1 files changed, 280 insertions, 0 deletions
diff --git a/tests/src/flicker/flicker.s b/tests/src/flicker/flicker.s
new file mode 100644
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+++ b/tests/src/flicker/flicker.s
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+;;;
+;; This is a simple example of sprite cycling, so sprites don't disappear on a
+;; scanline overflow. This example is extremely simple, and on a real game you'd
+;; take into account lots of things that would make this process less tedious
+;; and less cycle consuming.
+;; Taken from https://github.com/mssola/code.nes.
+
+.segment "HEADER"
+    .byte 'N', 'E', 'S', $1A
+
+    .byte $02, $01
+
+    .byte $00
+    .byte $00
+
+.segment "VECTORS"
+    .addr nmi, reset, irq
+
+.segment "CHARS"
+.incbin "../../assets/diskun.chr"
+
+.segment "CODE"
+
+.include "../shared/diskun.s"
+
+.proc main
+    jsr Diskun::init_palettes
+    jsr init_sprites
+
+    cli
+    lda #%10010000
+    sta $2000                   ; PPUCTRL
+    lda #%00011110
+    sta $2001                   ; PPUMASK
+
+@main_game_loop:
+    ;; NOTE: the logic is pretty simple: read the pad, move the player
+    ;; accordingly, and apply the flickering effect.
+    jsr joypad_read
+    jsr Diskun::update
+
+    ;; NOTE: comment this `jsr` out if you want to see what happens if no
+    ;; flickering effect is applied (spoiler alert: the last character will
+    ;; suddenly disappear :P).
+    ;;
+    ;; NOTE: this function is called mindlessly. In a real game you'd try to
+    ;; detect scanline overflows, or you will try to be more careful with sprite
+    ;; priorities and apply the flickering more carefully. Here it's applied for
+    ;; (almost) every sprite on each frame and we roll with it.
+    jsr apply_flicker
+
+    lda #%10000000
+    ora $20
+    sta $20
+@wait_for_render:
+    bit $20
+    bmi @wait_for_render
+
+    jmp @main_game_loop
+.endproc
+
+
+;;;
+;; NOTE: this is the actual meat of the example :D
+.proc apply_flicker
+    ;; We will store a 16-bit pointer to the first sprite that can be flickered.
+    ;; In this case, we will point to the first sprite from the first NPC. That
+    ;; is, we suppose that, for whatever reason, we don't want to apply the
+    ;; flickering effect for the player.
+    lda #$10
+    sta $40
+    lda #$02
+    sta $41
+
+    ;;;
+    ;; The algorithm is really straight-forward here: we will simply cycle the
+    ;; sprites starting from the one being pointed at $40-$41 until the last
+    ;; sprite. The main idea is that the OAM will show sprites on a scanline in
+    ;; order. Hence, those sprites that are left beyond the eigth on a scanline
+    ;; will suddenly disappear. Flickering is a technique by which sprites are
+    ;; rotated from OAM priority, and hence they won't disappear always, but
+    ;; just for a small amount of time: just enough to see them, creating a
+    ;; flickering effect.
+    ;;
+    ;; For this, we will reserve four bytes (1 sprite) for auxiliary temporary
+    ;; values ($42-$45) which will then be used in order to carry values along
+    ;; the cycle.
+
+    ;; We initialize the auxiliary bytes with the data from the last sprite to
+    ;; be iterated since these are the values to be stored for the first sprite
+    ;; now (that is, the last element is now the first one).
+    lda $24C
+    sta $42
+    lda $24D
+    sta $43
+    lda $24E
+    sta $44
+    lda $24F
+    sta $45
+
+@loop:
+    ;; For each of the four bytes from a sprite, pick the value stored on its
+    ;; auxiliary byte and save the old value into the auxiliary byte afterwards.
+    ;; This way the current byte holds the value from the previous sprite, but
+    ;; its value will be carry on into the next sprite.
+    .repeat 4, I
+        ldy #I
+        lda $42, y
+        pha
+        lda ($40), y
+        sta $42, y
+        pla
+        sta ($40), y
+    .endrepeat
+
+    ;; And move the sprite pointer 4 bytes (1 sprite). Note that we only need to
+    ;; move the low byte since we know beforehand that everything will be inside
+    ;; of the $2xx range.
+    lda $40
+    clc
+    adc #4
+    sta $40
+
+    ;; We know that the last byte from the last sprite is held at $24F. Thus, if
+    ;; the sprite pointer is already passed this point, we can break the loop.
+    ;; Otherwise just carry on.
+    cmp #$50
+    bne @loop
+
+    rts
+.endproc
+
+;;;
+;; NOTE: and from here on stuff that is not relevant for sprite flickering.
+
+nmi:
+    bit $20
+    bpl @next
+
+    pha
+    txa
+    pha
+    tya
+    pha
+
+    jsr Diskun::nmi_update
+
+    lda #$00
+    sta $2003                   ; OAMADDR
+    lda #$02
+    sta $4014                   ; OAMDMA
+
+    bit $2002                   ; PPUSTATUS
+    lda #$00
+    sta $2005                   ; PPUSCROLL
+    sta $2005                   ; PPUSCROLL
+
+    lda #%01111111
+    and $20
+    sta $20
+
+    pla
+    tay
+    pla
+    tax
+    pla
+@next:
+    rti
+
+reset:
+    sei
+    cld
+
+    ldx #$40
+    stx $4017                   ; APU Frame Counter
+
+    ldx #$ff
+    txs
+
+    inx
+    stx $2000                   ; PPUCTRL
+    stx $2001                   ; PPUMASK
+    stx $4010                   ; APU DMC
+
+@vblankwait1:
+    bit $2002                   ; PPUSTATUS
+    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 $700, x
+    inx
+    bne @ram_reset_loop         ; if x overflows back to #00, then we are done.
+
+    lda #$ef
+@sprite_reset_loop:
+    sta $200, x
+    inx
+    bne @sprite_reset_loop
+
+    lda #$00
+    sta $2003                   ; OAMADDR
+    lda #$02
+    sta $4014                   ; OAMDMA
+
+@vblankwait2:
+    bit $2002                   ; PPUSTATUS
+    bpl @vblankwait2
+
+    lda #$3F
+    sta $2006                   ; PPUADDR
+    lda #$00
+    sta $2006                   ; PPUADDR
+
+    lda #$0F
+    ldx #$20
+@palettes_reset_loop:
+    sta $2007                   ; PPUDATA
+    dex
+    bne @palettes_reset_loop
+
+    jmp main
+
+irq:
+    rti
+
+.proc init_sprites
+    NUM_SPRITES = 20
+
+    lda #$40
+    sta Diskun::m_screen_y
+    lda #$46
+    sta Diskun::m_screen_x
+
+    ldx #$00
+@load_sprites_loop:
+    lda initial_sprite_data, x
+    sta $0200, x
+    inx
+    cpx #(4 * NUM_SPRITES)
+    bne @load_sprites_loop
+    rts
+initial_sprite_data:
+    ;; $200-$20F
+    .byte $40, $01, %00000000, $46
+    .byte $40, $01, %01000000, $4E
+    .byte $48, $11, %00000000, $46
+    .byte $48, $11, %01000000, $4E
+
+    ;; $210-$21F
+    .byte $60, $01, %00000000, $68
+    .byte $60, $01, %01000000, $70
+    .byte $68, $11, %00000000, $68
+    .byte $68, $11, %01000000, $70
+
+    ;; $220-$22F
+    .byte $60, $01, %00000000, $7A
+    .byte $60, $01, %01000000, $82
+    .byte $68, $11, %00000000, $7A
+    .byte $68, $11, %01000000, $82
+
+    ;; $230-$23F
+    .byte $60, $01, %00000000, $8C
+    .byte $60, $01, %01000000, $94
+    .byte $68, $11, %00000000, $8C
+    .byte $68, $11, %01000000, $94
+
+    ;; $240-$24F
+    .byte $60, $01, %00000000, $9E
+    .byte $60, $01, %01000000, $A6
+    .byte $68, $11, %00000000, $9E
+    .byte $68, $11, %01000000, $A6
+.endproc