ppcopcodes: Decrementer fixes.

- Always add a tick to the timeout calculation.
- Make sure value of decrementer is < 0 for timed decrementer exception.
- Don't recalculate decrementer write time stamp in the case of an
  immediate decrementer exception.

Author: joevt

cpu/ppc/ppcopcodes.cpp

@@ -881,15 +881,9 @@ static void update_timebase(uint64_t mask, uint64_t new_val)
 
 static uint32_t decrementer_timer_id = 0;
 
-static void update_decrementer(uint32_t oldval, uint32_t newval);
+static void update_decrementer(bool update_time_stamp, uint32_t oldval, uint32_t newval);
 
 static void trigger_decrementer_exception() {
-    decrementer_timer_id = 0;
-
-    uint32_t new_val = calc_dec_value();
-
-    update_decrementer(new_val, new_val);
-
     if (ppc_state.msr & MSR::EE) {
         dec_exception_pending = false;
         //LOG_F(WARNING, "decrementer exception triggered");
@@ -901,9 +895,27 @@ static void trigger_decrementer_exception() {
     }
 }
 
-static void update_decrementer(uint32_t oldval, uint32_t newval) {
-    dec_wr_value = newval;
-    dec_wr_timestamp = get_virt_time_ns();
+static void trigger_timed_decrementer_exception() {
+    decrementer_timer_id = 0;
+    uint32_t new_val = calc_dec_value();
+    if (new_val >= 0 && new_val != uint32_t(-1)) {
+        new_val = -1;
+    }
+    update_decrementer(true, new_val, new_val);
+    trigger_decrementer_exception();
+}
+
+static void trigger_immediate_decrementer_exception() {
+    decrementer_timer_id = 0;
+    update_decrementer(false, ppc_state.spr[SPR::DEC_S], ppc_state.spr[SPR::DEC_S]);
+    trigger_decrementer_exception();
+}
+
+static void update_decrementer(bool update_time_stamp, uint32_t oldval, uint32_t newval) {
+    if (update_time_stamp) {
+        dec_wr_value = newval;
+        dec_wr_timestamp = get_virt_time_ns();
+    }
 
     dec_exception_pending = false;
 
@@ -913,22 +925,28 @@ static void update_decrementer(uint32_t oldval, uint32_t newval) {
 
     if (bit_changed(oldval, newval, 31) && bit_set(newval, 31)) {
         decrementer_timer_id = TimerManager::get_instance()->add_immediate_timer(
-            trigger_decrementer_exception
+            trigger_immediate_decrementer_exception
         );
         return;
     }
 
-    // add one tick if newval is zero
-    if (!newval)
-        newval += (is_601) ? (1 << 7) : 1;
+    // A count of zero should timeout after one tick so add one to the count
+    // but make sure the count doesn't overflow.
+    if (is_601) {
+        if (newval < uint32_t(-0x80))
+            newval += 0x80;
+    } else {
+        if (newval < uint32_t(-1))
+            newval += 1;
+    }
 
     uint64_t time_out;
     uint32_t time_out_lo;
     _u32xu64(newval, tbr_period_ns, time_out, time_out_lo);
     //LOG_F(WARNING, "new decrementer value: 0x%08X, interrupt after %llu ns", newval, time_out);
     decrementer_timer_id = TimerManager::get_instance()->add_oneshot_timer(
         time_out,
-        trigger_decrementer_exception
+        trigger_timed_decrementer_exception
     );
 }
 
@@ -1058,8 +1076,8 @@ void dppc_interpreter::ppc_mtspr(uint32_t opcode) {
     case SPR::DEC_S:
         if (is_601)
             val &= ~0x7F;
-        update_decrementer(ppc_state.spr[DEC_S], val);
-        ppc_state.spr[DEC_S] = val;
+        update_decrementer(true, calc_dec_value(), val);
+        ppc_state.spr[SPR::DEC_S] = val;
         break;
     case SPR::TBL_S:
         update_timebase(0xFFFFFFFF00000000ULL, val);