299 lines
4.5 KiB
ArmAsm
299 lines
4.5 KiB
ArmAsm
; starfield tiny -- Apple II Hires
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; by Vince `deater` Weaver
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; 139 bytes -- lores version
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; 157 bytes -- initial hires
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; 155 bytes -- scale to more of full screen
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; 153 bytes -- blue/orange instead of purple/green
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; 149 bytes -- optimize 2nd plot arg shuffling
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NUMSTARS = 27 ; 27 good, 28+ not work ($1C)
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; zero page locations
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star_z = $00
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HGR_BITS = $1C
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; 1C-40 has some things used by hires
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oldx = $50
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oldy = $70
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star_x = $90
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star_y = $B0
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; D0+ used by HGR routines
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HGR_COLOR = $E4
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HGR_PAGE = $E6
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SAVEX = $F8
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TEMP = $F9
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QUOTIENT = $FA
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DIVISOR = $FB
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DIVIDEND = $FC
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XX = $FD
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YY = $FE
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FRAME = $FF
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; soft-switches
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; ROM routines
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HGR2 = $F3D8 ; set hires page2 and clear $4000-$5fff
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HGR = $F3E2 ; set hires page1 and clear $2000-$3fff
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HPLOT0 = $F457 ; plot at (Y,X), (A)
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HCOLOR1 = $F6F0 ; set HGR_COLOR to value in X
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PLOT = $F800 ; PLOT AT Y,A (A colors output, Y preserved)
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NEXTCOL = $F85F ; COLOR=COLOR+3
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SETCOL = $F864 ; COLOR=A
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SETGR = $FB40 ; set graphics and clear LO-RES screen
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BELL2 = $FBE4
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WAIT = $FCA8 ; delay 1/2(26+27A+5A^2) us
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small_starfield:
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;0GR:DIMV(64,48):FORZ=1TO48:FORX=0TO64:V(X,Z)=(X*4-128)/Z+20:NEXTX,Z
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jsr HGR2 ; A is ? after
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;===================================
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; draw the stars
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;===================================
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; there are ways to skip this, but on real hardware there's
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; no guarantee star_z will be in a valid state, so waste the bytes
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ldx #NUMSTARS
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make_orig_stars:
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jsr make_new_star
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dex
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bpl make_orig_stars
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;===================================
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; starloop
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;2FORP=0TO15
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big_loop:
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ldx #NUMSTARS
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; really tried hard not to have to set this value
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; hard to judge best value for this
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; lda #100
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; jsr WAIT
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ldy #30
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jsr BELL2 ; BEEP delays too
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; A now 0
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star_loop:
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; X=FF
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;===================
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; erase old star
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;4 COLOR=0
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lda #$00 ; color to black
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sta HGR_COLOR ; set HGR_COLOR to value in X
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;HPLOT O(P),Q(P)
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stx SAVEX
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ldy oldx,X ; get X valu into Y
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; tya
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; tax
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sty TEMP
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lda oldy,X
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ldx TEMP
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ldy #0
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jsr HPLOT0 ; plot at (Y,X), (A)
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ldx SAVEX
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;===========================
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; position Z
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; lda star_z,X
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; beq new_star ; should never happen
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; sta DIVISOR
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; DIVISOR always star_z,X so can hard code this in divide routine
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;==============================
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; get Y/Z
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; Y=V(B(P),Z(P))
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; get YY
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lda star_y,X ; get Y of star
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jsr do_divide
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adc #96
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; if off-screen then need new star
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cmp #192
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bcs new_star ; bge >39
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cmp #0
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bcc new_star ; if <0
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sta YY ; YY
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sta oldy,X ; ;save for next time to erase
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;==============================
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; get X/Z
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; X=V(A(P),Z(P))
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; get XX
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lda star_x,X ; get X of start
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jsr do_divide
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adc #128
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tay ; put XX in Y
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sty oldx,X ; save for next time to erase
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; if offscreen then draw new star
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; bmi new_star ; if <0
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; cpy #40
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; bcs new_star ; bge >40
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;========================
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; adjust Z
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;Z(P)=Z(P)-1
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dec star_z,X
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beq new_star ; if Z=0 new star
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; draw the star
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draw_star:
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; lda #$7f ; white (with green/purple highlights)
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; sta HGR_COLOR ; set HGR_COLOR to value in X
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dec HGR_COLOR ; smaller, but blue/orange highlights
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;===========================
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; actually plot the star
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;HPLOT X,Y
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; O(P)=X:Q(P)=Y
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stx SAVEX
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tya ; XX in Y
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tax ; XX now in X
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lda YY ; YY
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ldy #0
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jsr HPLOT0 ; plot at (Y,X), (A)
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ldx SAVEX
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jmp done_star ; bra
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new_star:
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jsr make_new_star ;
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done_star:
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;7NEXT
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dex
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bpl star_loop
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; GOTO2
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bmi big_loop ; bra
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;===========================
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; NEW STAR
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;===========================
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make_new_star:
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;IFX<0ORX>39ORY<0ORY>39ORZ(P)<1THEN
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; A(P)=RND(1)*64
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; B(P)=RND(1)*64
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; Z(P)=RND(1)*48+1:GOTO7
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ldy FRAME
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lda $F000,Y
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sta star_x,X ; random XX
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color_lookup:
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lda $F100,Y
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sta star_y,X ; random YY
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lda $F002,Y
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and #$7f ; random ZZ 0..127 (can't go negative or stars move backward)
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ora #$1 ; avoid 0
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sta star_z,X
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inc FRAME
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rts
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;=============================
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; do signed divide
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; the signed part is the pain
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;=============================
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; Z is in divisor
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; x/y is in A
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do_divide:
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; A was just loaded so flags still valid
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php
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bpl not_negative
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eor #$ff ; make positive for division
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clc ; is this necessary?
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adc #1
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not_negative:
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ldy #$ff ; QUOTIENT
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div_loop:
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iny ; inc QUOTIENT
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sec
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sbc star_z,X ; DIVIDEND=DIVIDEND-DIVISOR
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bpl div_loop
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; write out quotient
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tya ; lda QUOTIENT
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plp
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bpl pos_add
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eor #$ff
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; sec ; FIXME: made math inaccurate to save room
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; bcs do_add
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pos_add:
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clc
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do_add:
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early_out:
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rts
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; for BASIC bot load
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; need this to be at $3F5
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; it's at 8C, so 6D
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jmp small_starfield
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