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Let’s Make a Game!

Step 5: Automate Vertical Delay

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

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Original Blog Entry

For this update, we're going to double the Y range of the player objects. To use the new Y value for the 2LK we just need to divide it in half using the LSR command. The remainder of the divide, which ends up in the Carry flag, will conveniently tell us if we need to turn on Vertical Delay.


This routine preps the 2LK data for player0 and turns on VDELP0 if required (if you're wondering, VDELP0 is turned off in VerticalSync):

    ; prep Humanoid's Y position for 2LK 
        ldx #1              ; preload X for setting VDELPx
        lda ObjectY         ; get the human's Y position
        lsr                 ; divide by 2 for the 2LK position
        sta Temp            ; save for position calculations
        bcs NoDelay0        ; if carry is set we don't need Vertical Delay
        stx VDELP0          ; carry was clear, so set Vertical Delay
    ; HumanDraw = ARENA_HEIGHT + HUMAN_HEIGHT - Y position
        sbc Temp
        sta HumanDraw
    ; HumanPtr = HumanGfx + HUMAN_HEIGHT - 1 - Y position
        lda #<(HumanGfx + HUMAN_HEIGHT - 1)
        sbc Temp
        sta HumanPtr
        lda #>(HumanGfx + HUMAN_HEIGHT - 1)
        sbc #0
        sta HumanPtr+1

One minor problem with the prior 2LK was that player1 could not show up on the topmost scanline of the Arena:



Scanlines Closeup

To fix this, we'll modify the kernel to prime GRP1 before it enters the loop that draws the Arena:

        ldy #ARENA_HEIGHT+1 ; 2  7 - the arena will be 180 scanlines (from 0-89)*2
    ; prime GRP1 so player1 can appear on topmost scanline of the Arena        
        lda #BOX_HEIGHT-1   ; 2  9 - height of the box graphics, 
        dcp BoxDraw         ; 5 14 - Decrement BoxDraw and compare with height
        bcs DoDrawGrp1pre   ; 2 16 - (3 17) if Carry is Set, then box is on current scanline
        lda #0              ; 2 18 - otherwise use 0 to turn off player1
        .byte $2C           ; 4 22 - $2C = BIT with absolute addressing, trick that
                            ;        causes the lda (BoxPtr),y to be skipped
DoDrawGrp1pre:              ;   17 - from bcs DoDrawGRP1pre
        lda (BoxPtr),y      ; 5 22 - load the shape for the box
        sta GRP1            ; 3 25 - @0-22, update player1 to draw box
        dey                 ; 2 27
ArenaLoop:                  ;   13 - from bpl ArenaLoop

The 2LK calculations for player1 used to be the same as for player0, but now must be modified to compensate for the priming of GRP1:

    ; prep box's Y position for 2LK
        lda ObjectY+1       ; get the box's Y position
        adc #1              ; add 1 to compensate for priming of GRP1
        lsr                 ; divide by 2 for the 2LK position
        sta Temp            ; save for position calculations
        bcs NoDelay1        ; if carry is set we don't need Vertical Delay
        stx VDELP1          ; carry was clear, so set Vertical Delay
    ; BoxDraw = ARENA_HEIGHT + BOX_HEIGHT - Y position + 1
    ; the + 1 compensates for priming of GRP1
        lda #(ARENA_HEIGHT + BOX_HEIGHT +1)

        sbc Temp
        sta BoxDraw
    ; BoxPtr = BoxGfx + BOX_HEIGHT - 1 - Y position
        lda #<(BoxGfx + BOX_HEIGHT - 1)
        sbc Temp
        sta BoxPtr
        lda #>(BoxGfx + BOX_HEIGHT - 1)
        sbc #0
        sta BoxPtr+1

Added GRP1 priming which allows player1 to cover full Arena:



Scanlines Closeup

Lastly, I added a new Box graphic for player1:


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




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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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