Integrate AddressDiff.

6502emu/src/address.rs

@@ -28,6 +28,30 @@
 #[deriving(PartialEq, Eq, PartialOrd, Ord)]
 pub struct Address(pub u16);
 
+#[deriving(PartialEq, Eq, PartialOrd, Ord)]
+pub struct AddressDiff(pub u16);
+
+// The idea here is that it doesn't make sense to add two addresses, but it
+// does make sense to add an address and an "address-difference". (If this
+// is too annoying to work with we should let it go.)
+
+impl Add<AddressDiff, Address> for Address {
+    fn add(&self, &AddressDiff(rhs): &AddressDiff) -> Address {
+        let &Address(lhs) = self;
+
+        // We probably don't want to overflow when doing arithmetic in our own
+        // code.
+        debug_assert!({
+            match lhs.checked_add(&rhs) {
+                None => false,
+                _ => true
+            }
+        });
+
+        return Address(lhs + rhs);
+    }
+}
+
 impl Address {
 	pub fn to_u16(&self) -> u16 {
 		match *self {

6502emu/src/machine.rs

@@ -47,7 +47,6 @@ impl Machine {
 	}
 
 	// TODO akeeton: Implement binary-coded decimal.
-	// TODO akeeton: Add test.
 	pub fn add_with_carry(&mut self, value: i8) {
 		let a_before: i8 = self.registers.accumulator;
 		let c_before: u8 = self.registers.status.carry.to_bit();

6502emu/src/memory.rs

@@ -26,6 +26,8 @@
 // POSSIBILITY OF SUCH DAMAGE.
 
 use address::Address;
+use address::AddressDiff;
+use registers::StackPointer;
 
 pub static MEMORY_ADDRESS_BEGIN: Address = Address(0x0000);
 pub static MEMORY_ADDRESS_END:   Address = Address(0xffff);
@@ -57,7 +59,12 @@ impl Memory {
 		return old_value;
 	}
 
-	fn is_stack_address(address: &Address) -> bool {
+	pub fn is_stack_address(address: &Address) -> bool {
 		STACK_ADDRESS_BEGIN <= *address && *address <= STACK_ADDRESS_END
 	}
+
+	pub fn stack_pointer_to_address(&StackPointer(sp): &StackPointer) -> Address
+	{
+		STACK_ADDRESS_BEGIN + AddressDiff(sp as u16)
+	}
 }

6502emu/src/registers.rs

@@ -66,11 +66,14 @@ impl Status {
 	}
 }
 
+#[deriving(PartialEq, Eq, PartialOrd, Ord)]
+pub struct StackPointer(pub u8);
+
 pub struct Registers {
 	pub accumulator:     i8,
 	pub index_x:         i8,
 	pub index_y:         i8,
-	pub stack_pointer:   u8,
+	pub stack_pointer:   StackPointer,
 	pub program_counter: u16,
 	pub status:          Status
 }
@@ -82,7 +85,7 @@ impl Registers {
 			accumulator:     0,
 			index_x:         0,
 			index_y:         0,
-			stack_pointer:   STACK_ADDRESS_END.get_offset(),
+			stack_pointer:   StackPointer(STACK_ADDRESS_END.get_offset()),
 			program_counter: 0,
 			status:          Status::new()
 		}