scroll: Implement a roulette-like game

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>

1 files changed, 521 insertions, 0 deletions

diff --git a/scroll/roulette.s b/scroll/roulette.s
new file mode 100644
index 0000000..ecf2569
--- /dev/null
+++ b/scroll/roulette.s
@@ -0,0 +1,521 @@
+;;;
+;; Divide the screen in three rows and make them move in different
+;; directions/speed. This is achieved thanks to the MMC3 chip (check the
+;; `fx/blink.s` for further information on this chip). In particular, we are
+;; using the scanline IRQ mechanism provided by this chip to react to different
+;; parts of the screen being rendered:
+;;
+;;   1. The top of the screen moves fast on one direction.
+;;   2. The center of the screen moves fast on the opposite direction.
+;;   3. The bottom of the screen moves on the same direction as 1. but slower.
+;;
+;; This is achieved by setting an IRQ that hits on points 2. and 3., and then we
+;; manipulate the PPU scroll register mid frame. That is, all you see are just
+;; background elements being scrolled in different ways.
+;;
+;; This trick was used, for example, on Super Mario Bros. 3 for the roulette
+;; mini-game. That being said, usually games used this capability to handle
+;; scroll on the top part of the screen, and then resetting the scroll on the
+;; lower part, so they could show a status section (again, as Super Mario Bros.
+;; 3 does inside of a level).
+
+;; Include helpful definitions.
+.include "../shared/mmc3.s"
+
+;; Variables used on this example.
+.scope Vars
+    top_scroll    = $00
+    center_scroll = $01
+    bottom_scroll = $02
+    is_bottom     = $04         ; 0 -> scroll center; 1 -> scroll bottom
+.endscope
+
+.segment "HEADER"
+    .byte 'N', 'E', 'S', $1A
+    .byte $10                   ; 16 * 16 PRG-ROM (256KB)
+    .byte $10                   ; 16 * 8  CHR-ROM (128KB)
+    .byte $42, $08              ; Mapper 4, battery present, iNES 2.0 header
+    .res 8, 0
+
+.segment "VECTORS"
+    .addr nmi, reset, irq
+
+;;; NOTE: lots of banks, all of them empty since we don't need them :)
+
+.segment "PRG0_00"
+.byte $FF
+.segment "PRG0_01"
+.byte $FF
+.segment "PRG0_02"
+.byte $FF
+.segment "PRG0_03"
+.byte $FF
+.segment "PRG0_04"
+.byte $FF
+.segment "PRG0_05"
+.byte $FF
+.segment "PRG0_06"
+.byte $FF
+.segment "PRG0_07"
+.byte $FF
+.segment "PRG0_08"
+.byte $FF
+.segment "PRG0_09"
+.byte $FF
+.segment "PRG0_0A"
+.byte $FF
+.segment "PRG0_0B"
+.byte $FF
+.segment "PRG0_0C"
+.byte $FF
+.segment "PRG0_0D"
+.byte $FF
+.segment "PRG0_0E"
+.byte $FF
+.segment "PRG1_00"
+.byte $FF
+.segment "PRG1_01"
+.byte $FF
+.segment "PRG1_02"
+.byte $FF
+.segment "PRG1_03"
+.byte $FF
+.segment "PRG1_04"
+.byte $FF
+.segment "PRG1_05"
+.byte $FF
+.segment "PRG1_06"
+.byte $FF
+.segment "PRG1_07"
+.byte $FF
+.segment "PRG1_08"
+.byte $FF
+.segment "PRG1_09"
+.byte $FF
+.segment "PRG1_0A"
+.byte $FF
+.segment "PRG1_0B"
+.byte $FF
+.segment "PRG1_0C"
+.byte $FF
+.segment "PRG1_0D"
+.byte $FF
+.segment "PRG1_0E"
+.byte $FF
+
+;;; NOTE: the first fixed PRG bank will simply contain utilities for moving the
+;;; player around.
+.segment "FIXED"
+.include "../shared/diskun.s"
+
+;;; NOTE: the main bulk of this example. Comments only for the parts which are
+;;; specific to this example.
+.segment "TAIL"
+.include "../shared/ppu.s"
+
+reset:
+    sei
+    cld
+
+    ;; NOTE: as explained on the `basics/sprite.s` example, this is done to
+    ;; disable the APU frame IRQ. This is usually done without giving it a
+    ;; second thought, but it's specially relevant on the MMC3 chip because
+    ;; disabling this allows the `irq` handler to be able to assume that the
+    ;; only kind of IRQ available is a scanline one.
+    ldx #$40
+    stx $4017
+
+    ldx #$ff
+    txs
+
+    inx
+    stx $2000
+    stx $2001
+    stx $4010
+
+    ;;;
+    ;; NOTE: Setup MMC3. Nothing different from `fx/blink.s`. Take that example
+    ;; as a reference on how to configure the MMC3 chip and bank switching on
+    ;; it.
+
+    lda #$00
+    sta MMC3::MIRRORING
+    sta MMC3::IRQ_DISABLE
+
+    lda #$80
+    sta MMC3::RAM_PROTECT
+
+    BANK_REGISTER_SET 0, 0
+    BANK_REGISTER_SET 1, 2
+    BANK_REGISTER_SET 2, 4
+    BANK_REGISTER_SET 3, 5
+    BANK_REGISTER_SET 4, 6
+    BANK_REGISTER_SET 5, 7
+    BANK_REGISTER_SET 6, 0
+    BANK_REGISTER_SET 7, 1
+
+@vblankwait1:
+    bit $2002
+    bpl @vblankwait1
+
+    ldx #0
+    lda #0
+@ram_reset_loop:
+    sta $000, x
+    sta $100, x
+    sta $300, x
+    sta $400, x
+    sta $500, x
+    sta $600, x
+    sta $700, x
+    inx
+    bne @ram_reset_loop
+
+    lda #$ef
+@sprite_reset_loop:
+    sta $200, x
+    inx
+    bne @sprite_reset_loop
+
+    lda #$00
+    sta $2003
+    lda #$02
+    sta $4014
+
+@vblankwait2:
+    bit $2002
+    bpl @vblankwait2
+
+    lda #$3F
+    sta $2006
+    lda #$00
+    sta $2006
+
+    lda #$0F
+    ldx #$20
+@palettes_reset_loop:
+    sta $2007
+    dex
+    bne @palettes_reset_loop
+    jmp main
+
+;; The main function is used here only for further initialization purposes.
+.proc main
+    ;; Initialize both the palettes and the nametables.
+    jsr Diskun::init_palettes
+    jsr init_nametables
+
+    ;; NOTE: enable back interrupts so we can set them up later on `nmi` code.
+    cli
+
+    lda #%10010000
+    sta $2000
+    lda #%00011110
+    sta $2001
+
+@main_game_loop:
+    ;; NOTE: nothing :D
+
+    lda #%10000000
+    ora $20
+    sta $20
+@wait_for_render:
+    bit $20
+    bmi @wait_for_render
+
+    ;; NOTE: no game logic, everything happens on NMI and IRQ handlers.
+
+    jmp @main_game_loop
+.endproc
+
+;; NOTE: everything displayed on this example only happens on the background.
+;; Moreover, to give a more accurate illusion of the scrolling, the same
+;; background elements are repeated on the other nametable. That's why we have
+;; to set the data twice on each section: once for each nametable.
+.proc init_nametables
+    ;; Top left
+    WRITE_PPU_DATA $20A6, $01
+    WRITE_PPU_DATA $20C6, $11
+    WRITE_PPU_DATA $20A7, $02
+    WRITE_PPU_DATA $20C7, $12
+
+    WRITE_PPU_DATA $24A6, $01
+    WRITE_PPU_DATA $24C6, $11
+    WRITE_PPU_DATA $24A7, $02
+    WRITE_PPU_DATA $24C7, $12
+
+    ;; Top center
+    WRITE_PPU_DATA $20AF, $01
+    WRITE_PPU_DATA $20CF, $11
+    WRITE_PPU_DATA $23CB, %01000100
+    WRITE_PPU_DATA $20B0, $02
+    WRITE_PPU_DATA $20D0, $12
+    WRITE_PPU_DATA $23CC, %00010001
+
+    WRITE_PPU_DATA $24AF, $01
+    WRITE_PPU_DATA $24CF, $11
+    WRITE_PPU_DATA $27CB, %01000100
+    WRITE_PPU_DATA $24B0, $02
+    WRITE_PPU_DATA $24D0, $12
+    WRITE_PPU_DATA $27CC, %00010001
+
+    ;; Top right
+    WRITE_PPU_DATA $20B8, $01
+    WRITE_PPU_DATA $20D8, $11
+    WRITE_PPU_DATA $23CE, %00100010
+    WRITE_PPU_DATA $20B9, $02
+    WRITE_PPU_DATA $20D9, $12
+
+    WRITE_PPU_DATA $24B8, $01
+    WRITE_PPU_DATA $24D8, $11
+    WRITE_PPU_DATA $27CE, %00100010
+    WRITE_PPU_DATA $24B9, $02
+    WRITE_PPU_DATA $24D9, $12
+
+    ;; Center left
+    WRITE_PPU_DATA $21E6, $01
+    WRITE_PPU_DATA $2206, $11
+    WRITE_PPU_DATA $21E7, $02
+    WRITE_PPU_DATA $2207, $12
+
+    WRITE_PPU_DATA $25E6, $01
+    WRITE_PPU_DATA $2606, $11
+    WRITE_PPU_DATA $25E7, $02
+    WRITE_PPU_DATA $2607, $12
+
+    ;; Center center
+    WRITE_PPU_DATA $21EF, $01
+    WRITE_PPU_DATA $220F, $11
+    WRITE_PPU_DATA $21F0, $02
+    WRITE_PPU_DATA $2210, $12
+    WRITE_PPU_DATA $23DB, %01000000
+    WRITE_PPU_DATA $23E3, %00000100
+    WRITE_PPU_DATA $23DC, %00010000
+    WRITE_PPU_DATA $23E4, %00000001
+
+    WRITE_PPU_DATA $25EF, $01
+    WRITE_PPU_DATA $260F, $11
+    WRITE_PPU_DATA $25F0, $02
+    WRITE_PPU_DATA $2610, $12
+    WRITE_PPU_DATA $27DB, %01000000
+    WRITE_PPU_DATA $27E3, %00000100
+    WRITE_PPU_DATA $27DC, %00010000
+    WRITE_PPU_DATA $27E4, %00000001
+
+    ;; Center right
+    WRITE_PPU_DATA $21F8, $01
+    WRITE_PPU_DATA $2218, $11
+    WRITE_PPU_DATA $21F9, $02
+    WRITE_PPU_DATA $2219, $12
+    WRITE_PPU_DATA $23DE, %00100000
+    WRITE_PPU_DATA $23E6, %00000010
+
+    WRITE_PPU_DATA $25F8, $01
+    WRITE_PPU_DATA $2618, $11
+    WRITE_PPU_DATA $25F9, $02
+    WRITE_PPU_DATA $2619, $12
+    WRITE_PPU_DATA $27DE, %00100000
+    WRITE_PPU_DATA $27E6, %00000010
+
+    ;; Bottom left
+    WRITE_PPU_DATA $2326, $01
+    WRITE_PPU_DATA $2327, $02
+    WRITE_PPU_DATA $2346, $11
+    WRITE_PPU_DATA $2347, $12
+
+    WRITE_PPU_DATA $2726, $01
+    WRITE_PPU_DATA $2727, $02
+    WRITE_PPU_DATA $2746, $11
+    WRITE_PPU_DATA $2747, $12
+
+    ;; Bottom center
+    WRITE_PPU_DATA $232F, $01
+    WRITE_PPU_DATA $234F, $11
+    WRITE_PPU_DATA $2330, $02
+    WRITE_PPU_DATA $2350, $12
+    WRITE_PPU_DATA $23F3, %01000100
+    WRITE_PPU_DATA $23F4, %00010001
+
+    WRITE_PPU_DATA $272F, $01
+    WRITE_PPU_DATA $274F, $11
+    WRITE_PPU_DATA $2730, $02
+    WRITE_PPU_DATA $2750, $12
+    WRITE_PPU_DATA $27F3, %01000100
+    WRITE_PPU_DATA $27F4, %00010001
+
+    ;; Bottom right
+    WRITE_PPU_DATA $2338, $01
+    WRITE_PPU_DATA $2358, $11
+    WRITE_PPU_DATA $2339, $02
+    WRITE_PPU_DATA $2359, $12
+    WRITE_PPU_DATA $23F6, %00100010
+
+    WRITE_PPU_DATA $2738, $01
+    WRITE_PPU_DATA $2758, $11
+    WRITE_PPU_DATA $2739, $02
+    WRITE_PPU_DATA $2759, $12
+    WRITE_PPU_DATA $27F6, %00100010
+
+    rts
+.endproc
+
+;;;
+;; NOTE: for this example the NMI is a bit different than on other examples. It
+;; has to do mainly two things:
+;;   1. Set up a scanline IRQ so the scroll at the center/bottom is different.
+;;   2. Set the scroll for the top region.
+nmi:
+    bit $20
+    bpl @next
+
+    pha
+    txa
+    pha
+    tya
+    pha
+
+    ;; NOTE: no DMA transfer as usual since there are no sprites involved.
+
+    ;;;
+    ;; NOTE: setting up IRQs for scanline counting.
+
+    ;; Disable scanline IRQs and acknowledge any previous one. Technically
+    ;; speaking this is not needed because the only times we set up an IRQ we
+    ;; know it's going to be acknowledge where it is needed. That being said,
+    ;; let's be safe.
+    ldx #$00
+    stx MMC3::IRQ_DISABLE
+
+    ;; The screen is made up of 240 scan lines. Since we are dividing the screen
+    ;; by 3: 240 / 3 = 80. Hence, the next IRQ should happen on scanline 80,
+    ;; where we would need to update the scroll value through the
+    ;; `{center/bottom}_scroll` values instead. Moreover, note that
+    ;; `MMC3::IRQ_ENABLE` accepts any value, so the same value as the two other
+    ;; registers is just fine.
+    lda #80
+    sta MMC3::IRQ_LATCH
+    sta MMC3::IRQ_RELOAD
+    sta MMC3::IRQ_ENABLE
+
+    ;; The IRQ for scanline 80 has been set up. Now proceed with the scroll for
+    ;; the top section. The scroll will only happen on the X axis and it's going
+    ;; to be a bit fast.
+    bit $2002
+    lda Vars::top_scroll
+    clc
+    adc #2
+    sta Vars::top_scroll
+    sta $2005
+    lda #$00
+    sta $2005
+
+    ;; NOTE: the rest as usual.
+
+    lda #%01111111
+    and $20
+    sta $20
+
+    pla
+    tay
+    pla
+    tax
+    pla
+@next:
+    rti
+
+;;;
+;; NOTE: handle a scanline IRQ.
+;;
+;; Notice that we cannot at first glance know what kind of IRQ is hitting at the
+;; moment, but we have disabled the frame counter on our `reset` code, so on the
+;; context of the MMC3 chip the only thing left are scanline IRQs, which we have
+;; set up on `nmi` code.
+irq:
+    ;; Save current context.
+    pha
+    txa
+    pha
+    tya
+    pha
+
+    ;; Disable IRQs and acknowledge the current one.
+    ldx #$00
+    stx MMC3::IRQ_DISABLE
+
+    ;; What are we trying to scroll, exactly?
+    lda Vars::is_bottom
+    beq @scroll_right
+
+    ;; We are scrolling the bottom section, which scrolls in the same direction
+    ;; as the top one but a bit slower at that. Load the next scroll value on
+    ;; the `a` register and `Vars::bottom_scroll`.
+    lda Vars::bottom_scroll
+    clc
+    adc #1
+    sta Vars::bottom_scroll
+    ldy #0
+    sty Vars::is_bottom
+    jmp @do_scroll
+
+@scroll_right:
+    ;; We are scrolling the center, which works by going on the opposite
+    ;; direction as the top and bottom sections. Load the next scroll value on
+    ;; the `a` register and `Vars::center_scroll`.
+    lda Vars::center_scroll
+    sec
+    adc #$FD
+    sta Vars::center_scroll
+    ldy #1
+    sty Vars::is_bottom
+
+    ;; We are at the center, but there is still the bottom section to be
+    ;; scrolled differently. Hence, set a new scanline IRQ 80 lines ahead of
+    ;; where we are now. This is done in pretty much the same way as we did in
+    ;; `nmi` code.
+    ldx #80
+    stx MMC3::IRQ_LATCH
+    stx MMC3::IRQ_RELOAD
+    stx MMC3::IRQ_ENABLE
+
+@do_scroll:
+    ;; Regardless of the path, the `a` register contains the value for the
+    ;; scroll on the X axis. Store this value now and leave.
+    sta $2005
+    lda #$00
+    sta $2005
+
+    ;; Restore previous context.
+    pla
+    tay
+    pla
+    tax
+    pla
+
+    rti
+
+;;; NOTE: pretty much the same as `fx/blink.s` but we also copy the same data on
+;;; the second pattern table as that's the one being used for background
+;;; elements.
+
+.segment "CHARS"
+.incbin "../assets/diskun0.chr"
+.incbin "../assets/diskun1.chr"
+.incbin "../assets/diskun0.chr"
+.incbin "../assets/diskun1.chr"
+
+;; The 15 other 8KB portions are left empty.
+.res $2000, $00
+.res $2000, $00
+.res $2000, $00
+.res $2000, $00
+.res $2000, $00
+.res $2000, $00
+.res $2000, $00
+.res $2000, $00
+.res $2000, $00
+.res $2000, $00
+.res $2000, $00
+.res $2000, $00
+.res $2000, $00
+.res $2000, $00
+.res $2000, $00