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Let’s Make a Game!

Step 1: Generate a Stable Display

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

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Original Blog Entry

First things firsthead over to MiniDig - Best of Stella and download the Stella Programmer's Guide from the docs page. I've also attached it to my blog entry, but you should still check out what's available over at MiniDig.


The heart of the Atari is the Television Interface Adaptor (TIA). It's the video chip, sound generator, and also handles some of the controller input. As a video chip, the TIA is very unusual. Most video game systems have memory that holds the current state of the display. Their video chip reads that memory and uses that information to generate the display. But not the TIAmemory was very expensive at the time, so the TIA was designed with a handful of registers that contain just the information needed to draw a single scanline. It's up to our program to change those registers in real-time so that each scanline shows what its supposed to. It's also up to our program to generate a "sync signal" that tells the TV when its time to start generating a new image.


Turn to page 2 of the programmer's guide. You'll find the following diagram, which I've slightly modified:

Modified Diagram

The Horizontal Blank is part of each scanline, so we don't need to worry about generating it. Everything else though is up to us! We need to generate a sync signal over 3 scanlines, after which we need to wait 37 scanlines before we tell TIA to "turn on" the image output. After that we need up update TIA so each of the 192 scanlines that comprise visible portion of the display show what they're supposed to. Once that's done, we "turn off" the image output and wait 30 scanlines before we start all over again.


In the source code, available below, you can see the Main Program Loop which follows the diagram:

        jsr VerticalSync    ; Jump to SubRoutine VerticalSync
        jsr VerticalBlank   ; Jump to SubRoutine VerticalBlank
        jsr Kernel          ; Jump to SubRoutine Kernel
        jsr OverScan        ; Jump to SubRoutine OverScan
        jmp Main            ; JuMP to Main

Each of the subroutines handles what's needed, such as this section which generates the sync signal:

        lda #2      ; LoaD Accumulator with 2 so D1=1
        sta WSYNC   ; Wait for SYNC (halts CPU until end of scanline)
        sta VSYNC   ; Accumulator D1=1, turns on Vertical Sync signal
        sta WSYNC   ; Wait for Sync - halts CPU until end of 1st scanline of VSYNC
        sta WSYNC   ; wait until end of 2nd scanline of VSYNC
        lda #0      ; LoaD Accumulator with 0 so D1=0
        sta WSYNC   ; wait until end of 3rd scanline of VSYNC
        sta VSYNC   ; Accumulator D1=0, turns off Vertical Sync signal
        rts         ; ReTurn from Subroutine

Currently there's no game logic, so the VerticalBlank just waits for the 37 scanlines to pass:

        ldx #37         ; LoaD X with 37
        sta WSYNC       ; Wait for SYNC (halts CPU until end of scanline)
        dex             ; DEcrement X by 1
        bne vbLoop      ; Branch if Not Equal to 0
        rts             ; ReTurn from Subroutine

The Kernel is the section of code that draws the screen:

    ; turn on the display
        sta WSYNC       ; Wait for SYNC (halts CPU until end of scanline)
        lda #0          ; LoaD Accumulator with 0 so D1=0
        sta VBLANK      ; Accumulator D1=1, turns off Vertical Blank signal (image output on)
    ; draw the screen        
        ldx #192        ; Load X with 192
        sta WSYNC       ; Wait for SYNC (halts CPU until end of scanline)
        stx COLUBK      ; STore X into TIA's background color register
        dex             ; DEcrement X by 1
        bne KernelLoop  ; Branch if Not Equal to 0
        rts             ; ReTurn from Subroutine

For this initial build it just changes the background color so we can see that we're generating a stable picture:

Stable Picture

Like Vertical Blank, OverScan doesn't have anything to do besides turning off the image output, so it just waits for enough scanlines to pass so that the total scanline count is 262.

        sta WSYNC   ; Wait for SYNC (halts CPU until end of scanline)
        lda #2      ; LoaD Accumulator with 2 so D1=1
        sta VBLANK  ; STore Accumulator to VBLANK, D1=1 turns image output off
        ldx #27     ; LoaD X with 27
        sta WSYNC   ; Wait for SYNC (halts CPU until end of scanline)
        dex         ; DEcrement X by 1
        bne osLoop  ; Branch if Not Equal to 0
        rts         ; ReTurn from Subroutine

Anyway, download the source at the bottom of my blog entry and take a lookthere are comments galore.




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 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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