line of playfield


        SEG.U vars
        org $80         ; Allocate memory, PIA has 128 bytes of RAM ($80-$FF). Stack from $FF

verPosP0                ds      1       ; Vertical position of drivehead
pot0                    ds      1       ; ** PADDLE ** Player 0's paddle value

score                   ds      3       ; Player score (6 hex digits)
scorePtr0               ds      2       ; Pointer to score shapes
scorePtr1               ds      2
scorePtr2               ds      2
scorePtr3               ds      2
scorePtr4               ds      2
scorePtr5               ds      2

temp                    ds      1       ; Temporary variables for drawing the score
saveStack               ds      1
counter                 ds      1


        SEG code
        org $F000

Start
        SEI             ; Disable interrupts, if there are any.
        CLD             ; Clear BCD math bit.
        LDX  #$FF
        TXS             ; Set stack to beginning.

        ; Loop to clear memory and TIA registers
        LDA  #0
B1      STA  0,X
        DEX
        BNE  B1

        ; The above routine does not clear location 0, which is VSYNC

        JSR  GameInit           ;Initial variable and register setup
MainLoop
        JSR  VerticalBlank      ;Execute the vertical blank.
        JSR  CheckSwitches      ;Check console switches.
        JSR  GameCalc           ;Do calculations during Vblank
        JSR  DrawScreen         ;Draw the screen
        JSR  OverScan           ;Do more calculations during overscan
        JMP  MainLoop           ;Continue forever.


VerticalBlank  ;*********************** VERTICAL BLANK HANDLER
;
;Beginning of the frame - at the end of overscan.
;
        ; ** PADDLE **
        LDA  VB_DumpPots
        STA  VBLANK     ; Discharge paddle potentiometer

        LDA  #2         ;VBLANK was set at the beginning of overscan.
        STA  WSYNC
        STA  WSYNC
        STA  WSYNC
        STA  VSYNC      ;Begin vertical sync
        STA  WSYNC      ;First line of VSYNC
        STA  WSYNC      ;Second line of VSYNC

        ;Set the timer to go off just before the end of vertical blank space
        ;37 scanlines * 76 cycles = 2888 cycles / 64 = approx. 44
        LDA  #44
        STA  TIM64T
;
; End the VSYNC period.
;
        LDA  #0
        STA  WSYNC      ; Third line of VSYNC.
        STA  VSYNC      ; (0)
        RTS


CheckSwitches ;*************************** CONSOLE SWITCH HANDLER

        RTS

;
; Minimal game calculations, just to get the ball rolling.
;
GameCalc ;******************************* GAME CALCULATION ROUTINES

        LDA  #$30
        STA  CTRLPF             ;No Reflect,No Score,Low Priority,8 pixel-wide ball

        ; ** PADDLE **
        LDA  #0
        STA  pot0

        ;
        ; Set pointers to number data
        ; Score Score+1 Score+2
        ;
        LDX  #3-1
        LDY  #10
ScoreLoop
        LDA  score,X
        AND  #$0F               ; Mask it
        ASL                     ; Multiply by 8
        ASL
        ASL

        ADC  #<FontPage         ; Store the 2-byte pointer
        STA  scorePtr0,Y
        LDA  #0
        ADC  #>FontPage
        STA  scorePtr0+1,Y

        DEY
        DEY

        LDA  score,X
        AND  #$F0               ; Mask it
        LSR                     ; $00,$08,$10,$18,$20, ... ( / 8 = value)

        ADC  #<FontPage         ; Store the 2-byte pointer
        STA  scorePtr0,Y
        LDA  #0
        ADC  #>FontPage
        STA  scorePtr0+1,Y

        DEY
        DEY
        DEX
        BPL  ScoreLoop

        RTS


DrawScreen ;**************************** SCREEN DRAWING ROUTINES

        ;
        ; Initialize display variables.
        ;
        LDA  #$0
        STA  COLUBK     ;Background will be black.

        LDA  #$1E
        STA  COLUP0
        STA  COLUP1

        LDX  #3
        STX  NUSIZ0             ; Three copies of player, close together (for score)
        STX  NUSIZ1

WaitVBlank
        LDA  INTIM              ; Loop until timer is done - wait for the proper scanline
                                ; (i.e. somewhere in the last line of VBLANK)
        BNE  WaitVBlank         ; Whew! We're at the beginning of the frame
        LDA  #0
        STA  WSYNC              ; End the current scanline
        STA  HMOVE              ; Add horizontal motion to position sprites
        STA  VBLANK             ; End the VBLANK period with a zero, enable video, start recharge
        STA  WSYNC              ; Wait 2 scanlines before drawing score
        STA  WSYNC

        LDX  #22
Pause1
        DEX
        BNE  Pause1
        STA  RESP0

        STA  WSYNC
        LDX  #22
Pause2
        DEX
        BNE  Pause2
        NOP
        STA  RESP1
        STA  HMCLR
        LDA  #%10100000         ; P1 position 2 less than P0
        STA  HMP1
        LDA  #%11000000
        STA  HMP0
        STA  WSYNC
        STA  HMOVE
        STA  WSYNC


ScanLoop ; ================================== SCANNING LOOP
;
; Display the scores - 6 digit, courtesy of Robin Harbron
;
        LDA  #8-1              ; Lines to display
        STA  counter

        LDX  #9
Pause3
        DEX
        BNE  Pause3
        NOP
        NOP
        NOP

        TSX
        STX  saveStack          ; Save the stack pointer (Andrew Davie)
DrawScore
        LDY  counter            ; 3
        LDA  (scorePtr0),Y      ; 5
        STA  GRP0               ; 3 = 11

        BIT  $00

        LDA  (scorePtr1),Y      ; 5
        STA  GRP1               ; 3
        LDA  (scorePtr5),Y      ; 5
        TAX                     ; 2
        TXS                     ; 2
        LDA  (scorePtr2),Y      ; 5
        STA  temp               ; 3
        LDA  (scorePtr3),Y      ; 5
        TAX                     ; 2
        LDA  (scorePtr4),Y      ; 5 = 37

        LDY  temp               ; 3
        STY  GRP0               ; 3
        STX  GRP1               ; 3
        STA  GRP0               ; 3
        TSX                     ; 2
        STX  GRP1               ; 3 = 17

        DEC  counter            ; 5
        BPL  DrawScore          ; 3 =  8

        LDX  saveStack
        TXS

        LDA  #0
        STA  GRP0
        STA  GRP1

        ;
        ; Skip down a few lines
        ; Do other calculations here if necessary, we have lots of time
        ;

        LDY  #6
DrawWait
        STA  WSYNC
        DEY
        BNE  DrawWait

        LDX  #0
        STX  NUSIZ0     ; One copy of player
        STX  NUSIZ1

        LDY  #178
Kernel
        STA  WSYNC

        ; ** PADDLE **
        LDA  INPT0      ; Read paddle 0
        BMI  Charged    ; Capacitor already charged, skip increment
        INC  pot0       ; Increment paddle position value
Charged

        ;Position drive head on proper Y axis
        CPY  verPosP0
        BNE  NotYet

        ; Draw sprite
        LDX  #8-1
Next
        LDA  Sprite,X   ;Get the shape
        STA  GRP0
        STA  WSYNC      ;Double height
        STA  WSYNC
        DEY
        DEY
        DEX
        BPL  Next

        INY
        INX
        STX  GRP0      ;Disable graphics

NotYet
        DEY
        BNE  Kernel

        LDA  #2
        STA  WSYNC      ; Finish this scanline.
        STA  VBLANK     ; Make TIA output invisible.

        RTS


OverScan   ;***************************** OVERSCAN CALCULATIONS
;
; For the Overscan routine, one might take the time to process such
; things as collisions.
;

        ; ** PADDLE **
        ;
        ; Set new vertical position of drivehead based on paddle value
        ;
        LDX  pot0
        STX  verPosP0
        STX  score+2    ; Show paddle value as score for debugging purposes

;        CPX  #YTOP      ; If pot0 < YTOP, set to YTOP
;        BMI  PaddleCont
;        LDX  #YTOP
;        JMP  PaddleDone
;PaddleCont
;        CPX  #YBOTTOM   ; If pot0 > YBOTTOM, set to YBOTTOM
;        BMI  PaddleDone
;        LDX  #YBOTTOM
;PaddleDone
;        STX  verPosP0

        LDX  #30
KillLines
        STA  WSYNC
        DEX
        BNE  KillLines
        RTS


GameInit   ;***************************** INITIAL VARIABLE & REGISTER SETUP
        ;
        ; Clear data pointers
        ;
        LDX  #11
        LDA  #0
Clear2
        STA  scorePtr0,X
        DEX
        BPL  Clear2

        ;
        ; Initialize score value
        ;
        STA  score      ; A = 0
        STA  score+1
        STA  score+2

        ;LDA  #$AB
        ;STA  score
        ;LDA  #$CD
        ;STA  score+1
        ;LDA  #$EF
        ;STA  score+2

        RTS

;
; Must not cross page boundaries!!
; All shapes are upside down to allow decrementing by Y as both
; a counter and a shape index
;
        org $FE00 ; *********************** GRAPHICS DATA

;
; Numeric font
;
FontPage
Zero
                .byte   %00111100 ; |  XXXX  |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %00111100 ; |  XXXX  |

One
                .byte   %00111100 ; |  XXXX  |
                .byte   %00011000 ; |   XX   |
                .byte   %00011000 ; |   XX   |
                .byte   %00011000 ; |   XX   |
                .byte   %00011000 ; |   XX   |
                .byte   %00011000 ; |   XX   |
                .byte   %00111000 ; |  XXX   |
                .byte   %00011000 ; |   XX   |

Two
                .byte   %01111110 ; | XXXXXX |
                .byte   %01100000 ; | XX     |
                .byte   %01100000 ; | XX     |
                .byte   %00111100 ; |  XXXX  |
                .byte   %00000110 ; |     XX |
                .byte   %00000110 ; |     XX |
                .byte   %01000110 ; | X   XX |
                .byte   %00111100 ; |  XXXX  |

Three
                .byte   %00111100 ; |  XXXX  |
                .byte   %01000110 ; | X   XX |
                .byte   %00000110 ; |     XX |
                .byte   %00001100 ; |    XX  |
                .byte   %00001100 ; |    XX  |
                .byte   %00000110 ; |     XX |
                .byte   %01000110 ; | X   XX |
                .byte   %00111100 ; |  XXXX  |

Four
                .byte   %00001100 ; |    XX  |
                .byte   %00001100 ; |    XX  |
                .byte   %00001100 ; |    XX  |
                .byte   %01111110 ; | XXXXXX |
                .byte   %01001100 ; | X  XX  |
                .byte   %00101100 ; |  X XX  |
                .byte   %00011100 ; |   XXX  |
                .byte   %00001100 ; |    XX  |

Five
                .byte   %01111100 ; | XXXXX  |
                .byte   %01000110 ; | X   XX |
                .byte   %00000110 ; |     XX |
                .byte   %00000110 ; |     XX |
                .byte   %01111100 ; | XXXXX  |
                .byte   %01100000 ; | XX     |
                .byte   %01100000 ; | XX     |
                .byte   %01111110 ; | XXXXXX |

Six
                .byte   %00111100 ; |  XXXX  |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %01111100 ; | XXXXX  |
                .byte   %01100000 ; | XX     |
                .byte   %01100010 ; | XX   X |
                .byte   %00111100 ; |  XXXX  |

Seven
                .byte   %00011000 ; |   XX   |
                .byte   %00011000 ; |   XX   |
                .byte   %00011000 ; |   XX   |
                .byte   %00011000 ; |   XX   |
                .byte   %00001100 ; |    XX  |
                .byte   %00000110 ; |     XX |
                .byte   %01000010 ; | X    X |
                .byte   %01111110 ; | XXXXXX |

Eight
                .byte   %00111100 ; |  XXXX  |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %00111100 ; |  XXXX  |
                .byte   %00111100 ; |  XXXX  |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %00111100 ; |  XXXX  |

Nine
                .byte   %00111100 ; |  XXXX  |
                .byte   %01000110 ; | X   XX |
                .byte   %00000110 ; |     XX |
                .byte   %00111110 ; |  XXXXX |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %00111100 ; |  XXXX  |

A
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %01111110 ; | XXXXXX |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %00111100 ; |  XXXX  |

B
                .byte   %01111100 ; | XXXXX  |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %01111100 ; | XXXXX  |
                .byte   %01111100 ; | XXXXX  |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100110 ; | XX  XX |
                .byte   %01111100 ; | XXXXX  |

C
                .byte   %00111100 ; |  XXXX  |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100000 ; | XX     |
                .byte   %01100000 ; | XX     |
                .byte   %01100000 ; | XX     |
                .byte   %01100000 ; | XX     |
                .byte   %01100110 ; | XX  XX |
                .byte   %00111100 ; |  XXXX  |

D
                .byte   %01111100 ; | XXXXX  |
                .byte   %01100110 ; | XX  XX |
                .byte   %01100010 ; | XX   X |
                .byte   %01100010 ; | XX   X |
                .byte   %01100010 ; | XX   X |
                .byte   %01100010 ; | XX   X |
                .byte   %01100110 ; | XX  XX |
                .byte   %01111100 ; | XXXXX  |

E
                .byte   %01111110 ; | XXXXXX |
                .byte   %01100000 ; | XX     |
                .byte   %01100000 ; | XX     |
                .byte   %01100000 ; | XX     |
                .byte   %01111110 ; | XXXXXX |
                .byte   %01100000 ; | XX     |
                .byte   %01100000 ; | XX     |
                .byte   %01111110 ; | XXXXXX |

F
                .byte   %01100000 ; | XX     |
                .byte   %01100000 ; | XX     |
                .byte   %01100000 ; | XX     |
                .byte   %01100000 ; | XX     |
                .byte   %01111110 ; | XXXXXX |
                .byte   %01100000 ; | XX     |
                .byte   %01100000 ; | XX     |
                .byte   %01111110 ; | XXXXXX |

Sprite
                .byte  %01111110 ; | XXXXXX |
                .byte  %10000001 ; |X      X|
                .byte  %10111101 ; |X XXXX X|
                .byte  %10100101 ; |X X  X X|
                .byte  %10000001 ; |X      X|
                .byte  %10100101 ; |X X  X X|
                .byte  %01000010 ; | X    X |
                .byte  %00111100 ; |  XXXX  |


        .byte "J. Grand"


        org $FFFC

        .word  Start
        .word  Start