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diff --git a/tests/src/shared/joypad.s b/tests/src/shared/joypad.s
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+;; Taken from https://github.com/mssola/code.nes.
+
+.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
+
+    ;; After running a `joypad_read_*` function these two variables will contain
+    ;; the given result.
+    m_buttons1 = $22
+    m_buttons2 = $23
+.endscope
+
+;;;
+;; Read the first joypad. This method is fast but it might be vulnerable to the
+;; DPCM bug (see: https://www.nesdev.org/wiki/Controller_reading_code).
+joypad_unsafe_read:
+    ldx #$00
+    ;; NOTE: fallthrough
+
+;;;
+;; Read the joypad as indexed by the X register (0 for controller 1; 1 for
+;; controller 2). This method is fast but it might be vulnerable to the DPCM bug
+;; (see: https://www.nesdev.org/wiki/Controller_reading_code).
+joypad_unsafe_read_x:
+    ;; Start the latch process.
+    lda #$01
+    sta Joypad::JOYPAD1
+    sta Joypad::m_buttons1, x   ; Bit as a guard for the loop below.
+    lsr a
+    sta Joypad::JOYPAD1
+
+    ;; Now the joypad is ready to accept reads.
+@joypad_unsafe_read_x_loop:
+    lda Joypad::JOYPAD1, x
+    and #%00000011              ; Ignore bits other than controller.
+    cmp #$01                    ; Set carry if and only if nonzero.
+    rol Joypad::m_buttons1, x   ; Carry -> bit 0; bit 7 -> Carry
+    bcc @joypad_unsafe_read_x_loop
+    rts
+
+;;;
+;; Safely read the first controller via a re-read algorithm.
+joypad_read:
+    ldx #$00
+
+    ;; NOTE: uncomment these two lines to also read safely the second
+    ;; controller.
+    ;;
+    ;; jsr joypad_read_x
+    ;; inx
+
+    ;; NOTE: fallthrough
+
+;;;
+;; Safely read via a re-read algorithm the joypad as indexed by the X register
+;; (0 for controller 1; 1 for controller 2).
+joypad_read_x:
+    jsr joypad_unsafe_read_x
+
+    ;; The main idea around a re-read algorithm is that you read the controller
+    ;; "unsafely" once, then you do it again and compare both reads. If they
+    ;; were the same then we are on the safe side. Otherwise we would need to
+    ;; loop until we get two identical reads. This sounds bad but in practice
+    ;; it's not so much (and hey, if it worked for Super Mario Bros. 3, it
+    ;; should work for us too :P). Otherwise there is the algorithm via OAM DMA,
+    ;; but it sure is tricky.
+@joypad_read_x_reread:
+    lda Joypad::m_buttons1, x
+    pha
+    jsr joypad_unsafe_read_x
+    pla
+    cmp Joypad::m_buttons1, x
+    bne @joypad_read_x_reread
+
+    rts