From 77b56c50e6466f18aee809edcad43d55660f0efb Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Fri, 13 May 2022 14:02:56 -0400
Subject: [PATCH] Ensure you can't trace into divide-by-zero, etc.

---
 InstructionSets/M68k/Executor.hpp              |  3 +++
 .../Implementation/ExecutorImplementation.hpp  | 18 ++++++++++++++----
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/InstructionSets/M68k/Executor.hpp b/InstructionSets/M68k/Executor.hpp
index d0663ddef..e9ca48b72 100644
--- a/InstructionSets/M68k/Executor.hpp
+++ b/InstructionSets/M68k/Executor.hpp
@@ -113,7 +113,10 @@ template <Model model, typename BusHandler> class Executor {
 				CPU::SlicedInt32 stack_pointers[2];
 				uint32_t instruction_address;
 				uint16_t instruction_opcode;
+
+				// Things that are ephemerally duplicative of Status.
 				int active_stack_pointer = 0;
+				Status::FlagT should_trace = 0;
 
 				// Bus state.
 				int interrupt_input = 0;
diff --git a/InstructionSets/M68k/Implementation/ExecutorImplementation.hpp b/InstructionSets/M68k/Implementation/ExecutorImplementation.hpp
index 95dfd907b..add5cc1d9 100644
--- a/InstructionSets/M68k/Implementation/ExecutorImplementation.hpp
+++ b/InstructionSets/M68k/Implementation/ExecutorImplementation.hpp
@@ -69,6 +69,9 @@ void Executor<model, BusHandler>::run_for_instructions(int count) {
 			state_.status.trace_flag = 0;
 			state_.did_update_status();
 
+			// Ensure no tracing occurs into the exception.
+			state_.should_trace = 0;
+
 			// Push status and the program counter at instruction start.
 			state_.template write<uint16_t>(sp.l - 14, code);
 			state_.template write<uint32_t>(sp.l - 12, faulting_address);
@@ -322,6 +325,14 @@ void Executor<model, BusHandler>::State::run(int &count) {
 			status.interrupt_level = interrupt_input;
 		}
 
+		// Capture the trace bit, indicating whether to trace
+		// after this instruction.
+		//
+		// If an exception occurs, this value will be cleared, but
+		// it'll persist across mere status register changes for
+		// one instruction's duration.
+		should_trace = status.trace_flag;
+
 		// Read the next instruction.
 		instruction_address = program_counter.l;
 		instruction_opcode = read_pc<uint16_t>();
@@ -345,10 +356,6 @@ void Executor<model, BusHandler>::State::run(int &count) {
 			}
 		}
 
-		// Capture the trace bit, indicating whether to trace
-		// after this instruction.
-		const auto should_trace = status.trace_flag;
-
 		// Temporary storage.
 		CPU::SlicedInt32 operand_[2];
 		EffectiveAddress effective_address_[2];
@@ -415,6 +422,9 @@ void Executor<model, BusHandler>::State::raise_exception(int index) {
 	write<uint16_t>(sp.l - 6, previous_status);
 	sp.l -= 6;
 
+	// Ensure no tracing occurs into the exception.
+	should_trace = 0;
+
 	// Fetch the new program counter.
 	program_counter.l = read<uint32_t>(address);
 }