Let’s Make a Game!

Step 4: 2 Line Kernel

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

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Original Blog Entry

Let's review the TIA Timing diagram from last time:


Timing Diagram


We used that to determine when we could safely update the playfield data in order to draw the score and timer. For moveable objects (player0, player1, missile0, missile1 and ball) if you update their graphics during the Visible Screen (cycles 23-76) you run the risk of shearing. For something that's moving fast, like the snowball in Stay Frosty 2, shearing may be an acceptable design compromise:

Stay Frosty 2

That snowball should be square, but the left edge has sheared due to the ball object being updated mid-scanline.


To prevent shearing we need to update the objects on cycles 0-22. There's a lot of calculations to be done in the kernel to draw just one player. For Collect I'm using DoDraw, which looks like this for drawing player0:

        lda #HUMAN_HEIGHT-1 ; 2  2 - height of the humanoid graphics, subtract 1 due to starting with 0
        dcp HumanDraw       ; 5  7 - Decrement HumanDraw and compare with height
        bcs DoDrawGrp0      ; 2  9 - (3 10) if Carry is Set, then humanoid is on current scanline
        lda #0              ; 2 11 - otherwise use 0 to turn off player0
        .byte $2C           ; 4 15 - $2C = BIT with absolute addressing, trick that
                            ;        causes the lda (HumanPtr),y to be skipped
DoDrawGrp0:                 ;   10 - from bcs DoDrawGrp0
        lda (HumanPtr),y    ; 5 15 - load the shape for player0
        sta GRP0            ; 3 18 - update player0 to draw Human

That's 18 cycles to draw a single player. One way to make it easier to fit all the code in is to use a 2 Line Kernel (2LK). In a 2LK we update the TIA's registers over 2 scanlines in order to build the display. For Collect, the current routines are updating them like this:

  1. player0, playfield
  2. player1, playfield


The actual code looks like this:

ldy #ARENA_HEIGHT   ; 2  7 - the arena will be 180 scanlines (from 0-89)*2        
ArenaLoop:                  ;   13 - from bpl ArenaLoop
    ; continuation of line 2 of the 2LK
    ; this precalculates data that's used on line 1 of the 2LK
        lda #HUMAN_HEIGHT-1 ; 2 15 - height of the humanoid graphics, subtract 1 due to starting with 0
        dcp HumanDraw       ; 5 20 - Decrement HumanDraw and compare with height
        bcs DoDrawGrp0      ; 2 22 - (3 23) if Carry is Set, then humanoid is on current scanline
        lda #0              ; 2 24 - otherwise use 0 to turn off player0
        .byte $2C           ; 4 28 - $2C = BIT with absolute addressing, trick that
                            ;        causes the lda (HumanPtr),y to be skipped
DoDrawGrp0:                 ;   23 - from bcs DoDrawGrp0
        lda (HumanPtr),y    ; 5 28 - load the shape for player0
        sta WSYNC           ; 3 31
    ; start of line 1 of the 2LK
        sta GRP0            ; 3  3 - @ 0-22, update player0 to draw Human
        ldx #%11111111      ; 2  5 - playfield pattern for vertical alignment testing
        stx PF0             ; 3  8 - @ 0-22
    ; precalculate data that's needed for line 2 of the 2LK        
        lda #HUMAN_HEIGHT-1 ; 2 10 - height of the humanoid graphics, 
        dcp BoxDraw         ; 5 15 - Decrement BoxDraw and compare with height
        bcs DoDrawGrp1      ; 2 17 - (3 18) if Carry is Set, then box is on current scanline
        lda #0              ; 2 19 - otherwise use 0 to turn off player1
        .byte $2C           ; 4 23 - $2C = BIT with absolute addressing, trick that
                            ;        causes the lda (BoxPtr),y to be skipped
DoDrawGrp1:                 ;   18 - from bcs DoDrawGRP1
        lda (BoxPtr),y      ; 5 23 - load the shape for the box
        sta WSYNC           ; 3 26
    ; start of line 2 of the 2LK
        sta GRP1            ; 3  3 - @0-22, update player1 to draw box
        ldx #0              ; 2  5 - PF pattern for alignment testing
        stx PF0             ; 3  8 - @0-22
        dey                 ; 2 10 - decrease the 2LK loop counter
        bpl ArenaLoop       ; 2 12 - (3 13) branch if there's more Arena to draw

If you look at that closely, you'll see I'm splitting DoDraw a bit so that this is how the 2LK works:

  1. update player0, update playfield, precalc player1 data for next scanline
  2. update player1, update playfield, precalc player0 data for next scanline


By pre-calculating data during the visible portion of the scanline, we'll have more time during the critical 0-22 cycles for when we add the other objects.


Since we're updating the players on every other scanline, each byte of graphic data is displayed twice (compare the thickness of the humanoid pixels with the red lines drawn with the playfield). Also, the players never line up as they're never updated on the same scanlines:



Scanlines Closeup

The designers of the TIA planned for this by adding a Vertical Delay feature to the players and ball (though sadly not the missiles). The TIA registers for this are VDELP0, VDELP1 and VDELBL. For this update to Collect, I've tied the Vertical Delay to the difficulty switches, putting the switch in position A will turn on the delay for that player so we can experiment with how that works. For the next update I'll set the Vertical Delay based on the Y position of the player (this also means the maximum Y value will be double that of this build).


Left Difficulty A, Right Difficulty B so VDELP0 = 1 and VDELP1 = 0. Sprites line up with the same Y:

Same Y


Scanlines Closeup

Left Difficulty B, Right Difficulty A so VDELP0 = 0 and VDELP1 = 1. Sprites line up when player1's Y = player0's Y + 1:

Sprites line up when player1's Y = player0's Y + 1.


Scanlines Closeup

The code that preps the data used by DoDraw looks like this:

; HumanDraw = ARENA_HEIGHT + HUMAN_HEIGHT - Y position
        sbc ObjectY
        sta HumanDraw
        ; HumanPtr = HumanGfx + HUMAN_HEIGHT - 1 - Y position
        lda #<(HumanGfx + HUMAN_HEIGHT - 1)
        sbc ObjectY
        sta HumanPtr
        lda #>(HumanGfx + HUMAN_HEIGHT - 1)
        sbc #0
        sta HumanPtr+1
        ; BoxDraw = ARENA_HEIGHT + HUMAN_HEIGHT - Y position
        sbc ObjectY+1
        sta BoxDraw
        ; BoxPtr = HumanGfx + HUMAN_HEIGHT - 1 - Y position
        lda #<(HumanGfx + HUMAN_HEIGHT - 1)
        sbc ObjectY+1
        sta BoxPtr
        lda #>(HumanGfx + HUMAN_HEIGHT - 1)
        sbc #0
        sta BoxPtr+1

        .byte %00011100
        .byte %00011000
        .byte %00011000
        .byte %00011000
        .byte %01011010
        .byte %01011010
        .byte %00111100
        .byte %00000000
        .byte %00011000
        .byte %00011000
HUMAN_HEIGHT = * - HumanGfx    

The graphics are much easier to see using my mode file for jEdit:

Human GFx

I'm sure some of you are wondering why the human graphics are upside down. If you wanted to loop through something 10 times, you'd normally think to write the code like this:

ldy #0
  ; do some work
  cpy #10
  bne Loop

But the 6507 does an automatic check for 0 (as well as positive/negative) which lets you save 2 cycles of processing time by eliminating the CPY command:

ldy #10
  ; do some work
  bne Loop

Alternatively, if your initial value is less than 128, you can use this:

ldy #(10-1)
  ; do some work
  bpl Loop

Making the loop count down instead of up saves 2 cycles, but doing so requires the graphics to be upside down. 2 cycles doesn't sound like much, but in a scanline that's 2.6% of your processing time and saving it might be what allows you to update everything you want. In Kernels I've written, I often use every cycleand that includes eliminating the sta WSYNC to buy back 3 cycles of processing time. See the reposition kernels in this post about Draconian.


I've also added joystick support that will let you move around the players. Pressing FIRE will slow down the movement, making it easier to line things up. The score (on the left) is used to display player0's Y position, and the timer is used for player1. As an added bonus, I'm showing how you can save ROM space by creating graphics that only face in one direction by using REFP0 and REFP1 (REFlect Player) to make the graphics face the other way. The routine's fairly sizable, so I'm not posting it here. Download the source code and check it out!


The ROM and the source are at the bottom of my blog entry.




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 Six Instruction Groups

The 6502 Instruction Set broken down into 6 groups.

6502 Instruction Set

Nice, simple instruction set in little boxes (not made out of ticky-tacky).



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

An easy-to-read page from The Second Book Of Machine Language.



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.

NMOS 6502 Opcodes

By John Pickens. Updated by Bruce Clark.



Guide to 6502 Assembly Language Programming by Andrew Jacobs

Below are direct links to the most important pages.


Goes over each of the internal registers and their use.

Instruction Set

Gives a summary of whole instruction set.

Addressing Modes

Describes each of the 6502 memory addressing modes.

Instruction Reference

Describes the complete instruction set in detail.



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. Stella finally got Atari 2600 quality sound in December of 2018. Until version 6.0, the game sounds in Stella were mostly OK, but not great. Now it's almost impossible to tell the difference between the sound effects in Stella and a real Atari 2600.




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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Interesting videos:

Charlie Robinson Talks About the Best Way to Wake Up Normies

Georgia Guidestones Explained

The Men Who Own Everything


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