cpu - Fix timing of delay when enabling interrupts

cpu/cpu.cpp

@@ -191,8 +191,6 @@ void Cpu::handleInterrupts()
   u8 si = state.IE & state.IF & INT_MASK;
 
   if (state.IME == IME_SCHEDULED)
-    state.IME = IME_DELAYED;
-  else if (state.IME == IME_DELAYED)
     state.IME = IME_ON;
   else if (state.IME == IME_ON && si != 0)
     {

cpu/cpu.h

@@ -43,11 +43,22 @@ enum InterruptType : u8
   INT_MASK = 0x1F,
 };
 
+
+/**
+ IME - Interrupt Master Enable
+
+ An EI instruction will enable the interrupts, but delayed. During the next instruction after EI,
+ interrupts are still disabled. For this to be emulated we use a small state machine. which works as follows
+
+ instruction EI - sets IME_SCHEDULED
+ handleInterrupts -> IME_SCHEDULED to IME_ON (but no call to isr yet)
+ instruction any - is IME_ON, but no chance for call to isr yet
+ handleInterrupts -> is IME_ON, do a call to isr if necessary
+ */
 enum IME_state
 {
   IME_OFF,
   IME_SCHEDULED,
-  IME_DELAYED,
   IME_ON,
 };