add timer implementation

1 files changed, 54 insertions, 18 deletions

diff --git a/ops.yaml b/ops.yaml
index 4e32ce8..7acf2f1 100644
--- a/ops.yaml
+++ b/ops.yaml
@@ -7629,10 +7629,9 @@ templates:
       } gpu;
 
       struct {
-        uint32_t clock; // FIXME: uint16_t?
-        uint8_t div,
-                tima,
-                tma,
+        uint16_t div;
+        uint32_t tima; // NOTE: cannot be uint16_t because of 10-bit shift
+        uint8_t tma,
                 tac;
       } timer;
 
@@ -7646,6 +7645,22 @@ templates:
       } cpu;
     } gb_t;
 
+    static uint8_t
+    timer_get_shift(
+      const gb_t * const ctx
+    ) {
+      switch (ctx->timer.tac & 0x3) {
+      case 0:
+        return 10; /* CPU clock / 1024, 4096Hz */
+      case 1:
+        return 4; /* CPU clock / 16, 262144Hz */
+      case 2:
+        return 6; /* CPU clock / 64, 65536Hz */
+      case 3:
+        return 8; /* CPU clock / 256, 16384Hz */
+      }
+    }
+
     #define PORT_P1   0xFF00
 
     // timer registers
@@ -7716,11 +7731,11 @@ templates:
 
         break;
       case PORT_DIV:
-        return ctx->timer.div;
+        return (ctx->timer.div & 0xFF00) >> 8;
 
         break;
       case PORT_TIMA:
-        return ctx->timer.tima;
+        return (ctx->timer.tima >> timer_get_shift(ctx)) & 0xFF;
 
         break;
       case PORT_TMA:
@@ -9167,22 +9182,29 @@ templates:
       gb_t * const ctx,
       const uint16_t clock
     ) {
-      // TODO:
-      UNUSED(ctx);
-      UNUSED(clock);
-      // ctx->timer.clock += clock;
-      // ctx->timer.div += (ctx->timer.clock
-      // const uint16_t new_div = ((uint16_t) ctx->timer.div) + (ctx->timer.clock / 32);
+      // increment div clock (high byte exposed via PORT_DIV)
+      ctx->timer.div += clock;
+
+      if (ctx->timer.tac & (1 << 2)) {
+        const uint8_t shift = timer_get_shift(ctx);
+        const uint32_t new_tima = ctx->timer.tima + clock;
+        const uint8_t old_of = (ctx->timer.tima >> (shift + 8)) & 0xFF,
+                      new_of = (new_tima >> (shift + 8)) & 0xFF;
+
+        if (new_of != old_of) {
+          // reset TIMA register to value of TMA
+          ctx->timer.tima = (ctx->timer.tma << shift);
+
+          // trigger timer interrupt
+          mmu_wb(ctx, PORT_IF, mmu_rb(ctx, PORT_IF) & (1 << 2));
+        }
+      }
     }
 
-    void
-    gb_step(
+    static uint16_t
+    cpu_step(
       gb_t * const ctx
     ) {
-      // TODO: handle halt
-      // handle pending interrupts
-      gb_handle_interrupts(ctx);
-
       // get pc, and current op
       const uint16_t old_pc = cpu_rw(ctx, RW_PC),
                      op = mmu_rb(ctx, old_pc);
@@ -9205,7 +9227,21 @@ templates:
 
       // increment cpu clock
       ctx->cpu.clock += clock;
+    }
+
+
+    void
+    gb_step(
+      gb_t * const ctx
+    ) {
+      // TODO: handle halt
+      // handle pending interrupts
+      gb_handle_interrupts(ctx);
+
+      // advance cpu
+      const uint16_t clock = cpu_step(ctx);
 
+      // advance timer
       timer_step(ctx, clock);
 
       // advance gpu