Atari 2600 Programming for Newbies
Session 8: Our First Kernel
By Andrew Davie (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
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Page 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 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
Be sure to check out the other assembly language tutorials and the general programming pages on this web site.
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Session 2: Television Display Basics
Sessions 3 & 6: The TIA and the 6502
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 14: Playfield Weirdness
Session 15: Playfield Continued
Session 16: Letting the Assembler do the Work
Sessions 17 & 18: Asymmetrical Playfields (Parts 1 & 2)
Session 20: Asymmetrical Playfields (Part 3)
Session 22: Sprites, Horizontal Positioning (Part 1)
Session 22: Sprites, Horizontal Positioning (Part 2)
Session 23: Moving Sprites Vertically
Session 25: Advanced Timeslicing
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.
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.