diff --git a/ceres-asm/src/lib.rs b/ceres-asm/src/lib.rs
index 6ca2483..e80a028 100644
--- a/ceres-asm/src/lib.rs
+++ b/ceres-asm/src/lib.rs
@@ -47,6 +47,7 @@ impl<'a> Assembler<'a> {
                 // cases that like, actually make sense to process
                 Token::Load => machine_code.push(self.load()?),
                 Token::Add => machine_code.push(self.add()?),
+                Token::Sub => machine_code.push(self.sub()?),
                 // cases that should straight up not happen
                 _ => return Err(CeresAsmError::LazyBadToken { token }),
             }
@@ -91,6 +92,20 @@ impl<'a> Assembler<'a> {
 
         Ok((opcode << 27) | (source_one << 8) | (source_two << 4) | dest)
     }
+
+    // sub instruction assembly
+    fn sub(&mut self) -> Result<u32> {
+        let opcode = 0b00011u32;
+
+        let token = self.next()?;
+        let source_one = token.register_index()? as u32;
+        let token = self.next()?;
+        let source_two = token.register_index()? as u32;
+        let token = self.next()?;
+        let dest = token.register_index()? as u32;
+
+        Ok((opcode << 27) | (source_one << 8) | (source_two << 4) | dest)
+    }
 }
 
 /// assembled machine code
@@ -196,6 +211,8 @@ pub enum Token {
     Load,
     #[token("add")]
     Add,
+    #[token("sub")]
+    Sub,
 
     // logos error
     #[error]
@@ -233,6 +250,7 @@ impl Token {
             // instructions
             Self::Load => "ld:".to_owned(),
             Self::Add => "add".to_owned(),
+            Self::Sub => "sub".to_owned(),
             // errors
             Self::Error => "ERR".to_owned(),
         }