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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.
m_joypad1 = $4016
;; m_joypad2 = $4017
;; After running a `read_*` function these two variables will contain the
;; given result.
zp_buttons1 = $22
zp_buttons2 = $23
;;;
;; 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_buttons1, x
tay
jsr Joypad::unsafe_read_x
tya
cmp Joypad::zp_buttons1, x
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_joypad1
sta Joypad::zp_buttons1, x ; Bit as a guard for the loop below.
lsr
sta Joypad::m_joypad1
;; Now the joypad is ready to accept reads.
@loop:
lda Joypad::m_joypad1, x
and #%00000011 ; Ignore bits other than controller.
cmp #$01 ; Set carry if and only if nonzero.
rol Joypad::zp_buttons1, x ; Carry -> bit 0; bit 7 -> Carry
bcc @loop
rts
.endproc
.endscope
;; Shortcut for reading the joypad from the first player safely.
.macro READ_JOYPAD1
ldx #$00
jsr Joypad::read_x
.endmacro
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