From 1b3acf9cd858250434b52369558ca58f9a1b2c6a Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Mon, 23 May 2022 08:18:37 -0400
Subject: [PATCH] Eliminate assumption.

---
 .../Implementation/68000Mk2Implementation.hpp | 23 +++++++++++--------
 .../Implementation/68000Mk2Storage.hpp        |  6 +++++
 2 files changed, 19 insertions(+), 10 deletions(-)

diff --git a/Processors/68000Mk2/Implementation/68000Mk2Implementation.hpp b/Processors/68000Mk2/Implementation/68000Mk2Implementation.hpp
index 1eb5b9aa0..4681f3520 100644
--- a/Processors/68000Mk2/Implementation/68000Mk2Implementation.hpp
+++ b/Processors/68000Mk2/Implementation/68000Mk2Implementation.hpp
@@ -1249,21 +1249,24 @@ void Processor<BusHandler, dtack_is_implicit, permit_overrun, signal_will_perfor
 				MoveToNextOperand(StoreOperand_bw);
 			}
 
-			// Assumption enshrined here: there are no write-only Dn
-			// byte operations. i.e. anything that should technically
-			// write back only a byte will have read from the register
-			// before the operation, making it safe to write back the
-			// entire word.
-			//
-			// However there are write-only Dn word operations, and
-			// the sign-extended top half needs to be kept for An.
 			switch(instruction_.mode(next_operand_)) {
-				case Mode::DataRegisterDirect:
-					registers_[instruction_.lreg(next_operand_)].w = operand_[next_operand_].w;
+				// Data register: write only the part of the word that has changed.
+				case Mode::DataRegisterDirect: {
+					const uint32_t write_mask = size_masks[int(instruction_.operand_size())];
+					const int reg = instruction_.reg(next_operand_);
+
+					registers_[reg].l =
+						(operand_[next_operand_].l & write_mask) |
+						(registers_[reg].l & ~write_mask);
+				}
 				MoveToNextOperand(StoreOperand_bw);
+
+				// Address register: always rewrite the whole word; the smaller
+				// result will have been sign extended.
 				case Mode::AddressRegisterDirect:
 					registers_[instruction_.lreg(next_operand_)] = operand_[next_operand_];
 				MoveToNextOperand(StoreOperand_bw);
+
 				default: break;
 			}
 
diff --git a/Processors/68000Mk2/Implementation/68000Mk2Storage.hpp b/Processors/68000Mk2/Implementation/68000Mk2Storage.hpp
index b133a2d67..a54de7610 100644
--- a/Processors/68000Mk2/Implementation/68000Mk2Storage.hpp
+++ b/Processors/68000Mk2/Implementation/68000Mk2Storage.hpp
@@ -112,6 +112,12 @@ struct ProcessorBase: public InstructionSet::M68k::NullFlowController {
 		{ 2, 2, 2, 2, 2, 2, 2, 2, },
 		{ 4, 4, 4, 4, 4, 4, 4, 4, },
 	};
+
+	// A lookup table that ensures write-back to data registers affects
+	// only the correct bits.
+	static constexpr uint32_t size_masks[3] = { 0xff, 0xffff, 0xffff'ffff };
+
+	// Assumptions used by the lookup tables above:
 	static_assert(int(InstructionSet::M68k::DataSize::Byte) == 0);
 	static_assert(int(InstructionSet::M68k::DataSize::Word) == 1);
 	static_assert(int(InstructionSet::M68k::DataSize::LongWord) == 2);