Merge pull request #28 from amw-zero/ark-bvc

Implement BVC instruction

src/machine.rs

@@ -163,6 +163,12 @@ impl Machine {
                 self.branch_if_positive(addr);
             }
 
+            (instruction::BVC, instruction::UseRelative(rel)) => {
+                let addr = self.registers.program_counter
+                         + AddressDiff(rel as i32);
+                self.branch_if_overflow_clear(addr);
+            }
+
             (instruction::CLC, instruction::UseImplied) => {
                 self.registers.status.and(!PS_CARRY);
             }
@@ -595,6 +601,12 @@ impl Machine {
         }
     }
 
+    fn branch_if_overflow_clear(&mut self, addr: Address) {
+        if !self.registers.status.contains(PS_OVERFLOW) {
+            self.registers.program_counter = addr;
+        }
+    }
+
     fn push_on_stack(&mut self, val: u8) {
         let addr = self.registers.stack_pointer.to_address();
         self.memory.set_byte(addr, val);
@@ -981,3 +993,16 @@ fn branch_if_positive_test() {
     machine.branch_if_positive(Address(0xABCD));
     assert_eq!(machine.registers.program_counter, Address(0xABCD));
 }
+
+#[test]
+fn branch_if_overflow_clear_test() {
+    let mut machine = Machine::new();
+
+    machine.registers.status.insert(PS_OVERFLOW);
+    machine.branch_if_overflow_clear(Address(0xABCD));
+    assert_eq!(machine.registers.program_counter, Address(0));
+
+    machine.registers.status.remove(PS_OVERFLOW);
+    machine.branch_if_overflow_clear(Address(0xABCD));
+    assert_eq!(machine.registers.program_counter, Address(0xABCD));
+}