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Let’s Make a Game!

Step 8: Select and Reset Support

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

Original Blog Entry

For this update we're adding initial support for the Select and Reset buttons. For this we're adding a new RAM variable called GameState to keep track of "Game Active" vs "Game Over".

    ; D7, 1=Game Active, 0=Game Over
GameState:      ds 1    ; stored in $A7

We're going to use D7 to denote the state as we can easily test D7 (as well as D6) by using the BIT command. You can see this in the revised Vertical Blank routine were we test GameState to determine if UpdateTimer and ProcessJoystick should be skipped over:

VerticalBlank:
        jsr ProcessSwitches
        bit GameState
        bpl NotActive
        jsr UpdateTimer
        jsr ProcessJoystick
NotActive:        
        jsr PositionObjects
        jsr SetObjectColors
        jsr PrepScoreForDisplay
        rts             ; ReTurn from Subroutine

ProcessSwitches will check SWCHB to see if RESET is pressed. If so, it'll start up a new game. If not, it'll check if SELECT is pressed, and if so cancel an active game.

ProcessSwitches:
        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      
        rts
        
NotReset:
        lsr             ; D1 is now in C
        bcs NotSelect
        lda #0
        sta GameState   ; clear D7 to signify Game Over
        
NotSelect:
        rts

In the next update ProcessSwitches will be expanded upon so that the Select routine will let you select a game variation (and if you check the source you'll see a new Arena layout is already in place for that).

 

In order to visually show you that the game is over, I've revised the Color routines to color cycle if the game is not active.

SetObjectColors:
        lda #$FF
        sta Temp2       ; default to color mask
        and ColorCycle  ; color cycle
        bit GameState
        bpl SOCgameover
        lda #0          ; if game is active, no color cycle
SOCgameover:
        sta Temp
        ldx #4          ; we're going to set 5 colors (0-4)
        ldy #4          ; default to the color entries in the table (0-4)
        lda SWCHB       ; read the state of the console switches
        and #%00001000  ; test state of D3, the TV Type switch
        bne SOCloop     ; if D3=1 then use color
        ldy #$0f
        sty Temp2       ; set B&W mask
        ldy #9          ; else use the b&w entries in the table (5-9)
SOCloop:        
        lda Colors,y    ; get the color or b&w value
        eor Temp        ; color cycle
        and Temp2       ; B&W mask
        sta COLUP0-1,x  ; and set it
        dey             ; decrease Y
        dex             ; decrease X 
        bne SOCloop     ; Branch Not Equal to Zero
        lda Colors,y    ; get the Arena color
        eor Temp        ; color cycle
        and Temp2       ; B&W mask
        sta ArenaColor  ; save in RAM for Kernal Usage
        
        rts             ; ReTurn from Subroutine

Color cycle example:

Color Cycle

B&W Color Cycle example:

Black and White Color Cycle

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!

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.

 

 

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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.

 

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