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.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
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
tay
jsr joypad_unsafe_read_x
tya
cmp Joypad::m_buttons1, x
bne @joypad_read_x_reread
rts
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