Controller reading code

Basic Example

This describes an efficient method of reading the standard controller using ca65 syntax.

The result byte buttons should be placed in zero page to save a cycle each time through the loop.

; we reserve one byte for storing the data that is read from controller
.zeropage
buttons .res 1

When reading from JOYPAD* what is read might be different from $01/$00 for various reasons. (See Controller port registers.) In this code the only concern is bit 0 read from JOYPAD*..

JOYPAD1 = $4016
JOYPAD2 = $4017

This is the end result that will be stored in buttons. 1 if the button was pressed, 0 otherwise.

bit	7	6	5	4	3	2	1	0
button	A	B	Select	Start	Up	Down	Left	Right

This subroutine takes 132 cycles to execute but ignores the Famicom expansion controller. Many controller reading subroutines use the X or Y register to count 8 times through the loop. But this one uses a more clever ring counter technique: $01 is loaded into the result first, and once eight bits are shifted in, the 1 bit will be shifted out, terminating the loop.

; At the same time that we strobe bit 0, we initialize the ring counter
; so we're hitting two birds with one stone here
readjoy:
    lda #$01
    ; While the strobe bit is set, buttons will be continuously reloaded.
    ; This means that reading from JOYPAD1 will only return the state of the
    ; first button: button A.
    sta JOYPAD1
    sta buttons
    lsr a        ; now A is 0
    ; By storing 0 into JOYPAD1, the strobe bit is cleared and the reloading stops.
    ; This allows all 8 buttons (newly reloaded) to be read from JOYPAD1.
    sta JOYPAD1
loop:
    lda JOYPAD1
    lsr a	       ; bit 0 -> Carry
    rol buttons  ; Carry -> bit 0; bit 7 -> Carry
    bcc loop
    rts

Adding support for controllers on the Famicom's DA15 expansion port and for player 2's controller is straightforward.

.zeropage
buttons1: .res 1
buttons2: .res 1

.code
readjoy:
    lda #$01
    sta JOYPAD1
    sta buttons2  ; player 2's buttons double as a ring counter
    lsr a         ; now A is 0
    sta JOYPAD1
loop:
    lda JOYPAD1
    and #%00000011  ; ignore bits other than controller
    cmp #$01        ; Set carry if and only if nonzero
    rol buttons1    ; Carry -> bit 0; bit 7 -> Carry
    lda JOYPAD2     ; Repeat
    and #%00000011
    cmp #$01
    rol buttons2    ; Carry -> bit 0; bit 7 -> Carry
    bcc loop
    rts

DPCM Safety using Repeated Reads

If your code is intended to be used with APU DMC playback, this code will need to be altered. The NES occasionally glitches the controller port twice in a row if sample playback is enabled, and games using samples need to work around this. For example, Super Mario Bros. 3 reads each controller's data at least two times each frame. First it reads it as normal, then it reads it again. If the two results differ, it does the procedure all over.

One controller example:

readjoy_safe:
    jsr readjoy
reread:
    lda buttons
    pha
    jsr readjoy
    pla
    cmp buttons
    bne reread
    rts

Because repeated rereads can take a long time, another way is to just use the previous frame's button press data if the results differ. Note that this is a compromise: if the player presses or releases a button between the two reads, this will interpret that as a corrupt read. Unfortunately, there's no way for it to tell the difference.

DPCM Safety using OAM DMA

Because DPCM reads may only occur on an odd cycle, it is possible to get glitch-free controller reads by timing all $4016/$4017 reads to fall on even cycles. This is made possible by the behavior of OAM DMA: the first cycle after an OAM DMA is guaranteed to be an even cycle.

This is a relatively new technique, and is not entirely supported by emulators.

    lda #OAM
    sta $4014          ; ------ DMA ------
    ldx #1             ; even odd          <- strobe code must take an odd number of cycles total
    stx controller1    ; even odd even
    stx $4016          ; odd even odd even
    dex                ; odd even
    stx $4016          ; odd even odd even
read_loop:
    lda $4017          ; odd even odd EVEN <- loop code must take an even number of cycles total
    and #3             ; odd even
    cmp #1             ; odd even
    rol controller2, x ; odd even odd even odd even (X = 0; waste 1 cycle and 0 bytes for alignment)
    lda $4016          ; odd even odd EVEN
    and #3             ; odd even
    cmp #1             ; odd even
    rol controller1    ; odd even odd even odd
    bcc read_loop      ; even odd [even]

Directional Safety

To reject opposing presses (Up+Down and Left+Right), which are possible on a worn Control Pad:

    lda buttons1,x
    and #%00001010    ; Compare Up and Left...
    lsr a
    and buttons1,x    ; to Down and Right
    beq not_updown
        ; Use previous frame's directions
        lda buttons1,x
        eor last_frame_buttons1,x
        and #%11110000
        eor last_frame_buttons1,x
        sta buttons1,x
    not_updown:

To instead reject all diagonal presses, simulating a 4-way joystick:

    lda buttons1,x
    and #%00001111    ; If A & (A - 1) is nonzero, A has more than one bit set
    beq not_diagonal
    sec
    sbc #1
    and buttons1,x
    beq not_diagonal
        ; Use previous frame's directions
        lda buttons1,x
        eor last_frame_buttons1,x
        and #%11110000
        eor last_frame_buttons1,x
        sta buttons1,x
    not_diagonal:

Calculating Presses and Releases

To calculate newly pressed and newly released buttons:

    lda buttons1,x
    eor #%11111111
    and last_frame_buttons1,x
    sta released_buttons1,x
    lda last_frame_buttons1,x
    eor #%11111111
    and buttons1,x
    sta pressed_buttons1,x

Button Flags

It is helpful to define the buttons as a series of bit flags:

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

And then buttons can be checked as follows:

    lda buttons
    and #BUTTON_A | BUTTON_B
    beq notPressingAorB
    ; Handle presses.
notPressingAorB:

External Examples

• Forum post: Blargg's DMC-fortified controller read routine
• Forum post: Rahsennor's OAM-synchronized controller read
• Forum post: Drag's bitwise DMC-safe controller reading