base:8_sprite_starfield
8 Sprite Starfield
This piece of source, done in ACME shows you a very simple way to create a simple star field/space feature, using 8 sprites. Basically this is a simple routine that will wrap 8 stars across the screen, according to the speed table. Maybe some other time I'll work on a routine which will display more than 8 stars in its simplest form.
;=========================================================================== ;Simple sprite starfield by Richard Bayliss ;=========================================================================== sync = $0340 starpos = $0350 !to "8spritefield.prg",cbm * = $0900 sei jsr $ff81 ; Clear the screen lda #$00 ;Black border + screen sta $d020 sta $d021 lda #$ff sta $d015 ;Turn on all sprites lda #$00 sta $d017 ;No sprite expansion X sta $d01b ;Sprites in front of chars sta $d01d ;No sprite expansion Y ldx #$00 clr2000 lda #$00 sta $2000,x ;Fill $2000 with zero inx bne clr2000 lda #$01 ;Create a dot for the sprite starfield sta $2000 ldx #$00 setsprs lda #$80 ;Sprite object data from $2000-$2080 sta $07f8,x lda #$01 ;All sprites are white sta $d027,x inx cpx #$08 ;Do the sprite creation 8 times bne setsprs ldx #$00 positions lda postable,x ;Read label postable sta starpos+0,x ;Create data memory for current sprite position inx cpx #$10 bne positions lda #<irq ;You should know this bit already ;) sta $0314 lda #>irq sta $0315 lda #$00 sta $d012 lda #$7f sta $dc0d lda #$1b sta $d011 lda #$01 sta $d01a cli mainloop lda #$00 ;Synchronize the routines outside IRQ so that all routines run outside IRQ sta sync ;correctly lda sync waitsync cmp sync bne cont jmp waitsync cont jsr expdpos ;Call label xpdpos for sprite position x expansion jsr movestars ;Call label movestars for virtual sprite movement jmp mainloop expdpos ldx #$00 xpdloop lda starpos+1,x ;Read virtual memory from starpos (odd number values) sta $d001,x ;Write memory to the actual sprite y position lda starpos+0,x ;Read virtual memory from starpos (odd number values) asl ror $d010 ;increase the screen limit for sprite x position sta $d000,x ;Write memory to the actual sprite x position inx inx cpx #$10 bne xpdloop rts movestars ldx #$00 moveloop lda starpos+0,x ;Read from data table (starpos) clc adc starspeed+0,x sta starpos+0,x inx ; Add 2 to each value of the loop inx ; cpx #$10 ;Once reached 16 times rts else repeat moveloop bne moveloop rts irq inc $d019 ;You should also know this bit already lda #$00 sta $d012 lda #$01 sta sync jmp $ea31 ;Data tables for the sprite positions ; x y postable !byte $00,$38 ;We always keep x as zero, y is changeable !byte $00,$40 !byte $00,$48 !byte $00,$50 !byte $00,$58 !byte $00,$60 !byte $00,$68 !byte $00,$70 !byte $00,$78 ;Data tables for speed of the moving stars (erm dots) ;x y starspeed !byte $04,$00 ;Important. Remember that Y should always be zero. X is changable for !byte $05,$00 ;varied speeds of the moving stars. :) !byte $06,$00 !byte $07,$00 !byte $06,$00 !byte $04,$00 !byte $07,$00 !byte $05,$00 !byte $00,$00
base/8_sprite_starfield.txt · Last modified: 2015-04-17 04:30 by 127.0.0.1