From 7af81018c9b51ca8b930b06399fd0ca4b7ee0303 Mon Sep 17 00:00:00 2001
From: Adrian Conlon <Adrian.conlon@gmail.com>
Date: Thu, 1 Nov 2018 19:47:21 +0000
Subject: [PATCH] Modify rotate and shift instructions to be a little more
 understandable (6502/6809)

Signed-off-by: Adrian Conlon <Adrian.conlon@gmail.com>
---
 M6502/src/mos6502.cpp | 14 ++++++--------
 MC6809/src/mc6809.cpp | 25 +++++++++++--------------
 2 files changed, 17 insertions(+), 22 deletions(-)

diff --git a/M6502/src/mos6502.cpp b/M6502/src/mos6502.cpp
index f6778ab..ebe66ff 100644
--- a/M6502/src/mos6502.cpp
+++ b/M6502/src/mos6502.cpp
@@ -613,20 +613,18 @@ void EightBit::MOS6502::plp() {
 	P() = (pop() | RF) & ~BF;
 }
 
-uint8_t EightBit::MOS6502::rol(uint8_t operand) {
+uint8_t EightBit::MOS6502::rol(const uint8_t operand) {
 	const auto carryIn = carry();
 	setFlag(P(), CF, operand & Bit7);
-	operand <<= 1;
-	operand |= carryIn;
-	return through(operand);
+	const uint8_t result = (operand << 1) | carryIn;
+	return through(result);
 }
 
-uint8_t EightBit::MOS6502::ror(uint8_t operand) {
+uint8_t EightBit::MOS6502::ror(const uint8_t operand) {
 	const auto carryIn = carry();
 	setFlag(P(), CF, operand & Bit0);
-	operand >>= 1;
-	operand |= (carryIn << 7);
-	return through(operand);
+	const uint8_t result = (operand >> 1) | (carryIn << 7);
+	return through(result);
 }
 
 void EightBit::MOS6502::rti() {
diff --git a/MC6809/src/mc6809.cpp b/MC6809/src/mc6809.cpp
index baf48ef..610d5c5 100644
--- a/MC6809/src/mc6809.cpp
+++ b/MC6809/src/mc6809.cpp
@@ -787,10 +787,9 @@ uint8_t EightBit::mc6809::asl(uint8_t operand) {
 
 uint8_t EightBit::mc6809::asr(uint8_t operand) {
 	setFlag(CC(), CF, operand & Bit0);
-	operand >>= 1;
-	operand |= Bit7;
-	adjustNZ(operand);
-	return operand;
+	const uint8_t result = (operand >> 1) | Bit7;
+	adjustNZ(result);
+	return result;
 }
 
 void EightBit::mc6809::bit(const uint8_t operand, const uint8_t data) {
@@ -1007,23 +1006,21 @@ void EightBit::mc6809::pul(register16_t& stack, const uint8_t data) {
 	}
 }
 
-uint8_t EightBit::mc6809::rol(uint8_t operand) {
+uint8_t EightBit::mc6809::rol(const uint8_t operand) {
 	const auto carryIn = carry();
 	setFlag(CC(), CF, operand & Bit7);
 	setFlag(CC(), VF, ((operand & Bit7) >> 7) ^ ((operand & Bit6) >> 6));
-	operand <<= 1;
-	operand |= carryIn;
-	adjustNZ(operand);
-	return operand;
+	const uint8_t result = (operand << 1) | carryIn;
+	adjustNZ(result);
+	return result;
 }
 		
-uint8_t EightBit::mc6809::ror(uint8_t operand) {
+uint8_t EightBit::mc6809::ror(const uint8_t operand) {
 	const auto carryIn = carry();
 	setFlag(CC(), CF, operand & Bit0);
-	operand >>= 1;
-	operand |= (carryIn << 7);
-	adjustNZ(operand);
-	return operand;
+	const uint8_t result = (operand >> 1) | (carryIn << 7);
+	adjustNZ(result);
+	return result;
 }
 
 void EightBit::mc6809::rti() {