Merge pull request #16 from typelist/jam-bmi

Implement the BMI instruction

src/address.rs

@@ -37,6 +37,8 @@ pub struct Address(pub u16);
 impl Add<AddressDiff, Address> for Address {
     fn add(&self, &AddressDiff(rhs): &AddressDiff) -> Address {
         let &Address(lhs) = self;
+
+        // TODO akeeton: Do a checked cast.
         Address(((lhs as i32) + rhs) as u16)
     }
 }

src/machine.rs

@@ -102,6 +102,13 @@ impl Machine {
                 self.jump(addr)
             },
 
+            (instruction::BMI, instruction::UseRelative(rel)) => {
+                let addr = self.registers.program_counter
+                         + AddressDiff(rel as i32);
+                log!(log::DEBUG, "branch if minus relative. address: {}", addr);
+                self.branch_if_minus(addr);
+            },
+
             (instruction::LDA, instruction::UseImmediate(val)) => {
                 log!(log::DEBUG, "load A immediate: {}", val);
                 self.load_accumulator(val as i8);
@@ -238,6 +245,12 @@ impl Machine {
     pub fn jump(&mut self, addr: Address) {
         self.registers.program_counter = addr;
     }
+
+    pub fn branch_if_minus(&mut self, addr: Address) {
+        if self.registers.status.contains(PS_NEGATIVE) {
+            self.registers.program_counter = addr;
+        }
+    }
 }
 
 impl std::fmt::Show for Machine {
@@ -249,7 +262,6 @@ impl std::fmt::Show for Machine {
 
 #[test]
 fn add_with_carry_test() {
-
     let mut machine = Machine::new();
 
     machine.add_with_carry(1);
@@ -404,3 +416,26 @@ fn jump_test() {
     machine.jump(addr);
     assert_eq!(machine.registers.program_counter, addr);
 }
+
+#[test]
+fn branch_if_minus_test() {
+    {
+        let mut machine      = Machine::new();
+        let registers_before = machine.registers;
+
+        machine.branch_if_minus(Address(0xABCD));
+        assert_eq!(machine.registers, registers_before);
+        assert_eq!(machine.registers.program_counter, Address(0));
+    }
+
+    {
+        let mut machine = Machine::new();
+
+        machine.registers.status.set_with_mask(PS_NEGATIVE, PS_NEGATIVE);
+        let registers_before = machine.registers;
+
+        machine.branch_if_minus(Address(0xABCD));
+        assert_eq!(machine.registers.status, registers_before.status);
+        assert_eq!(machine.registers.program_counter, Address(0xABCD));
+    }
+}

src/registers.rs

@@ -54,6 +54,7 @@ impl StatusArgs {
 }
 
 pub bitflags! {
+#[deriving(Show)]
     flags Status: u8 {
         const PS_NEGATIVE           = 0b10000000,
         const PS_OVERFLOW           = 0b01000000,
@@ -112,7 +113,7 @@ impl Status {
     }
 }
 
-#[deriving(PartialEq, Eq, PartialOrd, Ord)]
+#[deriving(PartialEq, Eq, PartialOrd, Ord, Show)]
 pub struct StackPointer(pub u8);
 
 impl StackPointer {
@@ -122,6 +123,7 @@ impl StackPointer {
     }
 }
 
+#[deriving(PartialEq, Eq, Show)]
 pub struct Registers {
     pub accumulator:     i8,
     pub index_x:         i8,