From 032aacee39b59d524f525e964094f91e617e5778 Mon Sep 17 00:00:00 2001
From: Michael LeBlanc <leblanc.michael@gmail.com>
Date: Fri, 8 Apr 2022 10:36:54 -0700
Subject: [PATCH] Fix errors in 6502 emulation

This commit fixes the following behaviors:

Indirect X zero page addressing should always wrap and stay on zero page
BRK should disable interrupts
ADC in decimal mode should carry between nibbles based on adjusted,
  not original values.
Writes of status register to the stack should always set M and should
  set B on BRK/PHP and ~B on IRQ/NMI

Found and tested against
https://github.com/Klaus2m5/6502_65C02_functional_tests/blob/master/6502_functional_test.a65
---
 src/m6502.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/src/m6502.c b/src/m6502.c
index 36af85e..41841c4 100755
--- a/src/m6502.c
+++ b/src/m6502.c
@@ -40,7 +40,7 @@
 #define Z 0x02
 #define C 0x01
 
-static unsigned char accumulator, xRegister, yRegister, statusRegister = 0x24, stackPointer;
+static unsigned char accumulator, xRegister, yRegister, statusRegister = I, stackPointer;
 static int IRQ = 0, NMI = 0;
 static unsigned short programCounter;
 static unsigned char btmp;
@@ -101,7 +101,7 @@ static void popProgramCounter(void)
 static void handleIRQ(void)
 {
 	pushProgramCounter();
-	memWrite((unsigned short)(0x100 + stackPointer), (unsigned char)(statusRegister & ~0x10));
+	memWrite((unsigned short)(0x100 + stackPointer), (unsigned char)((statusRegister & ~B) | M));
 	stackPointer--;
 	statusRegister |= I;
 	programCounter = memReadAbsolute(0xFFFE);
@@ -111,7 +111,7 @@ static void handleIRQ(void)
 static void handleNMI(void)
 {
 	pushProgramCounter();
-	memWrite((unsigned short)(0x100 + stackPointer), (unsigned char)(statusRegister & ~0x10));
+	memWrite((unsigned short)(0x100 + stackPointer), (unsigned char)((statusRegister & ~B) | M));
 	stackPointer--;
 	statusRegister |= I;
 	NMI = 0;
@@ -190,7 +190,7 @@ static void Ind(void)
 
 static void IndZeroX(void)
 {
-	ptr = xRegister + memRead(programCounter++);
+	ptr = (xRegister + memRead(programCounter++)) & 0xFF;
 	op = memRead(ptr);
 	op += memRead((unsigned short)((ptr + 1) & 0xFF)) << 8;
 	cycles += 3;
@@ -319,7 +319,7 @@ static void ADC(void)
 
 		tmp = (Op1 & 0x0F) + (Op2 & 0x0F) + (statusRegister & C ? 1 : 0);
 		accumulator = tmp < 0x0A ? tmp : tmp + 6;
-		tmp = (Op1 & 0xF0) + (Op2 & 0xF0) + (tmp & 0xF0);
+		tmp = (Op1 & 0xF0) + (Op2 & 0xF0) + (accumulator & 0xF0);
 		
 		if (tmp & 0x80)
 			statusRegister |= N;
@@ -617,7 +617,7 @@ static void PHA(void)
 
 static void PHP(void)
 {
-	memWrite((unsigned short)(0x100 + stackPointer), statusRegister);
+	memWrite((unsigned short)(0x100 + stackPointer), statusRegister | B | M);
 	stackPointer--;
 	cycles++;
 }
@@ -641,7 +641,7 @@ static void BRK(void)
 {
 	pushProgramCounter();
 	PHP();
-	statusRegister |= B;
+	statusRegister |= I;
 	programCounter = memReadAbsolute(0xFFFE);
 	cycles += 3;
 }