diff --git a/cpu/cpu.cpp b/cpu/cpu.cpp
index fd2fcd2..fa8345f 100644
--- a/cpu/cpu.cpp
+++ b/cpu/cpu.cpp
@@ -60,17 +60,23 @@ Cpu::Cpu(Mem_device* bus)
   reset();
 }
 
-void Cpu::step()
+unsigned int Cpu::step()
 {
-  if(state.stopped) return;
+  if(state.stopped) return 4;
+  if(handleInterrupts()) return 5;
 
-  if(!handleInterrupts()) // if no isr has been called, decode an instruction
-    {
-      if (state.halted)
-        processed_mcycles += 4;
-      else
-        executeInstruction();
-    }
+  // if no isr has been called, decode an instruction
+  if (state.halted) return 4;
+
+  return executeInstruction();
+}
+
+unsigned long Cpu::run(unsigned long mcycles)
+{
+  unsigned long processed_mcycles = 0;
+  while(processed_mcycles < mcycles)
+    processed_mcycles += step();
+  return processed_mcycles;
 }
 
 void Cpu::reset()
@@ -281,8 +287,6 @@ bool Cpu::handleInterrupts()
       state.IF &= ~it; // clear interrupt in IF
       doCall(isr);     // Call interrupt service routine
 
-      processed_mcycles += 5;
-
       return true;
     }
 
diff --git a/cpu/cpu.h b/cpu/cpu.h
index 02e9f06..dd50804 100644
--- a/cpu/cpu.h
+++ b/cpu/cpu.h
@@ -163,7 +163,7 @@ private:
   bool decodeCond(u8 cc);
 
   bool handleInterrupts();
-  void executeInstruction();
+  unsigned long executeInstruction();
 
   void reset();
 
@@ -172,12 +172,13 @@ public:
 
   Cpu_state state;
   Mem_device* bus;
-  unsigned long processed_mcycles;
   u16 last_instr_addr;
 
   void signalInterrupt(InterruptType it);
 
-  void step();
+  unsigned int step();
+
+  unsigned long run(unsigned long mcycles);
 };
 
 class CpuException : public EmulatorException {
diff --git a/cpu/decoder.cpp b/cpu/decoder.cpp
index a68cf39..d46f936 100644
--- a/cpu/decoder.cpp
+++ b/cpu/decoder.cpp
@@ -5,7 +5,7 @@ static inline u16 make_u16(u8 msb, u8 lsb)
   return (((u16)msb << 8) | (u16)lsb);
 }
 
-void Cpu::executeInstruction()
+unsigned long Cpu::executeInstruction()
 {
   last_instr_addr = state.PC;
   opcode op{ readPC8() };
@@ -458,5 +458,5 @@ void Cpu::executeInstruction()
         }
     }
 
-  processed_mcycles += mcycles;
+  return mcycles;
 }