scroll_multiple_screens.mfk

// compile with -t nes_small_v

import nes_joy

const word ppu_nametable_ram = $2000
const word ppu_pallete_ram = $3F00
bool scrolling_right
bool v_scrolling
byte v_scroll_dir
array oam_buffer [256] @$200 // sprite buffer
byte xscroll
byte yscroll
byte nametable
byte screencounter
byte border_draw_counter

void main() {
    init_sprites()
    load_palletes()
    draw_screen_nametable1(screen1.addr)
    xscroll = 0
    nametable = 0
    screencounter = 0
    scrolling_right = true
    border_draw_counter = 0
    v_scrolling = false
    v_scroll_dir = 0
    yscroll = 0
    ppu_set_scroll(0,0)
    ppu_wait_vblank()
    ppu_ctrl = %10010000   // enable NMI, sprites from Pattern Table 0, background from Pattern Table 1
    ppu_mask = %00011110   // enable sprites, enable background, no clipping on left side
    while(true) {}
}

void nmi() {
    //Push sprite information to the PPU through DMA transfer
    ppu_oam_dma_write(oam_buffer.addr.hi)
    
    if v_scrolling {
        v_scroll()
        ppu_set_scroll(xscroll,yscroll)
        return
    }
    
    read_joy1()
    if input_start != 0 {
        scrolling_right = false
    }
    else {
        scrolling_right = true
    }
    
    if input_dy > 0  && xscroll == 0 {
        v_scroll_dir = 0
        v_scrolling = true
    }
    if input_dy < 0  && xscroll == 0 {
        v_scroll_dir = 1
        v_scrolling = true
    }
    
    if xscroll & %00000111 == 0 {
        // on an 8 pixel boundary, so
        // load the appropriate tiles
        border_draw_counter = 1
    }
    
    if border_draw_counter == 1 {
        if scrolling_right {
            load_new_right_column()
        }
        border_draw_counter += 1
    }
    else if border_draw_counter == 2 {
        if not(scrolling_right) {
            load_new_left_column()
        }
        border_draw_counter = 0
    }
    
    if scrolling_right {
        xscroll += 1
    }
    else {
        xscroll -= 1
    }
    
    if scrolling_right {
        if xscroll == 0 {
            nametable ^= %00000001
            screencounter += 1
            if screencounter >= 4 {
                screencounter = 0
            }
        }
    }
    else {
        if xscroll == $FF {
            nametable ^= %00000001
            if screencounter == 0 {
                screencounter = 3
            }
            else {
                screencounter -= 1
            }
        }
    }

    ppu_set_scroll(xscroll,0)
    ppu_ctrl = %10010000 | nametable
    ppu_mask = %00011110
}

void irq() {
    
}

void v_scroll() {
    if v_scroll_dir == 0 {
        yscroll -= 2
    }
    else {
        yscroll += 2
    }
    
    if yscroll >= $F0 {
        if v_scroll_dir == 0 {
            yscroll = $EE
        }
        else {
            yscroll = 0
        }
    }
    
    if yscroll == 0 {
        v_scrolling = false
    }
}

void load_new_left_column() {
    pointer screen
    byte screencounter_next
    byte i
    byte xscroll_div_8
    byte nametable_temp
    byte alignment
    
    xscroll_div_8 = xscroll >> 3
    
    if xscroll_div_8 == 0  {
        if screencounter == 0 {
            screencounter_next = 3
        }
        else {
            screencounter_next = screencounter - 1
        }
        nametable_temp = nametable ^ %00000001
        alignment = $1F
    }
    else {
        screencounter_next = screencounter
        nametable_temp = nametable
        alignment = xscroll_div_8 - 1
    }
    
    if screencounter_next == 0 {
        screen = screen1.addr
    }
    if screencounter_next == 1 {
        screen = screen2.addr
    }
    if screencounter_next == 2 {
        screen = screen3.addr
    }
    if screencounter_next == 3 {
        screen = screen4.addr
    }
    
    screen += alignment

    //set ppu address increment to 32 so we can draw the left and right borders
    //(allows us to draw to the nametable in vertical strips rather than horizontal)
    ppu_ctrl = %10010100
    
    read_ppu_status()
    if nametable_temp == 0 {
        ppu_set_addr(ppu_nametable_ram + alignment)
    }
    else {
        ppu_set_addr(ppu_nametable_ram + $0400 + alignment)
    }
    
    for i,0,until,$1D {
        ppu_write_data(screen[0])
        screen += $20
    }
}

void load_new_right_column() {
    pointer screen
    byte screencounter_next
    byte i
    byte xscroll_div_8
    
    if screencounter == 3 {
        screencounter_next = 0
    }
    else {
        screencounter_next = screencounter + 1
    }
    
    if screencounter_next == 0 {
        screen = screen1.addr
    }
    if screencounter_next == 1 {
        screen = screen2.addr
    }
    if screencounter_next == 2 {
        screen = screen3.addr
    }
    if screencounter_next == 3 {
        screen = screen4.addr
    }
    
    xscroll_div_8 = xscroll >> 3
    screen += xscroll_div_8 //properly align the screen pointer

    //set ppu address increment to 32 so we can draw the left and right borders
    //(allows us to draw to the nametable in vertical strips rather than horizontal)
    ppu_ctrl = %10010100
    
    read_ppu_status()
    if nametable == 0 {
        ppu_set_addr(ppu_nametable_ram + $0400 + xscroll_div_8)
    }
    else {
        ppu_set_addr(ppu_nametable_ram + xscroll_div_8)
    }
    
    for i,0,until,$1D {
        ppu_write_data(screen[0])
        screen += $20
    }
}

void init_sprites() {
    byte i
    for i,0,to,255 {
        if (i & %00000011) == 0 {
            //each sprite takes up 4 bytes, and we want to edit
            //the y position of each sprite (0th byte)
            //so we use the %00000011 mask to write every 4th byte (every 0th sprite byte)
            
            oam_buffer[i] = $ef // move the sprite off screen
        }
        else {
            oam_buffer[i] = 0
        }
    }
}

inline asm void ppu_wait_vblank() {
    vblankwait:
        BIT $2002
        ! BPL vblankwait
    ? RTS
}

inline void draw_screen_nametable1(pointer screen) {
    word i
    read_ppu_status()
    ppu_set_addr(ppu_nametable_ram)
    for i,0,until,$3C0 {
        ppu_write_data(screen[i])
    }
}

inline void draw_screen_nametable2(pointer screen) {
    word i
    read_ppu_status()
    ppu_set_addr(ppu_nametable_ram+$0400)
    for i,0,until,$3C0 {
        ppu_write_data(screen[i])
    }
}

inline void load_palletes() {
    byte i
    read_ppu_status() // read PPU status to reset the high/low latch
    ppu_set_addr(ppu_pallete_ram) // point the PPU to palette ram
    for i,0,until,$20 {
        ppu_write_data(pallete[i])
    }
}

const array screen1 = [
    "11111111111111111111111111111111" ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "This is random text I'm typing i" ascii
]

const array screen2 = [
    "22222222222222222222222222222222" ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "n order to test out the scrollin" ascii
]

const array screen3 = [
    "33333333333333333333333333333333" ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "g. Hold down the start button to" ascii
]

const array screen4 = [
    "44444444444444444444444444444444" ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    "                                " ascii,
    " scroll backwards.End of screen." ascii
]

const array pallete = [
  $22,$29,$1A,$0F,  $22,$36,$17,$0F,  $22,$30,$21,$0F,  $22,$27,$17,$0F, //background palette
  $22,$1C,$15,$14,  $22,$02,$38,$3C,  $22,$1C,$15,$14,  $22,$02,$38,$3C //sprite palette
]

segment(chrrom) const array graphics @ $0000 = file("tiles.chr")