1 files changed, 101 insertions, 0 deletions
diff --git a/src/interrupts.s b/src/interrupts.s
new file mode 100644
index 0000000..8c53af7
--- /dev/null
+++ b/src/interrupts.s
@@ -0,0 +1,101 @@
+.segment "CODE"
+
+.proc nmi
+    ;; Should we skip it?
+    bit Globals::zp_flags
+
+    ;; If we are on a dev environment, account for any frame drops.
+    .ifdef PARTIAL
+        bpl @account_for_frame_drop
+    .else
+        bpl @end
+    .endif
+
+    ;; Save registers.
+    pha
+    txa
+    pha
+    tya
+    pha
+
+    ;; Sprite DMA.
+    lda #$00
+    sta OAM::m_address
+    lda #$02
+    sta OAM::m_dma
+
+    ;; Are we paused? If so skip timers, PAL handler and the likes.
+    lda #%00001000
+    and Globals::zp_flags
+    bne @ppu_registers
+
+    ;; PAL-specific code
+    .ifdef PAL
+        jsr Driver::pal_handler
+    .endif
+
+    ;; TODO: some actions here will depend on the status of the game...
+    lda Globals::zp_flags
+    and #%00000001
+    bne @ppu_registers
+
+    ;; Increase the random seed.
+    inc Prng::zp_rand
+
+    ;; Decrease title timer.
+    lda Title::zp_title_timer
+    beq @ppu_registers
+    dec Title::zp_title_timer
+
+@ppu_registers:
+    ;; Should we update PPU registers?
+    bit Globals::zp_flags
+    bvc @scroll
+
+    ;; Zero out the `ppu` flag.
+    lda #%10111111
+    and Globals::zp_flags
+    sta Globals::zp_flags
+
+    bit PPU::m_status
+
+    ;; Update the PPU control/mask registers with shadowed values.
+    lda PPU::zp_mask
+    sta PPU::m_mask
+    lda PPU::zp_control
+    sta PPU::m_control
+
+@scroll:
+    ;; Always reset the scroll just in case.
+    bit PPU::m_status
+    lda #$00
+    sta PPU::m_scroll
+    sta PPU::m_scroll
+
+    ;; Unblock the main code.
+    lda #%01111111
+    and Globals::zp_flags
+    sta Globals::zp_flags
+
+    ;; Restore registers.
+    pla
+    tay
+    pla
+    tax
+    pla
+
+@end:
+    rti
+
+    ;; If we are on a dev environment, account for any frame drops.
+.ifdef PARTIAL
+@account_for_frame_drop:
+    inc Debug::zp_frame_drops
+    rti
+.endif
+.endproc
+
+;; Unused.
+.proc irq
+    rti
+.endproc