Make Address take i32 so relative addresses work.

src/address.rs

@@ -29,7 +29,7 @@
 // does make sense to add an address and an "address-difference". (If this
 // is too annoying to work with we should let it go.)
 #[deriving(PartialEq, Eq, PartialOrd, Ord, Show)]
-pub struct AddressDiff(pub u16);
+pub struct AddressDiff(pub i32);
 
 #[deriving(PartialEq, Eq, PartialOrd, Ord, Show)]
 pub struct Address(pub u16);
@@ -37,7 +37,7 @@ pub struct Address(pub u16);
 impl Add<AddressDiff, Address> for Address {
     fn add(&self, &AddressDiff(rhs): &AddressDiff) -> Address {
         let &Address(lhs) = self;
-        Address(lhs + rhs)
+        Address(((lhs as i32) + rhs) as u16)
     }
 }
 

src/instruction.rs

@@ -216,12 +216,12 @@ impl AddressingMode {
             AbsoluteX => {
                 // Use [u8, ..2] from instruction as address, add X
                 // (Output: a 16-bit address)
-                UseAddress(arr_to_addr(arr) + AddressDiff(x as u16))
+                UseAddress(arr_to_addr(arr) + AddressDiff(x as i32))
             },
             AbsoluteY => {
                 // Use [u8, ..2] from instruction as address, add Y
                 // (Output: a 16-bit address)
-                UseAddress(arr_to_addr(arr) + AddressDiff(y as u16))
+                UseAddress(arr_to_addr(arr) + AddressDiff(y as i32))
             },
             Indirect => {
                 // Use [u8, ..2] from instruction as an address. Interpret the
@@ -248,7 +248,7 @@ impl AddressingMode {
                 let start = arr[0];
                 let slice = memory.get_slice(Address(start as u16),
                                              AddressDiff(2));
-                UseAddress(arr_to_addr(slice) + AddressDiff(y as u16))
+                UseAddress(arr_to_addr(slice) + AddressDiff(y as i32))
             },
         }
     }

src/machine.rs

@@ -29,7 +29,7 @@ use log;
 
 use std;
 
-use address::{AddressDiff};
+use address::{Address, AddressDiff};
 use instruction;
 use instruction::{DecodedInstr};
 use memory::Memory;

src/registers.rs

@@ -118,7 +118,7 @@ pub struct StackPointer(pub u8);
 impl StackPointer {
     pub fn to_address(&StackPointer(sp): &StackPointer) -> Address
     {
-        STACK_ADDRESS_LO + AddressDiff(sp as u16)
+        STACK_ADDRESS_LO + AddressDiff(sp as i32)
     }
 }