dos33fsprogs/mode7/starfield_demo.s

.include "zp.inc"

;===========
; CONSTANTS
;===========

NUMSTARS	EQU	16


	;=====================
	; Starfield
	;=====================

	;================================
	; Clear screen and setup graphics
	;================================

	jsr	clear_screens		 ; clear top/bottom of page 0/1
	jsr     set_gr_page0

	; Initialize the 2kB of multiply lookup tables
	jsr	init_multiply_tables


	;===============
	; Init Variables
	;===============
	lda	#0
	sta	DRAW_PAGE
	sta	RANDOM_POINTER

	ldy	#NUMSTARS
init_stars:
	jsr	random_star
	dey
	bpl	init_stars

	;===========================
	;===========================
	; Main Loop
	;===========================
	;===========================

starfield_loop:

	;===============
	; clear screen
	;===============
	jsr	clear_top

	;===============
	; draw stars
	;===============

	ldx	#NUMSTARS

draw_stars:
	stx	XX
	txa
	tay

	; calculate color

	lda	#$ff
	sta	COLOR

	sty	YPOS

	; calculate x value, stars[i].x/stars[i].z
	; put 1/stars[i].z in NUM1H:NUM1L and multiply

	ldy	YPOS
	sty	XPOS

	lda	star_z,Y
	sta	NUM1H		; I

	lda	#0		; F
	sta	NUM1L

	; load stars[i].x into NUM2H:NUM2L
	; NUM2L is always zero


	lda	star_x,Y
	sta	NUM2H

	lda	#0
	sta	NUM2L
	sec			; don't reuse old values
	jsr	multiply

	; integer result in X
	txa
	clc
	adc	#20
;	sta	XPOS


	; calculate y value, stars[i].y/stars[i].z

	lda	#0		; I
	sta	NUM1H
	lda	#0		; F
	sta	NUM1L

	lda	#1
	sta	NUM2H
	lda	#2
	sta	NUM2L
	sec			; don't reuse old values
	jsr	multiply

	; integer result in X
	txa
	clc
	adc	#20

	tay			; put Y value in Y to plot
	lda	XPOS		; reload X value to plot

	;================================
	; plot routine
	;================================
	; put address in GBASL/GBASH
	; Xcoord in A, Ycoord in Y

	sta	TEMPY
	lda	gr_offsets,Y	; lookup low-res memory address		; 4
        clc								; 2
        adc	TEMPY							; 3
        sta	GBASL							; 3
        iny								; 2

        lda	gr_offsets,Y                                            ; 4
        adc	DRAW_PAGE	; add in draw page offset		; 3
        sta	GBASH							; 3
								;===========
								;

	lda	TEMPY
	and	#$1
	bne	plot_odd
plot_even:
	lda	COLOR
	and	#$f0
	sta	COLOR
	lda	(GBASL),Y
	and	#$0f
	jmp	plot_write
plot_odd:
	lda	COLOR
	and	#$0f
	sta	COLOR
	lda	(GBASL),Y
	and	#$f0

plot_write:
	ldy	#0
	ora	COLOR
	sta	(GBASL),Y


	;==============================
	ldx	XX

	dex
	bpl	draw_stars


starfield_keyboard:

	jsr	get_key		; get keypress				; 6

	lda	LASTKEY							; 3

	cmp	#('Q')		; if quit, then return
	bne	skipskip
	rts

skipskip:

	;==================
	; flip pages
	;==================

	jsr	page_flip						; 6

	;==================
	; loop forever
	;==================

	jmp	starfield_loop						; 3


; matches scroll_row1 - row3
star_x	EQU	$8A00
star_y	EQU	$8B00
star_z	EQU	$8C00

	;===================
	; star number in Y
	; FIXME: increment at end?
	; X trashed
random_star:
	; random x location
	ldx	RANDOM_POINTER
	lda	random_table,X
	inc	RANDOM_POINTER
	sta	star_x,Y

	; random y location
	ldx	RANDOM_POINTER
	lda	random_table,X
	inc	RANDOM_POINTER
	sta	star_y,Y

	; random z location
	ldx	RANDOM_POINTER
	lda	random_table,X
	inc	RANDOM_POINTER
	and	#$3f
	sta	star_z,Y

	rts

;===============================================
; External modules
;===============================================

.include "../asm_routines/hlin_clearscreen.s"
.include "../asm_routines/pageflip.s"
.include "../asm_routines/gr_setpage.s"
.include "../asm_routines/keypress.s"
.include "../asm_routines/gr_putsprite.s"
.include "../asm_routines/text_print.s"

;===============================================
; Variables
;===============================================


.include "../asm_routines/multiply_fast.s"


;======================
; some "random" numbers
;======================
random_table:
	.byte	103,198,105,115, 81,255, 74,236, 41,205,186,171,242,251,227, 70
	.byte	124,194, 84,248, 27,232,231,141,118, 90, 46 ,99, 51,159,201,154
	.byte	102, 50, 13,183, 49, 88,163, 90, 37, 93,  5, 23, 88,233, 94,212
	.byte	171,178,205,198,155,180, 84, 17, 14,130,116, 65, 33, 61,220,135
	.byte	112,233, 62,161, 65,225,252,103, 62,  1,126,151,234,220,107,150
	.byte	143, 56, 92, 42,236,176, 59,251, 50,175, 60, 84,236, 24,219, 92
	.byte     2, 26,254, 67,251,250,170, 58,251, 41,209,230,  5, 60,124,148
	.byte	117,216,190, 97,137,249, 92,187,168,153, 15,149,177,235,241,179
	.byte	  5,239,247,  0,233,161, 58,229,202, 11,203,208, 72, 71,100,189
	.byte	 31, 35, 30,168, 28,123,100,197, 20,115, 90,197, 94, 75,121, 99
	.byte	 59,112,100, 36, 17,158,  9,220,170,212,172,242, 27, 16,175, 59
	.byte	 51,205,227, 80, 72, 71, 21, 92,187,111, 34, 25,186,155,125,245
	.byte	 11,225, 26, 28,127, 35,248, 41,248,164, 27, 19,181,202, 78,232
	.byte	152, 50, 56,224,121, 77, 61, 52,188, 95, 78,119,250,203,108,  5
	.byte	172,134, 33, 43,170, 26, 85,162,190,112,181,115, 59,  4, 92,211
	.byte	 54,148,179,175,226,240,228,158, 79, 50, 21, 73,253,130, 78,169

z_table:
	; 1/16.0 - 1/12.25
	.byte	$10,$10,$10,$10,$11,$11,$11,$11,$12,$12,$12,$13,$13,$14,$14,$14
	; 1/12.0 - 1/8.25
	.byte	$15,$15,$16,$16,$17,$17,$18,$18,$19,$1A,$1A,$1B,$1C,$1D,$1E,$1F
	; 1/8.0 - 1/4.25
	.byte	$20,$21,$22,$23,$24,$25,$27,$28,$2A,$2C,$2E,$30,$33,$35,$38,$3C
	; 1/4.0 - 1/0.25
	.byte	$40,$44,$49,$4E,$55,$5D,$66,$71,$80,$92,$AA,$CC,$00,$55,$00,$00