Let’s Make a Game!
Step 11: Add the Ball Object
By Darrell Spice, Jr. (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
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Original Blog Entry
The 2LK has been revised to display the ball object. Like player0, the ball needs to be primed before the ArenaLoop begins:
; prime ENABL so ball can appear on topmost scanline of Arena
ldx #1 ; 2 2 - D1=0, so ball will be off
lda #BOX_HEIGHT-1 ; 2 4 - height of box graphic
dcp BallDraw ; 5 9 - Decrement BallDraw and compare with height
bcs DoEnablPre ; 2 11 - (3 12) if Carry is Set, then ball is on current scanline
.byte $24 ; 3 14 - $24 = BIT with zero page addressing, trick that
; causes the inx to be skipped
DoEnablPre: ; 12 - from bcs DoEnablPre
inx ; 2 14 - D1=1, so ball will be ON
stx ENABL ; 3 17
; prime GRP0 so player0 can appear on topmost scanline of the Arena
lda #HUMAN_HEIGHT-1 ; 2 19 - height of player0 graphics,
dcp Player0Draw ; 5 24 - Decrement Player0Draw and compare with height
bcs DoDrawGrp0pre ; 2 26 - (3 27) if Carry is Set, then player0 is on current scanline
lda #0 ; 2 28 - otherwise use 0 to turn off player0
.byte $2C ; 4 32 - $2C = BIT with absolute addressing, trick that
; causes the lda (Player0Ptr),y to be skipped
DoDrawGrp0pre: ; 27 - from bcs DoDrawGRP0pre
lda (Player0Ptr),y ; 5 32 - load the shape for player0
sta GRP0 ; 3 35 - @0-22, update player0 graphics
dey ; 2 37
ArenaLoop: ; 37 - (currently 11 from bpl ArenaLoop)
tya ; 2 39 - 2LK loop counter in A for testing
and #%11 ; 2 41 - test for every 4th time through the loop,
bne SkipX ; 2 43 - (3 44) branch if not 4th time
inc ArenaIndex ; 5 48 - if 4th time, increase index so new playfield data is used
SkipX: ; 48 - use 48 as it's the longest path here
; continuation of line 2 of the 2LK
; this precalculates data that's used on line 1 of the 2LK
lda #HUMAN_HEIGHT-1 ; 2 50 - height of the humanoid graphics, subtract 1 due to starting with 0
dcp Player1Draw ; 5 55 - Decrement Player1Draw and compare with height
bcs DoDrawGrp1 ; 2 57 - (3 58) if Carry is Set, then player1 is on current scanline
lda #0 ; 2 59 - otherwise use 0 to turn off player1
.byte $2C ; 4 63 - $2C = BIT with absolute addressing, trick that
; causes the lda (Player1Ptr),y to be skipped
DoDrawGrp1: ; 58 - from bcs DoDrawGrp1
lda (Player1Ptr),y ; 5 63 - load the shape for player1
sta WSYNC ; 3 66
;---------------------------------------
; start of line 1 of the 2LK
sta GRP1 ; 3 3 - @0-22, update player1 graphics
ldx ArenaIndex ; 3 6
lda ArenaPF0,x ; 4 10 - get current scanline's playfield pattern
sta PF0 ; 3 13 - @0-22 and update it
lda ArenaPF1,x ; 4 17 - get current scanline's playfield pattern
sta PF1 ; 3 20 - @71-28 and update it
lda ArenaPF2,x ; 4 24 - get current scanline's playfield pattern
sta PF2 ; 3 27 - @60-39
; precalculate data that's needed for line 2 of the 2LK
ldx #1 ; 2 29 - D1=0, so ball will be off
lda #BOX_HEIGHT-1 ; 2 31 - height of box graphic
dcp BallDraw ; 5 36 - Decrement BallDraw and compare with height
bcs DoEnabl ; 2 38 - (3 39) if Carry is Set, then ball is on current scanline
.byte $24 ; 3 41 - $24 = BIT with zero page addressing, trick that
; causes the inx to be skipped
DoEnabl: ; 39 - from bcs DoEnablPre
inx ; 2 41 - D1=1, so ball will be ON
lda #HUMAN_HEIGHT-1 ; 2 43 - height of the box graphics,
dcp Player0Draw ; 5 48 - Decrement Player0Draw and compare with height
bcs DoDrawGrp0 ; 2 50 - (3 51) if Carry is Set then player0 is on current scanline
lda #0 ; 2 52 - otherwise use 0 to turn off player0
.byte $2C ; 4 56 - $2C = BIT with absolute addressing, trick that
; causes the lda (Player0Ptr),y to be skipped
DoDrawGrp0: ; 51 - from bcs DoDrawGRP0
lda (Player0Ptr),y ; 5 56 - load the shape for player0
sta WSYNC ; 3 59
;---------------------------------------
; start of line 2 of the 2LK
sta GRP0 ; 3 3 - @0-22, update player0 graphics
stx ENABL ; 3 6 - @0-22, update ball graphics
dey ; 2 8 - decrease the 2LK loop counter
bne ArenaLoop ; 2 10 - (3 11) branch if there's more Arena to draw
sty PF0 ; 3 13 - Y is 0, blank out playfield
sty PF1 ; 3 16 - Y is 0, blank out playfield
sty PF2 ; 3 19 - Y is 0, blank out playfield
rts ; 6 25 - ReTurn from Subroutine
I then modified RandomLocation to set all objects to the same location for comparison:
RandomLocation:
...
; for alignment test, set to (100, 100)
lda #100
sta ObjectX,x
sta ObjectY,x
rts
This revealed a minor quirk with the TIA—namely that when objects are set to the same X position, missiles and the ball end up 1 pixel to the left of where a player ends up (player1 is the green square and it's directly on top of the red ball).
This is a known issue and the solution is to increase the X value by 1 to compensate. I did so by adding this to the end of RandomLocation:
RandomLocation:
...
cpx #2
bcc RLdone
inc ObjectX,x ; missile and ball objects need their X adjusted
RLdone:
rts
And now player1 and the ball line up:
The NewGame routine revised last time already sets the ball to a random X-Y location, so all that's left to make it show up is to revise PositionObjects. Two changes are needed, first is to set the X position of the ball object by starting the POloop with X=4 (in the prior build X was initialized to 1), then prep a new variable BallDraw that's used by the 2LK to draw the ball object on the correct scanlines.
PositionObjects:
ldx #4 ; position all objects
POloop
lda ObjectX,x ; get the object's X position
jsr PosObject ; set coarse X position and fine-tune amount
dex ; DEcrement X
bpl POloop ; Branch PLus so we position all objects
sta WSYNC ; wait for end of scanline
sta HMOVE ; use fine-tune values to set final X positions
...
; prep ball's Y position for 2LK
ldx #1 ; preload X for setting VDELBL
lda ObjectY+4 ; get the balls's Y position
clc
adc #1 ; add 1 to compensate for priming of ball
lsr ; divide by 2 for the 2LK position
sta Temp ; save for position calculations
bcs NoDelayBL ; if carry is set we don't need Vertical Delay
stx VDELBL ; carry was clear, so set Vertical Delay
NoDelayBL:
; BallDraw = ARENA_HEIGHT + BOX_HEIGHT - Y position + 1
; the + 1 compensates for priming of ENABL
lda #(ARENA_HEIGHT + BOX_HEIGHT + 1)
sec
sbc Temp
sta BallDraw
rts
Lastly, Overscan's been updated to process collisions with the ball.
OverScan:
...
TestCollisions:
; Test left player collisions
...
bit CXP0FB ; N = player0/playfield, V=player0/ball
...
notP0PF: ; oVerflow flag is not affected by lda or sta
bvc notP0BL ; if V is off, then player0 did not collide with ball
ldx #4 ; which box was collected
jsr CollectBox ; update score and reposition box
notP0BL:
...
RightPlayer:
; Test right player collisions
...
bit CXP1FB ; N = player1/playfield, V=player1/ball
...
notP1PF: ; oVerflow flag is not affected by lda or sta
bvc notP1BL ; if V is off, then player1 did not collide with ball
ldx #4 ; which box was collected
jsr CollectBox ; update score and reposition box
notP1BL:
2 player games are now playable:
And 1 player games have 2 boxes to collect:
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 3: Score and Timer Display
Step 5: Automate Vertical Delay
Step 8: Select and Reset Support
Step 11: Add the Ball Object
Step 12: Add the Missile Objects
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 6502 Instruction Set broken down into 6 groups.
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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.
An easy-to-read page from The Second Book Of Machine Language.
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Guide to 6502 Assembly Language Programming by Andrew Jacobs
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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 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.
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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.
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