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Let’s Make a Game!
Step 12: Add the Missile Objects
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 both missile objects. Missile objects do not support Vertical Delay, so there's no need to prime ENAM0 or ENAM1.
ArenaLoop: ; 17 - (currently 17 from bpl ArenaLoop)
; continuation of line 2 of the 2LK
; this precalculates data that's used on line 1 of the 2LK
tya ; 2 19 - 2LK loop counter in A for testing
and #%11 ; 2 21 - test for every 4th time through the loop,
bne SkipX ; 2 23 - (3 24) branch if not 4th time
inc ArenaIndex ; 5 28 - if 4th time, increase index so new playfield data is used
SkipX: ; 28 - use 28 as it's the longest path to here
ldx #1 ; 2 30 - D1=0, so missile1 will be off
lda #BOX_HEIGHT-1 ; 2 32 - height of box graphic
dcp Missile1Draw ; 5 37 - Decrement Missile1Draw and compare with height
bcs DoEnam1 ; 2 39 - (3 40) if Carry is Set, then missile1 is on current scanline
.byte $24 ; 3 42 - $24 = BIT with zero page addressing, trick that
; causes the inx to be skipped
DoEnam1: ; 40 - from bcs DoEnam1
inx ; 2 42 - D1=1, so ball will be ON
lda #HUMAN_HEIGHT-1 ; 2 44 - height of the humanoid graphics, subtract 1 due to starting with 0
dcp Player1Draw ; 5 49 - Decrement Player1Draw and compare with height
bcs DoDrawGrp1 ; 2 51 - (3 52) if Carry is Set, then player1 is on current scanline
lda #0 ; 2 53 - otherwise use 0 to turn off player1
.byte $2C ; 4 57 - $2C = BIT with absolute addressing, trick that
; causes the lda (Player1Ptr),y to be skipped
DoDrawGrp1: ; 52 - from bcs DoDrawGrp1
lda (Player1Ptr),y ; 5 57 - load the shape for player1
sta WSYNC ; 3 60
;---------------------------------------
; start of line 1 of the 2LK
sta GRP1 ; 3 3 - @0-22, update player1 graphics
stx ENAM1 ; 3 6 - @0-22, update missile1 graphics
ldx ArenaIndex ; 3 9
lda ArenaPF0,x ; 4 13 - get current scanline's playfield pattern
sta PF0 ; 3 16 - @0-22 and update it
lda ArenaPF1,x ; 4 20 - get current scanline's playfield pattern
sta PF1 ; 3 23 - @71-28 and update it
lda ArenaPF2,x ; 4 27 - get current scanline's playfield pattern
sta PF2 ; 3 30 - @60-39
; precalculate data that's needed for line 2 of the 2LK
ldx #1 ; 2 32 - D1=0, so missile0 will be off
lda #BOX_HEIGHT-1 ; 2 34 - height of box graphic
dcp Missile0Draw ; 5 39 - Decrement Missile0Draw and compare with height
bcs DoEnam0 ; 2 41 - (3 42) if Carry is Set, then missile0 is on current scanline
.byte $24 ; 3 44 - $24 = BIT with zero page addressing, trick that
; causes the inx to be skipped
DoEnam0: ; 42 - from bcs DoEnam0
inx ; 2 44 - D1=1, so ball will be ON
stx Temp ; 3 47 - save for line 2
ldx #1 ; 2 49 - D1=0, so ball will be off
lda #BOX_HEIGHT-1 ; 2 51 - height of box graphic
dcp BallDraw ; 5 56 - Decrement BallDraw and compare with height
bcs DoEnabl ; 2 58 - (3 59) if Carry is Set, then ball is on current scanline
.byte $24 ; 3 61 - $24 = BIT with zero page addressing, trick that
; causes the inx to be skipped
DoEnabl: ; 59 - from bcs DoEnablPre
inx ; 2 61 - D1=1, so ball will be ON
lda #HUMAN_HEIGHT-1 ; 2 63 - height of the box graphics,
dcp Player0Draw ; 5 68 - Decrement Player0Draw and compare with height
bcs DoDrawGrp0 ; 2 70 - (3 71) if Carry is Set then player0 is on current scanline
lda #0 ; 2 72 - otherwise use 0 to turn off player0
.byte $2C ; 4 76 - $2C = BIT with absolute addressing, trick that
; causes the lda (Player0Ptr),y to be skipped
; start of line 2 of the 2LK
DoDrawGrp0: ; 71 - from bcs DoDrawGRP0
lda (Player0Ptr),y ; 5 76 - load the shape for player0
;---------------------------------------
; 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
lda Temp ; 3 9 - get the precalced data for missile0
sta ENAM0 ; 3 12 - @0-22, update missile0 graphics
dey ; 2 14 - decrease the 2LK loop counter
bne ArenaLoop ; 2 16 - (3 17) branch if there's more Arena to draw
sty PF0 ; 3 19 - @0-22, Y is 0, blank out playfield
sty PF1 ; 3 22 - @71-28, Y is 0, blank out playfield
sty PF2 ; 3 25 - @60-39, Y is 0, blank out playfield
rts ; 6 31 - ReTurn from Subroutine
The 2 lines of the 2LK now work like this:
- updates player1, missile1, playfield, precalcs player0, missile0, ball for line 2
- updates player0, missile0, ball, precalcs player1, missile1 and playfield index for line 1
We didn't have enough time to update missile0 and missile1 on both lines of the 2LK. Since the TIA doesn't offer a vertical delay feature for the missiles, they can never line up. This is just like how the players didn't line up when VDELP0 and VDELP1 were not used. For our game, this isn't a problem.
One thing to notice about the revised 2LK is the 1st line uses exactly 76 cycles, so it no longer ends with a sta WSYNC.
PositionObjects has been modified to initialize 2 new variables, Missile0Draw and Missile1Draw, that are used by the 2LK to control when the missiles are drawn.
PositionObjects:
...
; prep missile0's Y position for 2LK
lda ObjectY+2 ; get the missile's Y position
lsr ; divide by 2 for 2LK
sta Temp ; save for position calculation
; Missile0Draw = ARENA_HEIGHT + BOX_HEIGHT - Y position
lda #(ARENA_HEIGHT + BOX_HEIGHT)
sec
sbc Temp
sta Missile0Draw
; prep missile1's Y position for 2LK
lda ObjectY+3 ; get the missile's Y position
lsr ; divide by 2 for 2LK
sta Temp ; save for position calculation
; Missile0Draw = ARENA_HEIGHT + BOX_HEIGHT - Y position
lda #(ARENA_HEIGHT + BOX_HEIGHT)
sec
sbc Temp
sta Missile1Draw
The routines are a little simpler than the others (for player0, player1 and ball) as the missiles don't have a Vertical Delay to set up. This is how the Arena looks with all the boxes drawn:
Not exactly right, is it? One thing we forgot to do was set the width of the missiles! We'd done that for the ball back in step 9 when we set the upper nybble of CTRLPF to 3:
TimerBar:
...
lda Variation ; 3 20
lsr ; 2 22 - which Arena to show
tay ; 2 24 - set for index
ldx ArenaOffset,y ; 4 28 - set X for which arena to draw
lda ArenaPF0,x ; 4 32 - reflect and priority for playfield
and #%00000111 ; 2 34 - get the lower 3 bits for CTRLPF
ora #%00110000 ; 2 36 - set ball to display as 8x pixel
sta CTRLPF ; 3 39
Which is why the ball was the right width when we added it in step 11. The same setting is needed for the missiles, but it goes into NUSIZ0 and NUSIZ1 instead:
VerticalSync:
...
lda #$30 ;
sta NUSIZ0 ; set missile0 to be 8x
sta NUSIZ1 ; set missile1 to be 8x
...
And now it looks correct:
OverScan was updated to detect collisions with the boxes drawn by the missiles:
OverScan:
...
notP0BL:
bit CXM0P ; V=player0/missile0
bvc notP0M0 ; if V is off then player0 did not collide with missile0
ldx #2 ; which box was collected
jsr CollectBox ; update score and reposition box
notP0M0:
bit CXM1P ; N=player0/missile1
bpl notP0M1 ; if N is off then player0 did not collide with missile1
ldx #3 ; which box was collected
jsr CollectBox ; update score and reposition box
notP0M1:
...
notP1BL:
bit CXM0P ; N=player1/missile0
bpl notP1M0 ; if N is off then player1 did not collide with missile0
ldx #2 ; which box was collected
jsr CollectBox ; update score and reposition box
notP1M0:
bit CXM1P ; V=player1/missile1
bvc notP1M1 ; if V is off then player1 did not collide with missile1
ldx #3 ; which box was collected
jsr CollectBox ; update score and reposition box
notP1M1:
A test run of the players collecting the boxes:
Lastly, on the off chance that somebody might roll the score, a new Digit graphic has been added:
And a minor change was made to CollectBox to display it and end the game if a player collected 100 boxes.
CollectBox:
sed ; SEt Decimal flag
clc ; CLear Carry bit
lda #1 ; 1 point per box
adc Score,y ; add to player's current score
bcc Not100 ; if the Carry is clear, score did not roll
sta GameState ; stop the game (A holds 0)
lda #$BB ; B image is !! to show that score rolled
Not100:
sta Score,y ; and save it
cld ; CLear Decimal flag
jsr RandomLocation ; move box to new location
rts
To test it, I started up a game and used Stella's debugger to change the score to 99:
Then collected another box:
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 12: Add the Missile Objects
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