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Atari 2600 Programming for Newbies

Session 8: Our First Kernel

By Andrew Davie (adapted by Duane Alan Hahn)

Table of Contents

Original Session

We're going to jump right in, now that we know what a kernel needs to do. Seen below, and in the attached file, is the source code for a working '2600 kernel. It displays the image you see here. Not bad for just a few lines of code. Over the next few sessions we'll learn how to modify this code, and assemble itand, of course, what all those strange words mean.

 

For now, have a look at the structure of the code and note how closely it relates to the structure of the TV frame diagram in the earlier sessions:

Don't expect to understand everythingwe'll walk through every line soon. For now, all you need to know is that the "sta WSYNC" is where the 6502 is telling the TIA to halt the 6502 until the start of the next horizontal blank period (which is at the start of the next scanline, at TIA color clock 0). So each of those lines is where one complete scanline has been sent to the TV by the TIA. Have a close look at those lines, and see how there are 3, followed by 37 (vertical blank period), followed by 192 (picture) followed by 30 (overscan)and how this exactly matches our TV frame diagram, above.

 

Yes, this is a complete kernel. It's not that difficult!

 

 

 

 

 

Source Code


            processor 6502

            include "vcs.h"

            include "macro.h"



            SEG

            ORG $F000



Reset

StartOfFrame



   ; Start of vertical blank processing



            lda #0

            sta VBLANK



            lda #2

            sta VSYNC

            

               ; 3 scanlines of VSYNCH signal...



                sta WSYNC

                sta WSYNC

                sta WSYNC



            lda #0

            sta VSYNC           



               ; 37 scanlines of vertical blank...



                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

            





               ; 192 scanlines of picture...

 

                ldx #0

                REPEAT 192; scanlines



                    inx

                    stx COLUBK

                    sta WSYNC



                REPEND



 

            lda #%01000010

            sta VBLANK                     ; end of screen - enter blanking



               ; 30 scanlines of overscan...



                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC

                sta WSYNC



            jmp StartOfFrame





            ORG $FFFA



            .word Reset          ; NMI

            .word Reset          ; RESET

            .word Reset          ; IRQ



    	END

I tried to make the code sample above as understandable as possible. It is certainly not the most efficient codefor it uses too many bytes of ROM to achieve its effect. But we're learning, and what's important right now is understanding how things work.

 

 

Here's a screenshot:

Here's the .bin file to use with an emulator:

kernel_01.bin

 

 

 

 

 

Summary

Next session we'll have a look at how to actually assemble this code using DASM, and how to make modifications so you can play with it and test it on the emulator to see what effect your changes have.

 

 

 

Other Assembly Language Tutorials

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

 

 

< Previous Session

 

 

Next Session >

 

 

 

 

Session Links

Session 1: Start Here

Session 2: Television Display Basics

Sessions 3 & 6: The TIA and the 6502

Session 4: The TIA

Session 5: Memory Architecture

Session 7: The TV and our Kernel

Session 8: Our First Kernel

Session 9: 6502 and DASM - Assembling the Basics

Session 10: Orgasm

Session 11: Colorful Colors

Session 12: Initialization

Session 13: Playfield Basics

Session 14: Playfield Weirdness

Session 15: Playfield Continued

Session 16: Letting the Assembler do the Work

Sessions 17 & 18: Asymmetrical Playfields (Parts 1 & 2)

Session 19: Addressing Modes

Session 20: Asymmetrical Playfields (Part 3)

Session 21: Sprites

Session 22: Sprites, Horizontal Positioning (Part 1)

Session 23: Moving Sprites Vertically

Session 24: Some Nice Code

Session 25: Advanced Timeslicing

 

 

 

 

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.

 

 

Atari 2600 BASIC

If assembly language is too hard for you, try batari Basic. It's a BASIC-like language for creating Atari 2600 games. It's the faster, easier way to make Atari 2600 games.

Try 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.

 

Use any example programs at your own risk. I am not responsible if they blow up your computer or melt your Atari 2600. Use assembly language at your own risk. I am not responsible if assembly language makes you cry or gives you brain damage.

 

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