Let’s Make a Game!

Step 7: Draw the Playfield

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

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Original Blog Entry

For this update, we're going to modify the Arena Loop to draw the Arena using the playfield. The new Arena loop has these new changes:

ArenaLoop:                  ;   27 - (currently 7 from bpl ArenaLoop)
        tya                 ; 2 29 - 2LK loop counter in A for testing
        and #%11            ; 2 31 - test for every 4th time through the loop,
        bne SkipX           ; 2 33 (3 34) branch if not 4th time
        inx                 ; 2 35 - if 4th time, increase X so new playfield data is used
SkipX:                      ;   35 - use 35 as it's the longest path here        
    ; start of line 1 of the 2LK
        sta GRP1            ; 3  3 - @0-22, update player1 graphics
        lda ArenaPF0,x      ; 4  7 - get current scanline's playfield pattern
        sta PF0             ; 3 10 - @0-22 and update it
        lda ArenaPF1,x      ; 4 14 - get current scanline's playfield pattern
        sta PF1             ; 3 17 - @71-28 and update it
        lda ArenaPF2,x      ; 4 21 - get current scanline's playfield pattern
        sta PF2             ; 3 24 - @60-39
    ; start of line 2 of the 2LK
        sta GRP0            ; 3  3 - @0-22, update player0 graphics
        dey                 ; 2  5 - decrease the 2LK loop counter
        bne ArenaLoop       ; 2  7 - (3  8) branch if there's more Arena to draw
        sty PF0             ; 3 10 - Y is 0, blank out playfield
        sty PF1             ; 3 13 - Y is 0, blank out playfield
        sty PF2             ; 3 16 - Y is 0, blank out playfield
        rts                 ; 6 22 - ReTurn from Subroutine

The first change is we're using X as an index into the playfield graphic data. We're changing X every fourth time thru the 2LK, so each byte of playfield data will be used over 8 scanlines. This saves a bit of ROM.


Second change is all 3 playfield registers (PF0, PF1 and PF2) are now updated, and they're only updated on line 1 of our 2LK.


Third change is on line 2, the bpl ArenaLoop is now a bne ArenaLoop else the bottom row of playfield data was only used for 2 scanlines instead of 8. We also blank out the playfield registers when we are done drawing the playfield. The bne change also impacted OverscanTIM64T was originally set to 32, it's now set to 35.


The playfield data looks like this in jEdit:

Playfield Data

And this onscreen:

Playfield Data On Screen

Lastly we added some collision detection code. Some space was allocated in RAM:

   ;save player locations for playfield collision logic
SavedX:         ds 2    ; stored in $A1-A2
SavedY:         ds 2    ; stored in $A3-A4

Then the Process Joystick routines save the current X and Y values before processing the joystick:

        ldy ObjectX,x   ; save original X location so the player can be
        sty SavedX,x    ;   bounced back upon colliding with the playfield
        ldy ObjectY,x   ; save original Y location so the player can be
        sty SavedY,x    ;   bounced back upon colliding with the playfield

Finally OverScan was modified to move the players back to their previous X and Y location if a collision was detected:

; Test if player collided with playfield
        bit CXP0FB      ; N = player0/playfield, V=player0/ball
        bpl notP0PF     ; if N is off, then player0 did not collide with playfield
        lda SavedX      ; recall saved X
        sta ObjectX     ; and move player back to it
        lda SavedY      ; recall saved Y
        sta ObjectY     ; and move player back to it
        bit CXP1FB      ; N = player1/playfield, V=player1/ball
        bpl notP1PF     ; if N is off, then player1 did not collide with playfield
        lda SavedX+1    ; recall saved X
        sta ObjectX+1   ; and move player back to it
        lda SavedY+1    ; recall saved Y
        sta ObjectY+1   ; and move player back to it

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






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Table of Contents for Let’s Make a Game!


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.

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.




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.




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.




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.



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