CPU: Support stalling CPU for bus activity such as DMA

src/core/cpu_core.h

@@ -35,6 +35,11 @@ public:
 
   TickCount GetPendingTicks() const { return m_pending_ticks; }
   void ResetPendingTicks() { m_pending_ticks = 0; }
+  void AddPendingTicks(TickCount ticks)
+  {
+    m_pending_ticks += ticks;
+    m_downcount -= ticks;
+  }
 
   void SetDowncount(TickCount downcount) { m_downcount = (downcount < m_downcount) ? downcount : m_downcount; }
   void ResetDowncount() { m_downcount = MAX_SLICE_SIZE; }

src/core/system.cpp

@@ -296,6 +296,11 @@ void System::SetDowncount(TickCount downcount)
   m_cpu->SetDowncount(downcount);
 }
 
+void System::StallCPU(TickCount ticks)
+{
+  m_cpu->AddPendingTicks(ticks);
+}
+
 void System::SetController(u32 slot, std::shared_ptr<PadDevice> dev)
 {
   m_pad->SetController(slot, std::move(dev));

src/core/system.h

@@ -58,6 +58,9 @@ public:
   void SetDowncount(TickCount downcount);
   void Synchronize();
 
+  // Adds ticks to the global tick counter, simulating the CPU being stalled.
+  void StallCPU(TickCount ticks);
+
   void SetController(u32 slot, std::shared_ptr<PadDevice> dev);
   void SetMemoryCard(u32 slot, std::shared_ptr<PadDevice> dev);