Useful
Inventions
Favorite
Quotes
Game
Design
Atari
Memories
Personal
Pages

Let’s Make a Game!

Step 11: Add the Ball Object

By Darrell Spice, Jr. (adapted by Duane Alan Hahn)

Original Blog Entry

The 2LK has been revised to display the ball object. Like player0, the ball needs to be primed before the ArenaLoop begins:

   ; prime ENABL so ball can appear on topmost scanline of Arena
        ldx #1              ; 2  2 - D1=0, so ball will be off 
        lda #BOX_HEIGHT-1   ; 2  4 - height of box graphic
        dcp BallDraw        ; 5  9 - Decrement BallDraw and compare with height
        bcs DoEnablPre      ; 2 11 - (3 12) if Carry is Set, then ball is on current scanline
        .byte $24           ; 3 14 - $24 = BIT with zero page addressing, trick that
                            ;        causes the inx to be skipped
DoEnablPre:                 ;   12 - from bcs DoEnablPre
        inx                 ; 2 14 - D1=1, so ball will be ON
        stx ENABL           ; 3 17
        
    ; prime GRP0 so player0 can appear on topmost scanline of the Arena        
        lda #HUMAN_HEIGHT-1 ; 2 19 - height of player0 graphics, 
        dcp Player0Draw     ; 5 24 - Decrement Player0Draw and compare with height
        bcs DoDrawGrp0pre   ; 2 26 - (3 27) if Carry is Set, then player0 is on current scanline
        lda #0              ; 2 28 - otherwise use 0 to turn off player0
        .byte $2C           ; 4 32 - $2C = BIT with absolute addressing, trick that
                            ;        causes the lda (Player0Ptr),y to be skipped
DoDrawGrp0pre:              ;   27 - from bcs DoDrawGRP0pre
        lda (Player0Ptr),y  ; 5 32 - load the shape for player0
        sta GRP0            ; 3 35 - @0-22, update player0 graphics
        dey                 ; 2 37
        
ArenaLoop:                  ;   37 - (currently 11 from bpl ArenaLoop)
        tya                 ; 2 39 - 2LK loop counter in A for testing
        and #%11            ; 2 41 - test for every 4th time through the loop,
        bne SkipX           ; 2 43 - (3 44) branch if not 4th time
        inc ArenaIndex      ; 5 48 - if 4th time, increase index so new playfield data is used
SkipX:                      ;   48 - use 48 as it's the longest path here        
 
    ; continuation of line 2 of the 2LK
    ; this precalculates data that's used on line 1 of the 2LK
        lda #HUMAN_HEIGHT-1 ; 2 50 - height of the humanoid graphics, subtract 1 due to starting with 0
        dcp Player1Draw     ; 5 55 - Decrement Player1Draw and compare with height
        bcs DoDrawGrp1      ; 2 57 - (3 58) if Carry is Set, then player1 is on current scanline
        lda #0              ; 2 59 - otherwise use 0 to turn off player1
        .byte $2C           ; 4 63 - $2C = BIT with absolute addressing, trick that
                            ;        causes the lda (Player1Ptr),y to be skipped
DoDrawGrp1:                 ;   58 - from bcs DoDrawGrp1
        lda (Player1Ptr),y  ; 5 63 - load the shape for player1
        sta WSYNC           ; 3 66
;---------------------------------------
    ; start of line 1 of the 2LK
        sta GRP1            ; 3  3 - @0-22, update player1 graphics
        ldx ArenaIndex      ; 3  6
        lda ArenaPF0,x      ; 4 10 - get current scanline's playfield pattern
        sta PF0             ; 3 13 - @0-22 and update it
        lda ArenaPF1,x      ; 4 17 - get current scanline's playfield pattern
        sta PF1             ; 3 20 - @71-28 and update it
        lda ArenaPF2,x      ; 4 24 - get current scanline's playfield pattern
        sta PF2             ; 3 27 - @60-39
        
    ; precalculate data that's needed for line 2 of the 2LK    
        ldx #1              ; 2 29 - D1=0, so ball will be off 
        lda #BOX_HEIGHT-1   ; 2 31 - height of box graphic
        dcp BallDraw        ; 5 36 - Decrement BallDraw and compare with height
        bcs DoEnabl         ; 2 38 - (3 39) if Carry is Set, then ball is on current scanline
        .byte $24           ; 3 41 - $24 = BIT with zero page addressing, trick that
                            ;        causes the inx to be skipped
DoEnabl:                    ;   39 - from bcs DoEnablPre
        inx                 ; 2 41 - D1=1, so ball will be ON
    
        lda #HUMAN_HEIGHT-1 ; 2 43 - height of the box graphics, 
        dcp Player0Draw     ; 5 48 - Decrement Player0Draw and compare with height
        bcs DoDrawGrp0      ; 2 50 - (3 51) if Carry is Set then player0 is on current scanline
        lda #0              ; 2 52 - otherwise use 0 to turn off player0
        .byte $2C           ; 4 56 - $2C = BIT with absolute addressing, trick that
                            ;        causes the lda (Player0Ptr),y to be skipped
DoDrawGrp0:                 ;   51 - from bcs DoDrawGRP0
        lda (Player0Ptr),y  ; 5 56 - load the shape for player0
        sta WSYNC           ; 3 59
;---------------------------------------
    ; start of line 2 of the 2LK
        sta GRP0            ; 3  3 - @0-22, update player0 graphics
        stx ENABL           ; 3  6 - @0-22, update ball graphics
        dey                 ; 2  8 - decrease the 2LK loop counter
        bne ArenaLoop       ; 2 10 - (3 11) branch if there's more Arena to draw
        sty PF0             ; 3 13 - Y is 0, blank out playfield
        sty PF1             ; 3 16 - Y is 0, blank out playfield
        sty PF2             ; 3 19 - Y is 0, blank out playfield
        rts                 ; 6 25 - ReTurn from Subroutine

I then modified RandomLocation to set all objects to the same location for comparison:

RandomLocation:
...        
    ; for alignment test, set to (100, 100)
        lda #100
        sta ObjectX,x
        sta ObjectY,x
        rts

This revealed a minor quirk with the TIAnamely that when objects are set to the same X position, missiles and the ball end up 1 pixel to the left of where a player ends up (player1 is the green square and it's directly on top of the red ball).

Arena

This is a known issue and the solution is to increase the X value by 1 to compensate. I did so by adding this to the end of RandomLocation:

RandomLocation:
...        
        cpx #2
        bcc RLdone
        inc ObjectX,x   ; missile and ball objects need their X adjusted
RLdone:        
        rts

And now player1 and the ball line up:

Arena

The NewGame routine revised last time already sets the ball to a random X-Y location, so all that's left to make it show up is to revise PositionObjects. Two changes are needed, first is to set the X position of the ball object by starting the POloop with X=4 (in the prior build X was initialized to 1), then prep a new variable BallDraw that's used by the 2LK to draw the ball object on the correct scanlines.

PositionObjects:
        ldx #4              ; position all objects
POloop        
        lda ObjectX,x       ; get the object's X position
        jsr PosObject       ; set coarse X position and fine-tune amount 
        dex                 ; DEcrement X
        bpl POloop          ; Branch PLus so we position all objects
        sta WSYNC           ; wait for end of scanline
        sta HMOVE           ; use fine-tune values to set final X positions
 
...
 

    ; prep ball's Y position for 2LK 
        ldx #1              ; preload X for setting VDELBL
        lda ObjectY+4       ; get the balls's Y position
        clc
        adc #1              ; add 1 to compensate for priming of ball        
        lsr                 ; divide by 2 for the 2LK position
        sta Temp            ; save for position calculations
        bcs NoDelayBL        ; if carry is set we don't need Vertical Delay
        stx VDELBL          ; carry was clear, so set Vertical Delay
NoDelayBL:        
    ; BallDraw = ARENA_HEIGHT + BOX_HEIGHT - Y position + 1
    ; the + 1 compensates for priming of ENABL    
        lda #(ARENA_HEIGHT + BOX_HEIGHT + 1)
        sec
        sbc Temp
        sta BallDraw        
        
        rts

Lastly, Overscan's been updated to process collisions with the ball.

OverScan:

...

TestCollisions:
    ; Test left player collisions

...

        bit CXP0FB      ; N = player0/playfield, V=player0/ball
...
notP0PF:                ; oVerflow flag is not affected by lda or sta
        bvc notP0BL     ; if V is off, then player0 did not collide with ball
        ldx #4          ; which box was collected
        jsr CollectBox  ; update score and reposition box
        
notP0BL:        

...
        
RightPlayer:        
    ; Test right player collisions  

...
        bit CXP1FB      ; N = player1/playfield, V=player1/ball
...
notP1PF:                ; oVerflow flag is not affected by lda or sta
        bvc notP1BL     ; if V is off, then player1 did not collide with ball
        ldx #4          ; which box was collected
        jsr CollectBox  ; update score and reposition box
        
notP1BL:

2 player games are now playable:

Arena

And 1 player games have 2 boxes to collect:

Arena

The ROM and the source are at the bottom of my blog entry.

 

 

 

 

 

< Previous Step

 

 

Next Step >

 

 

 

 

 

Table of Contents for Let’s Make a Game!

Introduction

Goals for this tutorial.

Step 1: Generate a Stable Display

On other systems, the video chip generates the display; on the 2600, your program generates the display.

Step 2: Timers

Improve the display generation by using the built-in timer.

Step 3: Score and Timer Display

Using the playfield to display information.

Step 4: 2 Line Kernel

Draw the player objects (sprites) on screen (X & Y location).

Step 5: Automate Vertical Delay

Finish the Y positioning of the player objects (sprites).

Step 6: Spec Change

Revise our goals.

Step 7: Draw the Playfield

Display an arena (like the mazes in Combat).

Step 8: Select and Reset Support

Using the Game Select and Game Reset console switches.

Step 9: Game Variations

How to implement game variations (number of players, different mazes).

Step 10: “Random Numbers”

How to randomize your game.

Step 11: Add the Ball Object

Draw the ball on screen (X & Y location).

Step 12: Add the Missile Objects

Draw the missiles on screen (X & Y location)

Step 13: Add Sound Effects

Let’s make some noise!

Step 14: Add Animation

Make the humans run instead of glide.

 

 

 

 

Useful Links

Easy 6502 by Nick Morgan

How to get started writing 6502 assembly language. Includes a JavaScript 6502 assembler and simulator.

 

 

Atari Roots by Mark Andrews (Online Book)

This book was written in English, not computerese. It's written for Atari users, not for professional programmers (though they might find it useful).

 

 

Machine Language For Beginners by Richard Mansfield (Online Book)

This book only assumes a working knowledge of BASIC. It was designed to speak directly to the amateur programmer, the part-time computerist. It should help you make the transition from BASIC to machine language with relative ease.

 

 

The Second Book Of Machine Language by Richard Mansfield (Online Book)

This book shows how to put together a large machine language program. All of the fundamentals were covered in Machine Language for Beginners. What remains is to put the rules to use by constructing a working program, to take the theory into the field and show how machine language is done.

 

 

6502 Instruction Set with Examples

A useful page from Assembly Language Programming for the Atari Computers.

 

 

6502.org

Continually strives to remain the largest and most complete source for 6502-related information in the world.

 

 

Guide to 6502 Assembly Language Programming by Andrew Jacobs

Below are direct links to the most important pages.

 

 

Stella Programmer's Guide

HTMLified version.

 

 

Nick Bensema's Guide to Cycle Counting on the Atari 2600

Cycle counting is an important aspect of Atari 2600 programming. It makes possible the positioning of sprites, the drawing of six-digit scores, non-mirrored playfield graphics and many other cool TIA tricks that keep every game from looking like Combat.

 

 

How to Draw A Playfield by Nick Bensema

Atari 2600 programming is different from any other kind of programming in many ways. Just one of these ways is the flow of the program.

 

 

Cart Sizes and Bankswitching Methods by Kevin Horton

The "bankswitching bible." Also check out the Atari 2600 Fun Facts and Information Guide and this post about bankswitching by SeaGtGruff at AtariAge.

 

 

Atari 2600 Specifications

Atari 2600 programming specs (HTML version).

 

 

Atari 2600 Programming Page (AtariAge)

Links to useful information, tools, source code, and documentation.

 

 

MiniDig

Atari 2600 programming site based on Garon's "The Dig," which is now dead.

 

 

TIA Color Charts and Tools

Includes interactive color charts, an NTSC/PAL color conversion tool, and Atari 2600 color compatibility tools that can help you quickly find colors that go great together.

 

 

The Atari 2600 Music and Sound Page

Adapted information and charts related to Atari 2600 music and sound.

 

 

Game Standards and Procedures

A guide and a check list for finished carts.

 

 

Stella

A multi-platform Atari 2600 VCS emulator. It has a built-in debugger to help you with your works in progress or you can use it to study classic games.

 

 

JAVATARI

A very good emulator that can also be embedded on your own web site so people can play the games you make online. It's much better than JStella.

 

 

batari Basic Commands

If assembly language seems a little too hard, don't worry. You can always try to make Atari 2600 games the faster, easier way with batari Basic.

 

 

Back to Top

 

Disclaimer

View this page and any external web sites at your own risk. I am not responsible for any possible spiritual, emotional, physical, financial or any other damage to you, your friends, family, ancestors, or descendants in the past, present, or future, living or dead, in this dimension or any other.

 

Home Inventions Quotations Game Design Atari Memories Personal Pages About Site Map Contact Privacy Policy Tip Jar