1 files changed, 82 insertions, 0 deletions
diff --git a/src/timer.c b/src/timer.c
new file mode 100644
index 0000000..d4ee69f
--- /dev/null
+++ b/src/timer.c
@@ -0,0 +1,82 @@
+#include "timer.h"
+
+// Which DIV bit feeds the TIMA increment for each TAC clock-select.
+static const int tac_bit[4] = { 9, 3, 5, 7 };
+
+static bool timer_signal(Timer *t) {
+    if (!(t->tac & 0x04)) return false;
+    return (t->div >> tac_bit[t->tac & 3]) & 1;
+}
+
+static void set_div(GB *gb, u16 newdiv) {
+    Timer *t = &gb->timer;
+    bool old = timer_signal(t);
+    t->div = newdiv;
+    bool now = timer_signal(t);
+    // falling edge -> increment TIMA
+    if (old && !now) {
+        if (t->tima == 0xFF) {
+            t->tima = 0;
+            t->overflow_pending = true;
+            t->reload_delay = 4; // reload TMA after 4 T-cycles
+        } else {
+            t->tima++;
+        }
+    }
+}
+
+void timer_tick(GB *gb, int tcycles) {
+    Timer *t = &gb->timer;
+    for (int i = 0; i < tcycles; i++) {
+        if (t->overflow_pending) {
+            t->reload_delay--;
+            if (t->reload_delay == 0) {
+                t->tima = t->tma;
+                t->overflow_pending = false;
+                gb_request_interrupt(gb, INT_TIMER);
+            }
+        }
+        set_div(gb, t->div + 1);
+    }
+}
+
+u8 timer_read(GB *gb, u16 addr) {
+    Timer *t = &gb->timer;
+    switch (addr) {
+        case 0xFF04: return t->div >> 8;
+        case 0xFF05: return t->tima;
+        case 0xFF06: return t->tma;
+        case 0xFF07: return t->tac | 0xF8;
+    }
+    return 0xFF;
+}
+
+void timer_write(GB *gb, u16 addr, u8 val) {
+    Timer *t = &gb->timer;
+    switch (addr) {
+        case 0xFF04:
+            set_div(gb, 0);   // writing DIV resets it (may cause edge)
+            break;
+        case 0xFF05:
+            // writing during reload delay cancels the reload
+            if (t->reload_delay != 0) t->overflow_pending = false;
+            if (!(t->overflow_pending && t->reload_delay == 0))
+                t->tima = val;
+            break;
+        case 0xFF06:
+            t->tma = val;
+            break;
+        case 0xFF07: {
+            bool old = timer_signal(t);
+            t->tac = val & 0x07;
+            bool now = timer_signal(t);
+            if (old && !now) {
+                // disabling can also cause an increment (glitch)
+                if (t->tima == 0xFF) {
+                    t->tima = 0; t->overflow_pending = true; t->reload_delay = 4;
+                } else t->tima++;
+            }
+            break;
+        }
+    }
+}