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8bitworkshop/presets/vcs/examples/missiles.a
2018-09-30 21:07:45 -04:00

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processor 6502
include "vcs.h"
include "macro.h"
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;
; Besides the two 8x1 sprites ("players") the TIA has
; two "missiles" and one "ball", which are just variable-length
; dots or dashes. They have similar positioning and display
; requirements, so we're going to make a subroutine that can
; set the horizontal position of any of them.
; But we can also use the HMPx/HMOVE registers directly to move the
; objects by small offsets without using this routine every time.
;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
org $f000
counter equ $81
; Initialize and set initial offsets of objects.
start CLEAN_START
lda #10
ldx #0
jsr SetHorizPos ; set player 0 horiz. pos
inx
lda #130
jsr SetHorizPos ; set player 1 horiz. pos
inx
lda #40
jsr SetHorizPos ; set missile 0 horiz. pos
lda #$10
sta NUSIZ0 ; make missile 0 2x-wide
inx
lda #70
jsr SetHorizPos ; set missile 1 horiz. pos
lda #$20
sta NUSIZ1 ; make missile 1 4x-wide
inx
lda #100
jsr SetHorizPos ; set ball horiz. pos
lda #$30
sta CTRLPF ; set ball 8x-wide
sta WSYNC
sta HMOVE
; We've technically generated an invalid frame because
; these operations have generated superfluous WSYNCs.
; But it's just at the beginning of our program, so whatever.
; Next frame loop
nextframe
VERTICAL_SYNC
; 36 lines of VBLANK
ldx #36
lvblank sta WSYNC
dex
bne lvblank
; Draw 192 scanlines
; We're going to draw both players, both missiles, and the ball
; straight down the screen. We can draw various kinds of vertical
; lines this way.
ldx #192
stx COLUBK ; set the background color
lda #0 ; A changes every scanline
ldy #0 ; Y is sprite data index
lvscan
sta WSYNC ; wait for next scanline
lda NUMBERS,y
sta GRP0 ; set sprite 0 pixels
sta GRP1 ; set sprite 1 pixels
tya ; we'll use the Y position, only the 2nd bit matters
sta ENAM0 ; enable/disable missile 0
sta ENAM1 ; enable/disable missile 1
sta ENABL ; enable/disable ball
iny
cpy #60
bne wrap1 ; wrap Y at 60 to 0
ldy #0
wrap1
dex
bne lvscan ; repeat next scanline until finished
; Clear all colors to black before overscan
stx COLUBK
stx COLUP0
stx COLUP1
stx COLUPF
; 30 lines of overscan
ldx #25
lvover sta WSYNC
dex
bne lvover
; Move all the objects by a different offset using HMP/HMOVE registers
; We'll hard-code the offsets in a table for now
ldx #0
hmoveloop
lda MOVEMENT,x
sta HMCLR
sta HMP0,x
sta WSYNC
sta HMOVE
inx
cpx #5
bcc hmoveloop
; This loop also gave us 5 extra scanlines = 30 total
; Cycle the sprite colors for the next frame
inc counter
lda counter
sta COLUP0
clc
ror
sta COLUP1
clc
ror
sta COLUPF
jmp nextframe
; SetHorizPos - Sets the horizontal position of an object.
; The X register contains the index of the desired object:
; X=0: player 0
; X=1: player 1
; X=2: missile 0
; X=3: missile 1
; X=4: ball
; This routine does a WSYNC and HMOVE before executing,
; so whatever you do here will not take effect until you
; call the routine again or do your own WSYNC and HMOVE.
SetHorizPos
sta WSYNC ; start a new line
sta HMOVE ; apply the previous fine position(s)
sta HMCLR ; reset the old horizontal position(s)
sec ; set carry flag
DivideLoop
sbc #15 ; subtract 15
bcs DivideLoop ; branch until negative
eor #7 ; calculate fine offset
asl
asl
asl
asl
sta RESP0,x ; fix coarse position
sta HMP0,x ; set fine offset
rts ; return to caller
; Hard-coded values for movement registers
MOVEMENT
.byte $f0 ; +1 pixels
.byte $e0 ; +2 pixels
.byte $c0 ; +4 pixels
.byte $10 ; -1 pixels
.byte $20 ; -2 pixels
; Bitmap pattern for digits
NUMBERS ;;{w:8,h:6,count:10,brev:1};;
.byte $EE,$AA,$AA,$AA,$EE,$00
.byte $22,$22,$22,$22,$22,$00
.byte $EE,$22,$EE,$88,$EE,$00
.byte $EE,$22,$66,$22,$EE,$00
.byte $AA,$AA,$EE,$22,$22,$00
.byte $EE,$88,$EE,$22,$EE,$00
.byte $EE,$88,$EE,$AA,$EE,$00
.byte $EE,$22,$22,$22,$22,$00
.byte $EE,$AA,$EE,$AA,$EE,$00
.byte $EE,$AA,$EE,$22,$EE,$00
; Epilogue
org $fffc
.word start
.word start