Controller reading code
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 ; bit0 -> Carry rol buttons ; Carry -> bit0; 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 #$03 ; ignore bits other than controller cmp #$01 ; Set carry if and only if nonzero rol buttons1 ; Carry -> bit0; bit 7 -> Carry lda JOYPAD2 ; Repeat and #$03 cmp #$01 rol buttons2 ; Carry -> bit0; 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. 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:
last_frame_buttons1 = $00 last_frame_buttons2 = $01 first_read_buttons1 = $02 first_read_buttons2 = $03 readjoy_safe: lda buttons2 sta last_frame_buttons2 lda buttons1 sta last_frame_buttons1 ; Read the controllers once and stash the result jsr readjoy lda buttons2 sta first_read_buttons2 lda buttons1 sta first_read_buttons1 ; Read the controllers again and compare jsr readjoy ldx #1 cleanup_loop: ; Ignore read values if a bit deletion occurred lda buttons1,x cmp first_read_buttons1,x beq not_glitched lda last_frame_buttons,x sta buttons1,x not_glitched: dex bpl cleanup_loop rts
Note that 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.Cite error: Closing
- Forum post: Rahsennor's OAM-synchronized controller read