; Written in 2019 by Rob McMullen, https://playermissile.com/scrolling_tutorial/
; Copyright and related rights waived via CC0: https://creativecommons.org/publicdomain/zero/1.0/
        *= $3000

.include "hardware.s"

delay = 15

init
        jsr init_font

        lda #<dlist_coarse_mode4
        sta SDLSTL
        lda #>dlist_coarse_mode4
        sta SDLSTL+1
        jsr fillscreen_coarse_test_pattern
;        lda #$80
;        ldx #24
;        jsr label_pages

loop    ldx #15         ; number of VBLANKs to wait
?start  lda RTCLOK+2    ; check fastest moving RTCLOCK byte
?wait   cmp RTCLOK+2    ; VBLANK will update this
        beq ?wait       ; delay until VBLANK changes it
        dex             ; delay for a number of VBLANKs
        bpl ?start

        ; enough time has passed, scroll one line
        jsr coarse_scroll_down

        jmp loop

; move viewport one line down by pointing display list start address
; to the address 40 bytes further in memory
coarse_scroll_down
        clc
        lda dlist_coarse_address
        adc #40
        sta dlist_coarse_address
        lda dlist_coarse_address+1
        adc #0
        sta dlist_coarse_address+1
        rts

; Simple display list to be used as coarse scrolling comparison
dlist_coarse_mode4
        .byte $70,$70,$70       ; 24 blank lines
        .byte $44               ; Mode 4 + LMS
dlist_coarse_address
        .byte $00,$80           ; screen address
        .byte 4,4,4,4,4,4,4,4   ; 21 more Mode 4 lines
        .byte 4,4,4,4,4,4,4,4
        .byte 4,4,4,4,4
        .byte $42,<static_text, >static_text ; 2 Mode 2 lines + LMS + address
        .byte $2
        .byte $41,<dlist_coarse_mode4,>dlist_coarse_mode4 ; JVB ends display list

        ;             0123456789012345678901234567890123456789
static_text
        .sbyte +$80, " ANTIC MODE 2, NOT SCROLLED, FIRST LINE "
        .sbyte       " ANTIC MODE 2, NOT SCROLLED, SECOND LINE"


.include "util_font.s"
.include "util_scroll.s"
.include "font_data_antic4.s"

; tell DOS where to run the program when loaded
        * = $2e0
        .word init