improve branching cycle accuracy

1 files 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