diff --git a/cpu/src/lib.rs b/cpu/src/lib.rs
index 01a4670..6f69701 100644
--- a/cpu/src/lib.rs
+++ b/cpu/src/lib.rs
@@ -503,11 +503,8 @@ where
         // The address, converted to a multiple of 4.
         let write_addr_index = index_to_word(write_addr);
 
-        // The array index of the word for the byte to write to
-        let index_of_write = index_of_byte(write_addr.into());
-
         // The Word to write, with one byte overwritten to the read byte
-        let cell_to_write = Word::zero_extend_byte(cell_byte, index_of_write);
+        let cell_to_write = Word::zero_extend_byte(cell_byte);
 
         state
             .mem_mut()
@@ -562,11 +559,8 @@ where
         // The address, converted to a multiple of 4.
         let write_addr_index = index_to_word(write_addr);
 
-        // The array index of the word for the byte to write to
-        let index_of_write = index_of_byte(write_addr.into());
-
         // The Word to write, with one byte overwritten to the read byte
-        let cell_to_write = Word::sign_extend_byte(cell_byte, index_of_write);
+        let cell_to_write = Word::sign_extend_byte(cell_byte);
 
         state
             .mem_mut()
diff --git a/machine/src/core.rs b/machine/src/core.rs
index 2bdd914..f507c4f 100644
--- a/machine/src/core.rs
+++ b/machine/src/core.rs
@@ -28,19 +28,18 @@ impl Word<u8> {
 }
 
 impl Word<u8> {
-    //TODO if the byte isn't the lowest byte then this doesn't make sense?
-    pub fn sign_extend_byte(byte: u8, loc: usize) -> Self {
+    pub fn sign_extend_byte(byte: u8) -> Self {
         let sign = byte as i8 >> 7;
         let mut result: [u8; MEMORY_CELL_BYTES] = [sign as u8; MEMORY_CELL_BYTES];
-        result[loc] = byte;
+        result[3] = byte;
         Self(result)
     }
 }
 
 impl Word<u8> {
-    pub fn zero_extend_byte(byte: u8, loc: usize) -> Self {
+    pub fn zero_extend_byte(byte: u8) -> Self {
         let mut result: [u8; MEMORY_CELL_BYTES] = [0; MEMORY_CELL_BYTES];
-        result[loc] = byte;
+        result[3] = byte;
         Self(result)
     }
 }