starbot: improve comments
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Vince Weaver
parent
45094f7b9f
commit
546cf5918c
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@ -20,9 +20,16 @@
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; trying all kinds of stuff, including using BELL for WAIT
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; 139 bytes -- just made math innacurate (remove sec in sign correction)
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; zero page locations
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COLOR = $30
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star_z = $60
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oldx = $70
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oldy = $80
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star_x = $90
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star_y = $A0
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NEGATIVE = $F9
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QUOTIENT = $FA
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DIVISOR = $FB
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@ -31,22 +38,12 @@ XX = $FD
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YY = $FE
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FRAME = $FF
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star_z = $60
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oldx = $70
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oldy = $80
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star_x = $90
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star_y = $A0
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;oldx = $1000
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;oldy = $1040
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;star_x = $2000 ; should be 0, not used as we never /0
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;star_y = $2040
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;star_z = $2080
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; soft-switches
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LORES = $C056 ; Enable LORES graphics
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; ROM routines
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HGR2 = $F3D8
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HGR = $F3E2
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PLOT = $F800 ; PLOT AT Y,A (A colors output, Y preserved)
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@ -65,10 +62,8 @@ small_starfield:
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; draw the stars
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;===================================
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; bit LORES
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; jsr SETGR
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; tax
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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 #15
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make_orig_stars:
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@ -79,19 +74,17 @@ make_orig_stars:
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;===================================
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; starloop
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;2FORP=0TO5
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;2FORP=0TO15
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big_loop:
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ldx #15
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; txa
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; tay
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; ldy #30
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; jsr $FBE4 ; BEEP
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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 #80
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; txa
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jsr WAIT
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; jsr $FBE4 ; BEEP delays too
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; A now 0
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@ -99,10 +92,10 @@ star_loop:
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; X=FF
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;===================
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; erase old
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; erase old star
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;4 COLOR=0
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lda #$00
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lda #$00 ; color to black
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sta COLOR
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;PLOT O(P),Q(P)
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@ -118,7 +111,7 @@ star_loop:
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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
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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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@ -130,6 +123,8 @@ star_loop:
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jsr do_divide
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; if off-screen then need new star
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bmi new_star ; if <0
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cmp #40
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bcs new_star ; bge >39
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@ -146,8 +141,9 @@ star_loop:
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jsr do_divide
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; sta XX
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tay
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tay ; put XX in Y
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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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@ -166,17 +162,22 @@ draw_star:
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; COLOR=15
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dec COLOR ; color from $00 (black) to $ff (white)
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; trouble causing code, wanted it two-tone
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; this will make it white or blue depending on if odd or even star
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; initially tried distance based color on Z, didn't look as good
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txa
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ror
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bcs not_far
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jsr NEXTCOL
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; ror
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; jsr NEXTCOL
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; lda #$55
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; sta COLOR ; FF -> 7F
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not_far:
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;===========================
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; actually plot the star
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;PLOT X,Y
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; O(P)=X:Q(P)=Y
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@ -199,14 +200,7 @@ done_star:
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dex
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bpl star_loop
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; lda #120
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; jsr WAIT ; A is 0 after
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; jsr $FBE2 ; BEEP
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; jsr $FBE4 ; BEEP
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; GOTO2
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; beq big_loop ; bra
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bmi big_loop ; bra
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@ -238,7 +232,8 @@ color_lookup:
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rts
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;=============================
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; do divide
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; do signed divide
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; the signed part is the pain
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;=============================
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; Z is in divisor
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; x/y is in A
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@ -248,9 +243,9 @@ do_divide:
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php
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bpl not_negative
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eor #$ff
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clc
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adc #1 ; invert
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eor #$ff ; make positive for division
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clc ; is this necessary?
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adc #1
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not_negative:
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@ -268,13 +263,13 @@ div_loop:
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bpl pos_add
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eor #$ff
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; sec
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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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adc #20
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adc #20 ; pre-adjust to have star origin mid screen
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early_out:
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rts
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