star.asm

*=$0801
!byte $0c,$08,$0a,$00,$9e,$20,$32,$33,$30,$34,$00,$00,$00 ; BASIC auto start at $0900 

screen_color = $f7
color = $f8
color_h = $f9
velocity = $fa
cursor = $fb ; locations in screen memory
cursor_h = $fc
cursor_clear = $fd 
cursor_clear_h = $fe
bitmask_clear = $ff

x_pos = $1200 ; +32
x_pos_h = $1220 ; +32
y_pos = $1240 ; +32
cursor_buffer = $1260 ; + 32
cursor_buffer_h = $1280 ; + 32
bitmask_buffer = $12a0 ; + 32

size = 32

*=$0900
      jsr init     
      jsr blank_video
      jsr blank_screen
      jsr init_starfield
move_loop
      jsr draw_stars
      jsr move_stars
      jsr update_star_colors
vsync_wait      
      lda $d012
      bne vsync_wait
      jmp move_loop

init
    lda $d018
    ora #$8 ; Set video base to 8192
    sta $d018

    lda #0 ; color
    sta $d020 ; set border color
    lda $d011
    ora #$20 ; set high res mode
    sta $d011
    rts

blank_video
      lda #$00
      sta cursor
      lda #$20
      sta cursor_h
      ldy #0      
videoloop
      lda #0
      sta (cursor), y
      iny
      bne videoloop ; loop unless high bit needs inc
      inc cursor_h
      lda cursor_h ; check end of range
      cmp #$40 ; 8000 bytes
      beq blank_screen ; finished
      jmp videoloop
      rts

blank_screen
      ldy #0
      lda #16
screenloop
      sta $0400, Y
      sta $0500, Y
      sta $0600, Y
      sta $0700, Y
      iny
      bne screenloop
      rts

init_starfield
      ldx #0
next_star
      jsr init_star
      inx
      cpx #size
      bcc next_star
      rts

draw_stars
      ldy #0
draw_star
      jsr plot_star ; Plot star index y, return with pixel bit index in x
      txa
      pha
      jsr set_color ; Set color in screen memory using star at index y 
      pla
      tax
      lda cursor_buffer_h, y
      beq save_cursor ; Skip if buffer is empty
      sta cursor_clear_h
      lda cursor_buffer, y
      sta cursor_clear
      lda bitmask_buffer, y
      sta bitmask_clear
      tya
      pha
      ldy #0
      lda (cursor_clear), y
      and bitmask_clear ; clear star
      sta (cursor_clear), y
      pla
      tay
save_cursor
      lda cursor
      sta cursor_buffer, y ; store coordinate in buffer
      lda cursor_h
      sta cursor_buffer_h, y
      lda x_bit_clear, x
      sta bitmask_buffer, y ; store bitmask in buffer
      iny
      cpy #size
      bcc draw_star
      rts

move_stars
       ldx #0
move_next_star
       jsr update_velocity ; Update velocity
       lda x_pos, x
       clc
       adc velocity, x
       sta x_pos, x
       lda x_pos_h, x
       adc #0
       sta x_pos_h, x
       inx
       cpx #size
       bcc move_next_star
       rts

update_star_colors
    ldx #0
update_color
    lda cursor_buffer_h, x
    beq color_loop_done
    lda cursor_buffer, x
    tax
    lda color_h, y
    sta screen_color
    lda color, y
    tay
    pha
    lda (color), y
    and #%00001111
    ora screen_color
    sta (color), y
    pla
    tay
color_loop_done
    inx
    cpx #size
    bcc update_color

    ; Implement your color enhancement logic here

    rts

color_loop_done
    inx
    cpx #size
    bcc update_color
    rts

init_star
    lda #0
    sta x_pos_h, x
    jsr rnd
    lda $d012        ; Use current raster line for randomness
    and #%00111111   ; Get a more random value for X
    clc
    adc x_pos, x
    sta x_pos, x
    lda #0
    adc x_pos_h, x
    sta x_pos_h, x
    jsr rnd
    lda $d012        ; Use current raster line for randomness
    and #%11000000   ; Get a different random value for Y
    sta y_pos, x
    rts

new_star
    jsr regen_y
    lda #63 ; move x to pos 319
    sta x_pos, x
    lda #1
    sta x_pos_h, x
continue_move
    inx
    cpx #size
    bcc update_vel
    rts

plot_star
    lda y_pos, y 
    lsr ; /8
    lsr
    lsr 
    sta cursor_h ; high = y / 8 * 256
    sta cursor
    lda #0
    ldx #6; low = y / 8 * 64
y_low_mul
    asl cursor 
    rol 
    dex
    bne y_low_mul
    clc
    adc cursor_h
    sta cursor_h
    lda y_pos, y
    and #%00000111
    clc
    adc cursor
    sta cursor
    lda cursor_h
    adc #0
    sta cursor_h
    lda x_pos, y
    and #%11111000
    clc
    adc cursor
    sta cursor
    lda cursor_h
    adc x_pos_h, y
    sta cursor_h
    lda x_pos, y 
    and #%00000111
    tax ; Lose contents of x
    lda cursor_h ; add video offset
    clc
    adc #$20 
    sta cursor_h
    tya
    pha
    ldy #0
    lda (cursor), y
    ora x_bit_set, x      
    sta (cursor), y
    pla
    tay
    rts

set_color
    lda y_pos, y ; / 8 
    lsr 
    lsr 
    lsr 
    tax
    lda y_screen_h, x ; Lookup *40 table
    sta color_h
    lda y_screen, x
    sta color
    lda x_pos_h, y ; / 8
    lsr ; (x < 320)
    lda x_pos, y
    ror
    lsr
    lsr
    clc
    adc color
    sta color
    lda #04 ; Add final carry and $0400
    adc color_h
    sta color_h
    lda y_pos, y ; Use low 4 bits for color
    asl 
    asl
    asl
    asl
    cmp #0
    bne save_color; Proceed for normal colors
    lda #16 ; Set black stars to white
save_color
    sta screen_color
    tya ; now set color in screen memory
    pha
    ldy #0
    lda (color), y
    and #%00001111
    ora screen_color
    sta (color), y
    pla
    tay
    rts

init_star
    lda #0
    sta x_pos_h, x
    jsr rnd
    lda $d012        ; Use current raster line for randomness
    and #%00111111   ; Get a more random value for X
    clc
    adc x_pos, x
    sta x_pos, x
    lda #0
    adc x_pos_h, x
    sta x_pos_h, x
    jsr rnd
    lda $d012        ; Use current raster line for randomness
    and #%11000000   ; Get a different random value for Y
    sta y_pos, x
    rts


regen_y
    jsr rnd
    lda $d012        ; Use current raster line for randomness
    and #%11000111   ; Get a more random value for Y
    sta y_pos, x
    rts

rnd
    txa
    pha
    jsr $e09a        ; Call the C64's built-in RNG
    eor $d012        ; XOR with the current raster line for added randomness
    pla
    tax
    rts

x_bit_set
    !byte $80,$40,$20,$10,$08,$04,$02,$01
x_bit_clear
    !byte $7F,$BF,$DF,$EF,$F7,$FB,$FD,$FE
y_screen ; *40 lookup for screen memory
    !byte $00,$28,$50,$78,$A0,$C8,$F0,$18
    !byte $40,$68,$90,$B8,$E0,$08,$30,$58
    !byte $80,$A8,$D0,$F8,$20,$48,$70,$98
    !byte $C0
y_screen_h
    !byte $00,$00,$00,$00,$00,$00,$00,$01
    !byte $01,$01,$01,$01,$01,$02,$02,$02
    !byte $02,$02,$02,$02,$02,$03,$03,$03
    !byte $03