basics: Add a flickering example

Add an example on how to flicker sprites so to not surpass the scanline
limit for them.

Signed-off-by: Miquel Sabaté Solà <msabate@suse.com>

2 files changed, 151 insertions, 0 deletions

diff --git a/shared/diskun.s b/shared/diskun.s
new file mode 100644
index 0000000..d8bab90
--- /dev/null
+++ b/shared/diskun.s
@@ -0,0 +1,95 @@
+;;; Simple sprite that can be moved with the dpad easily. Use it to have
+;;; something funny moving.
+
+.include "joypad.s"
+
+.scope Diskun
+    m_screen_x = $30
+    m_screen_y = $31
+
+    .proc init_palettes
+        lda #$3F
+        sta $2006                   ; PPUADDR
+        lda #$00
+        sta $2006                   ; PPUADDR
+
+        ldx #0
+    @load_palettes_loop:
+        lda palettes, x
+        sta $2007                   ; PPUDATA
+        inx
+        cpx #$20
+        bne @load_palettes_loop
+        rts
+    palettes:
+        DEFAULT_COLOR = $11
+
+        ;; Background
+        .byte DEFAULT_COLOR, $36, $17, $0F
+        .byte DEFAULT_COLOR, $00, $00, $00
+        .byte DEFAULT_COLOR, $00, $00, $00
+        .byte DEFAULT_COLOR, $00, $00, $00
+
+        ;; Foreground
+        .byte DEFAULT_COLOR, $28, $0F, $30
+        .byte DEFAULT_COLOR, $00, $00, $00
+        .byte DEFAULT_COLOR, $00, $00, $00
+        .byte DEFAULT_COLOR, $00, $00, $00
+
+        rts
+    .endproc
+
+    .proc update
+        lda #Joypad::BUTTON_UP
+        and Joypad::m_buttons1
+        beq @check_down
+
+        dec Diskun::m_screen_y
+        dec Diskun::m_screen_y
+        jmp @check_left
+    @check_down:
+        lda #Joypad::BUTTON_DOWN
+        and Joypad::m_buttons1
+        beq @check_left
+
+        inc Diskun::m_screen_y
+        inc Diskun::m_screen_y
+    @check_left:
+        lda #Joypad::BUTTON_LEFT
+        and Joypad::m_buttons1
+        beq @check_right
+
+        dec Diskun::m_screen_x
+        dec Diskun::m_screen_x
+    @check_right:
+        lda #Joypad::BUTTON_RIGHT
+        and Joypad::m_buttons1
+        beq @end
+
+        inc Diskun::m_screen_x
+        inc Diskun::m_screen_x
+    @end:
+        rts
+    .endproc
+
+    .proc nmi_update
+        lda Diskun::m_screen_x
+        sta $203
+        sta $20B
+        clc
+        adc #8
+        sta $207
+        sta $20F
+
+        lda Diskun::m_screen_y
+        sta $200
+        sta $204
+        clc
+        adc #8
+        sta $208
+        sta $20C
+
+        rts
+    .endproc
+.endscope
+
diff --git a/shared/joypad.s b/shared/joypad.s
new file mode 100644
index 0000000..2abf959
--- /dev/null
+++ b/shared/joypad.s
@@ -0,0 +1,56 @@
+;;; TODO: safe read
+
+.scope Joypad
+    ;; Button masks.
+    BUTTON_A      = 1 << 7
+    BUTTON_B      = 1 << 6
+    BUTTON_SELECT = 1 << 5
+    BUTTON_START  = 1 << 4
+    BUTTON_UP     = 1 << 3
+    BUTTON_DOWN   = 1 << 2
+    BUTTON_LEFT   = 1 << 1
+    BUTTON_RIGHT  = 1 << 0
+
+    ;; Port addresses for controllers.
+    JOYPAD1 = $4016
+    JOYPAD2 = $4017
+
+    ;; We keep all the information from controller from mainly two variables:
+    ;; m_buttons1 and m_buttons2; containing respectively the buttons pressed
+    ;; for this frame for both controllers. The m_inv_buttons ($21) is an
+    ;; internal variable and should not be used for anything outside of this
+    ;; usage.
+    m_inv_buttons = $21
+    m_buttons1    = $22
+    m_buttons2    = $23
+
+    ;; READ_CONTROLLER reads the input from the controller mapped into the given
+    ;; port, and saves the state into the given `buttons` address.
+    ;; Implementation taken from NESHacker's example of smb3-like movement.
+    .macro READ_CONTROLLER port, buttons
+        lda m_inv_buttons
+        tay
+        lda #1
+        sta port
+        sta m_inv_buttons
+        lsr
+        sta port
+    :
+        lda port
+        lsr
+        rol m_inv_buttons
+        bcc :-
+        tya
+        eor m_inv_buttons
+        and m_inv_buttons
+        sta buttons
+    .endmacro
+
+    ;; read sets the values for m_buttons1 and m_buttons2 as read from both
+    ;; controllers.
+    .proc read
+        READ_CONTROLLER JOYPAD1, m_buttons1
+        READ_CONTROLLER JOYPAD2, m_buttons2
+        rts
+    .endproc
+.endscope