From 4fcc8d48f4dd3f7be37d917826a148c02701d0e3 Mon Sep 17 00:00:00 2001
From: vin <git@vineetk.net>
Date: Wed, 3 Jul 2024 20:40:57 -0400
Subject: [PATCH] improve branching cycle accuracy

---
 cpu.c | 39 +++++++++++++++++++++++----------------
 1 file changed, 23 insertions(+), 16 deletions(-)

diff --git a/cpu.c b/cpu.c
index df9c852..bba0ed5 100644
--- a/cpu.c
+++ b/cpu.c
@@ -1,4 +1,5 @@
 #include <libgen.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -151,6 +152,20 @@ opcode_mem(enum addressing_mode mode)
 	return val;
 }
 
+static void
+branch(uint16_t addr, bool cond)
+{
+	if (!cond)
+		return;
+	cycles++;
+
+	addr += regs.pc;
+	if ((regs.pc + 1) & 0xFF00 != addr & 0xFF00)
+		cycles++;
+
+	regs.pc = addr;
+}
+
 void
 ADC(uint16_t arg)
 {
@@ -201,22 +216,19 @@ ASL(uint16_t mem)
 void
 BCC(uint16_t arg)
 {
-	if (regs.status.carry == 0)
-		regs.pc += arg, cycles++;
+	branch(arg, regs.status.carry == 0);
 }
 
 void
 BCS(uint16_t arg)
 {
-	if (regs.status.carry == 1)
-		regs.pc += arg, cycles++;
+	branch(arg, regs.status.carry == 1);
 }
 
 void
 BEQ(uint16_t arg)
 {
-	if (regs.status.zero == 1)
-		regs.pc += arg, cycles++;
+	branch(arg, regs.status.zero == 1);
 }
 
 void
@@ -232,22 +244,19 @@ BIT(uint16_t arg)
 void
 BMI(uint16_t arg)
 {
-	if (regs.status.negative == 1)
-		regs.pc += arg, cycles++;
+	branch(arg, regs.status.negative == 1);
 }
 
 void
 BNE(uint16_t arg)
 {
-	if (regs.status.zero == 0)
-		regs.pc += arg, cycles++;
+	branch(arg, regs.status.zero == 0);
 }
 
 void
 BPL(uint16_t arg)
 {
-	if (regs.status.negative == 0)
-		regs.pc += arg, cycles++;
+	branch(arg, regs.status.negative == 0);
 }
 
 void
@@ -263,16 +272,14 @@ BVC(uint16_t arg)
 {
 	//regs.status.overflow = (STATUS_TO_INT() & (1 << 6)) == 0;
 	//if (regs.status.overflow == 0)
-	if ((STATUS_TO_INT() & (1 << 6)) == 0)
-		regs.pc += arg, cycles++;
+	branch(arg, (STATUS_TO_INT() & (1 << 6)) == 0);
 }
 
 void
 BVS(uint16_t arg)
 {
 	regs.status.overflow = (STATUS_TO_INT() & (1 << 6)) != 0;
-	if (regs.status.overflow == 1)
-		regs.pc += arg, cycles++;
+	branch(arg, regs.status.overflow == 1);
 }
 
 void