====== ROL Starfield using $D018 ======

By Richard Bayliss

While I was programming a game called Hyper Duel. I wanted to do a simple star field. So I followed a note on one of the disks which had an example of the star field, and tried to implement one of my own. Here is the final result. <code>
;The ROL Star Field animation by Richard/TND (From Hyper Duel)


                                        !to "starfield",cbm
starcontrol = $0340



                                        * = $0810 ;SYS2064 to start

                        jsr $ff81
                        lda #$00
                        sta $d020
                        sta $d021
                                       
;Draw starfield to screen $0400
 
                        ldx #$00
.copyscreen       lda stardata,x
                        sta $0400,x
                        lda stardata+$100,x
                        sta $0500,x
                        lda stardata+$200,x
                        sta $0600,x
                        lda stardata+$2e8,x
                        sta $06e8,x
                        lda #$0f ;Light grey stars for now
                        sta $d800,x
                        sta $d900,x
                        sta $da00,x
                        sta $dae8,x
                        inx
                        bne .copyscreen

                        lda #<.irq
                        sta $0314
                        lda #>.irq
                        sta $0315
                        lda #$7f
                        sta $dc0d
                        lda #$31
                        sta $d012
                        lda #$1b
                        sta $d011
                        lda #$08
                        sta $d016
                        lda #$1c ;Char at $3000
                        sta $d018
                        lda #$01
                        sta $d01a ;Interrupt is on!
                        cli
                        jmp *   ;Continuous loop to itself!

.irq                   inc $d019 ;$D019 interrupt on
                        lda #$fc   ;Raster position 252
                        sta $d012
                        jsr .starfield
                        jmp $ea7e

;Star field source:
;Starfield

.starfield      inc .starcontrol
         lda .starcontrol
         cmp #$03
         beq .doscroll
         rts
.doscroll      lda #0
         sta .starcontrol
         
         ldx #0
         jsr .starscroll
         jsr .starscroll
         ldx #1
         jsr .starscroll
         ldx #2
         jsr .starscroll
         jsr .starscroll
         jsr .starscroll
         ldx #3
         jsr .starscroll
         ldx #4
         jsr .starscroll
         jsr .starscroll
         jsr .starscroll
         ldx #5
         jsr .starscroll
         jsr .starscroll
         ldx #6
         jsr .starscroll
         ldx #7
         jsr .starscroll
         jsr .starscroll
         rts
         
.starscroll      lda $3220,x
         rol $3208,x
         rol $3210,x
         rol $3218,x
         rol $3220,x
         sta $3208,x
         rts

;The starfield char data here. Hell yeah!
;That's what we want :o)

                  * = $3200
                  
                  ;Shift + * (Blank it)
                  !byte %00000000 ;Weird eh? Those are for the stars
                  !byte %00000000
                  !byte %00000000
                  !byte %00000000
                  !byte %00000000
                  !byte %00000000
                  !byte %00000000
                  !byte %00000000
                  ;Shift + A
                  !byte %00000000
                  !byte %00000000
                  !byte %00000000
                  !byte %00000000
                  !byte %00100000
                  !byte %00000000
                  !byte %00000000
                  !byte %00000000
                  !byte %01000000
                  ;Shift + B
                  !byte %00000000
                  !byte %00000000
                  !byte %00010000
                  !byte %00000000
                  !byte %00000000
                  !byte %00000000
                  !byte %00000100
                  !byte %00000000
                  ;Shift + C
                  !byte %00000000
                  !byte %01000000
                  !byte %00000000
                  !byte %00000000
                  !byte %00000000
                  !byte %00000000
                  !byte %00001000
                  !byte %00000000
                  ;Shift + D
                  !byte %00000010
                  !byte %00000000
                  !byte %00000000
                  !byte %00010000
                  !byte %00000000
                  !byte %00000000
                  !byte %00000000      

;Now for the screen data

                    *=$4000
stardata
!scr"BCDABCDABCDABCDABCDABCDABCDABCDABCDABCDA"
!scr"CDABCDABCDABCDABCDABCDABCDABCDABCDABCDAB"
!scr"ABCDABCDABCDABCDABCDABCDABCDABCDABCDABCD"
!scr"DABCDABCDABCDABCDABCDABCDABCDABCDABCDABC"
!scr"BCDABCDABCDABCDABCDABCDABCDABCDABCDABCDA"
!scr"CDABCDABCDABCDABCDABCDABCDABCDABCDABCDAB"
!scr"ABCDABCDABCDABCDABCDABCDABCDABCDABCDABCD"
!scr"DABCDABCDABCDABCDABCDABCDABCDABCDABCDABC"
!scr"BCDABCDABCDABCDABCDABCDABCDABCDABCDABCDA"
!scr"CDABCDABCDABCDABCDABCDABCDABCDABCDABCDAB"
!scr"ABCDABCDABCDABCDABCDABCDABCDABCDABCDABCD"
!scr"DABCDABCDABCDABCDABCDABCDABCDABCDABCDABC"
!scr"BCDABCDABCDABCDABCDABCDABCDABCDABCDABCDA"
!scr"CDABCDABCDABCDABCDABCDABCDABCDABCDABCDAB"
!scr"ABCDABCDABCDABCDABCDABCDABCDABCDABCDABCD"
!scr"DABCDABCDABCDABCDABCDABCDABCDABCDABCDABC"
!scr"BCDABCDABCDABCDABCDABCDABCDABCDABCDABCDA"
!scr"CDABCDABCDABCDABCDABCDABCDABCDABCDABCDAB"
!scr"ABCDABCDABCDABCDABCDABCDABCDABCDABCDABCD"
!scr"DABCDABCDABCDABCDABCDABCDABCDABCDABCDABC"
!scr"BCDABCDABCDABCDABCDABCDABCDABCDABCDABCDA"
!scr"CDABCDABCDABCDABCDABCDABCDABCDABCDABCDAB"
!scr"ABCDABCDABCDABCDABCDABCDABCDABCDABCDABCD"
!scr"DABCDABCDABCDABCDABCDABCDABCDABCDABCDABC"
!scr"BCDABCDABCDABCDABCDABCDABCDABCDABCDABCDA"
!scr"CDABCDABCDABCDABCDABCDABCDABCDABCDABCDAB"
!scr"ABCDABCDABCDABCDABCDABCDABCDABCDABCDABCD"
!scr"DABCDABCDABCDABCDABCDABCDABCDABCDABCDABC"
!byte 0,0,0,0,00,0
</code>