diff options
| -rw-r--r-- | basics/flicker.s | 2 | ||||
| -rw-r--r-- | basics/input.s | 5 | ||||
| -rw-r--r-- | scroll/include/joypad.s | 63 | ||||
| -rw-r--r-- | shared/joypad.s | 108 |
4 files changed, 74 insertions, 104 deletions
diff --git a/basics/flicker.s b/basics/flicker.s index b170d5e..640817a 100644 --- a/basics/flicker.s +++ b/basics/flicker.s @@ -36,7 +36,7 @@ @main_game_loop: ;; NOTE: the logic is pretty simply: read the pad, move the player ;; accordingly, and apply the flickering effect. - jsr Joypad::read + jsr joypad_read jsr Diskun::update ;; NOTE: comment this `jsr` out if you want to see what happens if no diff --git a/basics/input.s b/basics/input.s index dbb9ee6..6918487 100644 --- a/basics/input.s +++ b/basics/input.s @@ -8,6 +8,11 @@ ;; we are constantly polling it and filling it, the value will move constantly). ;; - $21: the previous status of the right arrow. ;; - $42: the counter which is incremented on each press of the right arrow button. +;; +;; Whenever you are done with this example, hop into the `shared/joypad.s` file, +;; which brings some other considerations when reading from controllers. That +;; is, the algorithm shown below is not entirely "safe" due to a hardware bug +;; which might give unreliable inputs on some spikes. ;;; ;;; diff --git a/scroll/include/joypad.s b/scroll/include/joypad.s deleted file mode 100644 index 8711c28..0000000 --- a/scroll/include/joypad.s +++ /dev/null @@ -1,63 +0,0 @@ -.segment "CODE" - -;;; -;; Joypad controller code. The following memory addresses are reserved: $21-$23. -;; -;; Memory address $21 is used for internal purposes, whereas $22 and $23 contain -;; the bitmask of the buttons that are pressed from each controller. -;;; - -.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 - - ;; We keep all the information from controller from mainly two variables: - ;; m_buttons1 and m_buttons2; containing respectively the buttons pressed - ;; for this frame for both controllers. The m_inv_buttons ($21) is an - ;; internal variable and should not be used for anything outside of this - ;; usage. - m_inv_buttons = $21 - m_buttons1 = $22 - m_buttons2 = $23 - - ;; READ_CONTROLLER reads the input from the controller mapped into the given - ;; port, and saves the state into the given `buttons` address. - ;; Implementation taken from NESHacker's example of smb3-like movement. - .macro READ_CONTROLLER port, buttons - lda m_inv_buttons - tay - lda #1 - sta port - sta m_inv_buttons - lsr - sta port - : - lda port - lsr - rol m_inv_buttons - bcc :- - tya - eor m_inv_buttons - and m_inv_buttons - sta buttons - .endmacro - - ;; read sets the values for m_buttons1 and m_buttons2 as read from both - ;; controllers. - .proc read - READ_CONTROLLER JOYPAD1, m_buttons1 - READ_CONTROLLER JOYPAD2, m_buttons2 - rts - .endproc -.endscope diff --git a/shared/joypad.s b/shared/joypad.s index 2abf959..7c3e44c 100644 --- a/shared/joypad.s +++ b/shared/joypad.s @@ -1,5 +1,3 @@ -;;; TODO: safe read - .scope Joypad ;; Button masks. BUTTON_A = 1 << 7 @@ -15,42 +13,72 @@ JOYPAD1 = $4016 JOYPAD2 = $4017 - ;; We keep all the information from controller from mainly two variables: - ;; m_buttons1 and m_buttons2; containing respectively the buttons pressed - ;; for this frame for both controllers. The m_inv_buttons ($21) is an - ;; internal variable and should not be used for anything outside of this - ;; usage. - m_inv_buttons = $21 - m_buttons1 = $22 - m_buttons2 = $23 - - ;; READ_CONTROLLER reads the input from the controller mapped into the given - ;; port, and saves the state into the given `buttons` address. - ;; Implementation taken from NESHacker's example of smb3-like movement. - .macro READ_CONTROLLER port, buttons - lda m_inv_buttons - tay - lda #1 - sta port - sta m_inv_buttons - lsr - sta port - : - lda port - lsr - rol m_inv_buttons - bcc :- - tya - eor m_inv_buttons - and m_inv_buttons - sta buttons - .endmacro - - ;; read sets the values for m_buttons1 and m_buttons2 as read from both - ;; controllers. - .proc read - READ_CONTROLLER JOYPAD1, m_buttons1 - READ_CONTROLLER JOYPAD2, m_buttons2 - rts - .endproc + ;; 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 a + 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 + pha + jsr joypad_unsafe_read_x + pla + cmp Joypad::m_buttons1, x + bne @joypad_read_x_reread + + rts |