By Andrew Davie (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
As an Amazon Associate I earn from qualifying purchases. (I get commissions for purchases made through certain links on this page.)
Page Table of Contents
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 it—and, 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 everything—we'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!
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 code—for 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:
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
Session 8: Our First Kernel
This book was written in English, not computerese. It's written for Atari users, not for professional programmers (though they might find it useful).
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 6502 Instruction Set broken down into 6 groups.
Nice, simple instruction set in little boxes (not made out of ticky-tacky).
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.
An easy-to-read page from The Second Book Of Machine Language.
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.
By John Pickens. Updated by Bruce Clark.
Below are direct links to the most important pages.
Goes over each of the internal registers and their use.
Gives a summary of whole instruction set.
Describes each of the 6502 memory addressing modes.
Describes the complete instruction set in detail.
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.
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.
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 programming specs (HTML version).
Links to useful information, tools, source code, and documentation.
Atari 2600 programming site based on Garon's "The Dig," which is now dead.
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.
Adapted information and charts related to Atari 2600 music and sound.
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.
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.
Some people appear to have a mental illness because they have a vitamin B deficiency. For example, the wife of a guy I used to chat with online had severe mood swings which seemed to be caused by food allergies or intolerances. She would became irrational, obnoxious, throw tantrums, and generally act like she had a mental illness. The horrid behavior stopped after she started taking a vitamin B complex. I’ve been taking #ad Jarrow B-Right for many years. It makes me much easier to live with.
Unfermented soy is bad! “When she stopped eating soy, the mental problems went away.” Fermented soy doesn’t bother me, but the various versions of unfermented soy (soy flour, soybean oil, and so on) that are used in all kinds of products these days causes a negative mental health reaction in me that a vitamin B complex can’t tame. The sinister encroachment of soy has made the careful reading of ingredients a necessity.
If you are overweight, have type II diabetes, or are worried about the condition of your heart, check out the videos by William Davis and Ivor Cummins. Most people should avoid wheat, not just those who have a wheat allergy or celiac disease. Check out these books: #ad Undoctored, #ad Wheat Belly, and #ad Eat Rich, Live Long.
Negative ions are good for us. You might want to avoid positive ion generators and ozone generators. Whenever I need a new air cleaner (with negative ion generator), I buy it from surroundair.com. A plain old air cleaner is better than nothing, but one that produces negative ions makes the air in a room fresher and easier for me to breathe. It also helps to brighten my mood.
Never litter. Toss it in the trash or take it home. Do not throw it on the ground.
Hydrofracking is bad for you, your family, your friends, and the environment.
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.