Let’s Make a Game!

Step 9: Game Variations

By Darrell Spice, Jr. (adapted by Duane Alan Hahn, a.k.a. Random Terrain)

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Original Blog Entry

It's common for Atari games to have a number of game variations. To simplify the logic, the variations are usually driven by individual bits and/or groups of bits within a single byte that holds the game variation. A good example of that would be Space Invaderscheck out the game matrix from the manual:


Space Invaders


A byte is comprised of 8 bits, usually numbered 0-7 where 7 is the leftmost bit as in 76543210.


For Space Invaders, the bits in the game variation are used in this fashion:


Humans start counting at 1, but computers start at 0, so if you select game variation 12, internally it's really 11. 11 in binary is %00001011, which means bits 0, 1 and 3 are all turned on so the game variation has Invisible Invaders, ZigZagging Bombs and Moving Shields. You can confirm that by looking at column 12 of the Game Matrix above.


With this update to Collect, we're using bits 1 and 0 to give us 4 game variations:


If we have space at the end of the project, I plan to add some additional arenas. If we can add 2 more we'd just start using bit 2 and let the game Variation go from 1-8:


If we can add 6 more we'd add bit 3 and let the game variation go from 1-16


The ProcessSwitches routine has been modified so that hitting Select will increment the new variable Variation. It will also limit Variation to only the values 0-3. After changing Variation, the left score will be set to show Variation+1 as Humans prefer to see 1-4 instead of 0-3. The Right Score will be used to show the # of players, either 1 or 2.

        lsr             ; D1 is now in C
        bcs NotSelect   ; if D1 was on, the SELECT switch was not held
        lda #0
        sta GameState   ; clear D7 to signify Game Over        
        ldx Variation   ; Get the Game Variation
        inx             ; and increase it
        txa             ; transfer it to A
        and #%00000011  ; limit Variation to 0-3
        sta Variation   ; save it
        tax             ; transfer it to X
        inx             ; and increase it by 1 for the human readable varation 1-4
        stx Score       ; save in Score so it shows on left side
        ldy #1          ; default to showing 1 player variation 
        lsr             ; D0 of Variation, # of players, now in Carry flag
        bcc Not2        ; if Carry is clear, then show 1 player 
        iny             ; else set Y to 2 to show 2 players
        ror Players     ; put Carry into D7 for BIT testing of # of players
        sty Score+1     ; show the human readable # of players on right side

The routine works, but when Select is pressed the game variation will rapidly change making it difficult to select a specific game variation. You can see that in the build in my blog entry.


To fix that, we'll add a SelectDelay variable so that holding down SELECT will only result in Variation changing at the rate of once per second. However, if the user rapidly presses/releases SELECT then Variation will also rapidly change.

        lda SWCHB       ; load in the state of the switches
        lsr             ; D0 is now in C
        bcs NotReset    ; if D0 was on, the RESET switch was not held
        jsr InitPos     ; Prep for new game
        lda #%10000000  
        sta GameState   ; set D7 on to signify Game Active  
        bne NotSelect   ; clear SelectDelay
        lsr             ; D1 is now in C
        bcs NotSelect   ; if D1 was on, the SELECT switch was not held
        lda #0
        sta GameState   ; clear D7 to signify Game Over        
        lda SelectDelay ; do we need to delay the Select switch?
        beq SelectOK    ; if delay is 0 then no
        dec SelectDelay ; else decrement the delay
        rts             ; and exit the subroutine
        lda #60         ; Set the Select Delay to 1 second
        sta SelectDelay ;
        ldx Variation   ; Get the Game Variation
        inx             ; and increase it
        txa             ; transfer it to A
        and #%00000011  ; limit Variation to 0-3
        sta Variation   ; save it
        tax             ; transfer it to X
        inx             ; and increase it by 1 for the human readable varation 1-4
        stx Score       ; save in Score so it shows on left side
        ldy #1          ; default to showing 1 player variation 
        lsr             ; D0 of Variation, # of players, now in Carry flag
        bcc Not2        ; if Carry is clear, then show 1 player 
        iny             ; else set Y to 2 to show 2 players
        ror Players     ; put Carry into D7 for BIT testing of # of players
        sty Score+1     ; show the human readable # of players on right side
        lda #0          ; clears SelectDelay if SELECT not held
        sta SelectDelay

The routine PositionObjects has been modified to use a Box Graphic for player1 if a 1 player game has been selected:

        lda Variation       ; get the game variation
        and #1              ; and find out if we're 1 or 2 player
    ; Player1Ptr = BoxGfx + HUMAN_HEIGHT - 1 - Y position
        lda ShapePtrLow,x
        sbc Temp
        sta Player1Ptr
        lda ShapePtrHi,x
        sbc #0
        sta Player1Ptr+1
        .byte <(BoxGfx + HUMAN_HEIGHT - 1)
        .byte <(HumanGfx + HUMAN_HEIGHT - 1)
        .byte >(BoxGfx + HUMAN_HEIGHT - 1)
        .byte >(HumanGfx + HUMAN_HEIGHT - 1)

The Kernel has also been modified so that the correct Arena will be drawn. A little bit after TimerBar: you'll find this:

        lda Variation       ; 3 20
        lsr                 ; 2 22 - which Arena to show
        tay                 ; 2 24 - set for index
        ldx ArenaOffset,y   ; 4 28 - set X for which arena to draw
        lda ArenaPF0,x      ; 4 32 - reflect and priority for playfield
        and #%00000111      ; 2 34 - get the lower 3 bits for CTRLPF
        ora #%00110000      ; 2 36 - set ball to display as 8x pixel 
        sta CTRLPF          ; 3 39
        .byte 0         ; Arena 1
        .byte 22        ; Arena 2

The lsr command shifts bit 1 down to bit 0 so that we end up with 0 or 1 for the Arena number. That's used to set X to either 0 or 22 via the command ldx ArenaOffset,y.


I also added code to update CTRLPF based on the first PF0 data byte for the selected Arena. CTRLPF uses its bits like this:


Since PF0 only uses bits 7654, also known as the upper nybble of the byte, we can use the lower nybble to hold extra information to specify whether or not the selected Arena uses Playfield Priority (as opposed to Player Priority) or has a Reflected Playfield(as opposed to Repeated Playfield). We could even specify Score Mode which would just color the two sides of the playfield to match the colors of the players (like in the score display).

ArenaPF0:   ; PF0 is drawn in reverse order, and only the upper nybble
        .byte %11110001 ; Arena 1   lower nybble controls playfield, set for REFLECT
        .byte %00010000
        .byte %00010000
        .byte %00010000
        .byte %11110100 ; Arena 2 - lower nybble controls playfield, set for PLAYFIELD PRIORITY
        .byte %00010000
        .byte %00010000
        .byte %00010000

Game Variation 2, Arena 1 with 2 players. Arena 1 features Reflected Playfield and Player Priority:

Arena 1

Game Variation 3, Arena 2 with 1 player. Arena 2 features Repeated Playfield and Playfield Priority:

Arena 2

Look at the left Humanoid's head in each screenshot to see the difference that setting Playfield Priority makes. You might remember this being used in some games like Combat where the planes go behind the clouds.


Just for fun, here's Arena 2 with SCORE mode set (I've moved the players to the side of the screen they didn't start on):

Arena 2

The code change for that is:

.byte %11110010 ; Arena 2 - lower nybble controls playfield, set for SCORE

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




Other Assembly Language Tutorials

Be sure to check out the other assembly language tutorials and the general programming pages on this web site.


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Table of Contents for Let’s Make a Game!


Step 1: Generate a Stable Display

Step 2: Timers

Step 3: Score and Timer Display

Step 4: 2 Line Kernel

Step 5: Automate Vertical Delay

Step 6: Spec Change

Step 7: Draw the Playfield

Step 8: Select and Reset Support

Step 9: Game Variations

Step 10: “Random Numbers”

Step 11: Add the Ball Object

Step 12: Add the Missile Objects

Step 13: Add Sound Effects

Step 14: Add Animation





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 Six Instruction Groups

The 6502 Instruction Set broken down into 6 groups.

6502 Instruction Set

Nice, simple instruction set in little boxes (not made out of ticky-tacky).



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

An easy-to-read page from The Second Book Of Machine Language.



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.

NMOS 6502 Opcodes

By John Pickens. Updated by Bruce Clark.



Guide to 6502 Assembly Language Programming by Andrew Jacobs

Below are direct links to the most important pages.


Goes over each of the internal registers and their use.

Instruction Set

Gives a summary of whole instruction set.

Addressing Modes

Describes each of the 6502 memory addressing modes.

Instruction Reference

Describes the complete instruction set in detail.



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. Stella finally got Atari 2600 quality sound in December of 2018. Until version 6.0, the game sounds in Stella were mostly OK, but not great. Now it's almost impossible to tell the difference between the sound effects in Stella and a real Atari 2600.




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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Thought Experiment: What Happens After the Jab?

The Truth About Polio and Vaccines

What Is Causing the Mysterious Self-Assembling Non-Organic Clots and Sudden Deaths?

Full Video of Tennessee House of Representatives Health Subcommittee Hearing Room 2 (The Doctors Start Talking at 33:28)



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Watch these two videos for more information:

CO2 is Greening The Earth

The Climate Agenda



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Charlie Robinson had some good advice about waking up normies (see the link to the video below). He said instead of verbally unloading or being nasty or acting like a bully, ask the person a question. Being nice and asking a question will help the person actually think about the subject.


Interesting videos:

Charlie Robinson Talks About the Best Way to Wake Up Normies

Georgia Guidestones Explained

The Men Who Own Everything


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