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Diffstat (limited to 'space/src/states/bullets.s')
| -rw-r--r-- | space/src/states/bullets.s | 181 |
1 files changed, 181 insertions, 0 deletions
diff --git a/space/src/states/bullets.s b/space/src/states/bullets.s new file mode 100644 index 0000000..af74ce7 --- /dev/null +++ b/space/src/states/bullets.s @@ -0,0 +1,181 @@ +;;; +;; Implements the pool of bullets available when pressing B. The main code +;; should just want to call `init` for initializing the memory space, and then +;; `update` on each main game loop iteration. The following addresses are +;; reserved: +;; -> $40-$41: internal data. +;; -> $0208-$0213: OAM data (this is a pool of three sprites). +;; +;; NOTE: the code in here touch the addresses on OAM directly, instead of +;; keeping them in some internal addresses on zero page. This is usually not a +;; good idea, and more so if we were to do collision checking and stuff like +;; that. That being said, because of its simplicity, it's not that big of a +;; deal. +.scope Bullets + ;; The number of bullets shown on screen for the current frame. + m_bullets_screen = $40 + + ;; Frame counter. See `FRAMES` below. + m_frames = $41 + + ;; How many frames have to pass to allow the user to shoot another bullet + ;; after the previous one. + FRAMES = 15 + + ;; Initializes the pool of bullets. + .proc init + ;; Initializing variables. + + lda #0 + sta m_bullets_screen + + lda #FRAMES + sta m_frames + + ;; Set X and Y positions off-screen for the three available slots. + + lda #$ff + sta $208 + sta $20C + sta $210 + sta $20B + sta $20F + sta $213 + + ;; All three sprites are instances of the same tile. + + lda #4 + sta $209 + sta $20D + sta $210 + + ;; Nothing special for these sprites, set attributes to 0. + + lda #0 + sta $20A + sta $20E + sta $212 + + rts + .endproc + + ;; Update bullets on screen, check if a new one has to be shown, etc. + .proc update + ;; If the frame counter has the same value as our allowed one, we can go + ;; into the `bullets_pressed` subroutine, otherwise we will skip it + ;; altogether. + lda m_frames + cmp #FRAMES + beq @check + inc m_frames + jmp @position + @check: + jsr bullets_pressed + @position: + jsr update_positions + + rts + .endproc + + ;; Shows a new bullet right in front of the ship if the player requested it + ;; and it is possible. + .proc bullets_pressed + ;; If we reached the maximum of bullets on screen, return early. + lda m_bullets_screen + cmp #3 + bne :+ + rts + : + ;; If the B button is not pressed, return early. + lda #Joypad::BUTTON_B + and Joypad::m_buttons1 + bne :+ + rts + : + ;; The following code will loop through the three bullet sprites and + ;; find one that is free (off-screen). If one could be found, it will + ;; assign it to the ship's position. + ldx #0 + @loop: + ;; Is the Y position on this sprite $FF? If so then it's free and we can + ;; jump into @set so we re-initialize the position for this bullet. + lda $208, x + cmp #$FF + beq @set + + ;; Oops, that was not the case. At this point, is this the last bullet? + ;; If so, we can go to the @end, otherwise we increase the index and try + ;; again. + cpx #8 + beq @end + inx + inx + inx + inx + jmp @loop + @set: + ;; At the current index we have a bullet to initialize. Hence, give it + ;; the Y value from the player and the X one (+4 so it's at the center + ;; of the ship on the X axis). + lda Player::m_screen_y + sta $208, x + lda Player::m_screen_x + clc + adc #4 + inx + inx + inx + sta $208, x + + ;; Reset the `m_frames` so to disallow too many bullets being shot at + ;; once, and increate the `m_bullets_screen` variable. + lda #0 + sta m_frames + inc m_bullets_screen + @end: + rts + .endproc + + ;; Updates the positions from the bullets being shown on screen. If one + ;; bullet goes off screen, then it's properly freed so it can be picked up + ;; by another B press. + .proc update_positions + ldx #0 + @loop: + ;; Do nothing if the bullet is free. + lda $208, x + cmp #$FF + beq @next + + ;; The indexed bullet is shown on screen, and we should make it move + ;; upward by 10 units. That being said, if its Y position is lesser or + ;; equal than 10, we can directly free it now. Otherwise, just update + ;; the Y value by performing this subtraction. + cmp #10 + bcc @free + beq @free + sec + sbc #10 + jmp @save + @free: + ;; This bullet should be freed, decrease `m_bullets_screen` and set `a` + ;; to an off-screen value. + dec m_bullets_screen + lda #$FF + @save: + ;; Either way you reach this, in `a` we have the Y value to be stored. + sta $208, x + @next: + ;; If we are already at the last bullet, jump to the @end. Otherwise + ;; increase the index and loop again. + cpx #8 + beq @end + inx + inx + inx + inx + jmp @loop + @end: + rts + .endproc +.endscope |