Merge pull request #13 from KallDrexx/loopback_instruction_fix

Loopback jumps should occur on the same cpu address of the jump

NESDecompiler.Core/Decompilation/DecompiledFunction.cs

@@ -32,13 +32,17 @@ public class DecompiledFunction
         JumpTargets = instructions
             .Where(x => jumpTargets.Contains(x.CPUAddress))
             .Where(x => x.Label != null)
+            .Where(x => x.SubAddressOrder == 0) // only real instructions should be jumped to
             .ToDictionary(x => x.CPUAddress, x => x.Label!);
 
         // We need to order the instructions so that the starting instruction is the first one encountered.
         // We can't just rely on the CPU address, because a function may jump to a code point earlier than
         // the first instruction.
+        var entryPointInstructions = instructions.Where(x => x.CPUAddress == address)
+            .Where(x => x.SubAddressOrder >= 0);
+
         var initialInstructions = instructions
-            .Where(x => x.CPUAddress >= address)
+            .Where(x => x.CPUAddress > address)
             .OrderBy(x => x.CPUAddress)
             .ThenBy(x => x.SubAddressOrder);
 
@@ -47,6 +51,17 @@ public class DecompiledFunction
             .OrderBy(x => x.CPUAddress)
             .ThenBy(x => x.SubAddressOrder); // real instructions before virtual ones
 
-        OrderedInstructions = initialInstructions.Concat(trailingInstructions).ToArray();
+        // If there was a loopback jump point at the function address, put that here. This is required
+        // because if an emulator is executing a virtual loopback instruction and an IRQ occurs, this
+        // will cause the virtual instruction to be saved to the stack, and that can cause the entry
+        // point to be wrong.
+        var loopbackInstructions = instructions.Where(x => x.CPUAddress == address)
+            .Where(x => x.SubAddressOrder < 0);
+
+        OrderedInstructions = entryPointInstructions
+            .Concat(initialInstructions)
+            .Concat(trailingInstructions)
+            .Concat(loopbackInstructions)
+            .ToArray();
     }
 }

NESDecompiler.Core/Decompilation/FunctionDecompiler.cs

@@ -26,7 +26,7 @@ public static class FunctionDecompiler
                     // This means a branch occurred that caused the flow to wrap around to instructions preceding
                     // the function entrance. This usually happens when there is a jump/branch to right before the
                     // entrypoint, usually due to decompiling in the middle of a loop. To fix this, we need to add
-                    // a jump back to the function entrypoint
+                    // a jump back to the function entrypoint.
                     if (functionAddress == 0x00)
                     {
                         const string message = "Wrap around instruction detected for a function at 0000, but that " +
@@ -41,12 +41,12 @@ public static class FunctionDecompiler
                     var jumpInstruction = new DisassembledInstruction
                     {
                         Info = InstructionSet.GetInstruction(0x4C),
-                        CPUAddress = (ushort)(nextAddress - 1),
+                        CPUAddress = nextAddress,
                         Bytes = [0x4C, (byte)addressLow, (byte)addressHigh],
                         TargetAddress = functionAddress,
 
-                        // Make sure they appear after any instruction that already occupies that address
-                        SubAddressOrder = 1,
+                        // Make sure they appear before the function address
+                        SubAddressOrder = -1,
                     };
 
                     instructions.Add(jumpInstruction);
@@ -92,7 +92,8 @@ public static class FunctionDecompiler
         // Add labels for any jump targets
         foreach (var instruction in instructions)
         {
-            if (jumpAddresses.Contains(instruction.CPUAddress))
+            // Only real instructions should have a label, virtual ones should not
+            if (jumpAddresses.Contains(instruction.CPUAddress) && instruction.SubAddressOrder == 0)
             {
                 instruction.Label = $"loc_{instruction.CPUAddress:X4}";
             }

NESDecompiler.Core/Disassembly/Disassembler.cs

@@ -78,7 +78,7 @@ namespace NESDecompiler.Core.Disassembly
         /// address location at runtime. Can be used to add runtime hooks or to work around
         /// decompilation issues. Should be 0 for all native instructions from a ROM.
         /// </summary>
-        public byte SubAddressOrder { get; set; }
+        public sbyte SubAddressOrder { get; set; }
 
         /// <summary>
         /// Returns a string representation of this instruction