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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 address for starting the latch process for both controllers and
;; reading from Joypad 1. The second Joypad can be accessed by reading from
;; $4017, which means that functions down below will simply use an indexed
;; load with the proper value on the index register (i.e. +1).
m_joypad = $4016
;; The previous reading from the latest read controller.
zp_prev = $21
;; After running a `read_*` function this variable will contain the given
;; result.
zp_buttons = $22
;;;
;; Safely read a controller via a re-read algorithm the joypad as indexed by
;; the X register (0 for controller 1; 1 for controller 2).
.proc 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.
@reread:
lda Joypad::zp_buttons
tay
jsr Joypad::unsafe_read_x
tya
cmp Joypad::zp_buttons
bne @reread
rts
.endproc
;;;
;; 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).
.proc unsafe_read_x
;; Start the latch process.
lda #$01
sta Joypad::m_joypad
sta Joypad::zp_buttons ; Bit as a guard for the loop below.
lsr
sta Joypad::m_joypad
;; Now the joypad is ready to accept reads.
@loop:
lda Joypad::m_joypad, x
and #%00000011 ; Ignore bits other than controller.
cmp #$01 ; Set carry if and only if nonzero.
rol Joypad::zp_buttons ; Carry -> bit 0; bit 7 -> Carry
bcc @loop
rts
.endproc
.endscope
;; Shortcut for reading the joypad indexed by 'x' (0 for controller 1; 1 for
;; controller 2).
.macro READ_JOYPAD_X
lda Joypad::zp_buttons
sta Joypad::zp_prev
jsr Joypad::read_x
.endmacro
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