; 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 = 5

horz_scroll = $91       ; variable used to store HSCROL value
horz_scroll_max = 4     ; ANTIC mode 4 has 4 color clocks

init
        jsr init_font

        lda #<dlist_hscroll_mode4
        sta SDLSTL
        lda #>dlist_hscroll_mode4
        sta SDLSTL+1
        jsr fillscreen_test_pattern
        lda #$80
        ldx #22
        jsr label_pages

        lda #0          ; initialize horizontal scrolling value
        sta horz_scroll
        sta HSCROL      ; initialize hardware register

loop    ldx #delay      ; 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 color clock
        jsr fine_scroll_left

        jmp loop

; scroll one color clock left and check if at HSCROL limit
fine_scroll_left
        inc horz_scroll
        lda horz_scroll
        cmp #horz_scroll_max ; check to see if we need to do a coarse scroll
        bcc ?done       ; nope, still in the middle of the character
        jsr coarse_scroll_left ; yep, do a coarse scroll...
        lda #0          ;  ...followed by reseting the HSCROL register
        sta horz_scroll
?done   sta HSCROL      ; store vertical scroll value in hardware register
        rts

; move viewport one byte to the left by pointing each display list start
; address to one byte lower in memory
coarse_scroll_left
        ldy #22         ; 22 lines to modify
        ldx #4          ; 4th byte after start of display list is low byte of address
?loop   dec dlist_hscroll_mode4,x
        inx             ; skip to next low byte which is 3 bytes away
        inx
        inx
        dey
        bne ?loop
        rts


.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