diff --git a/src/emu/dos.rs b/src/emu/dos.rs
new file mode 100644
index 0000000..28603f1
--- /dev/null
+++ b/src/emu/dos.rs
@@ -0,0 +1,9 @@
+use emu::pc::Bus;
+use emu::i8088::i8088;
+
+pub fn interrupt(cpu: &mut i8088, _bus: &mut Bus) {
+  let svc = cpu.a;
+  match svc {
+    _ => unimplemented!("dos service: {:#02X}\ncpu: {:#X?}", svc, cpu)
+  }
+}
diff --git a/src/emu/i8088.rs b/src/emu/i8088.rs
index 353238d..683526d 100644
--- a/src/emu/i8088.rs
+++ b/src/emu/i8088.rs
@@ -50,8 +50,9 @@ pub struct Flags {
 }
 
 mod ops {
-  use emu::i8088::{LValue, RValue, segoff_to_addr};
   use emu::byteorder::{ByteOrder, LittleEndian};
+  use emu::dos;
+  use emu::i8088::{Bus, LValue, RValue, i8088, segoff_to_addr};
 
   pub fn call(ip: &mut u16, ss: u16, sp: &mut u16, mem: &mut [u8], addr: i16) {
     let target = ip.wrapping_add(addr as u16);
@@ -59,6 +60,13 @@ mod ops {
     *ip = target;
   }
 
+  pub fn int(cpu: &mut i8088, bus: &mut Bus, num: &u8) {
+    match num {
+      0x21 => dos::interrupt(cpu, bus),
+      _ => unimplemented!("interrupt: {:02X}\ncpu: {:#X?}", num, cpu)
+    }
+  }
+
   pub fn mov<T>(dst: &mut impl LValue<T>, src: &impl RValue<T>) {
     dst.write(src.read());
   }
@@ -130,14 +138,14 @@ impl<'a> RValue<u8> for RegHi<'a> {
 macro_rules! step {
   // Base case: all args processed and ready to call op
   (@code ( $($args:tt)* ),
-         $cpu:ident, $bus:ident, $name:ident, $cycles:literal,
+         $cpu:expr, $bus:expr, $name:ident, $cycles:literal,
          ()) => {
     ops::$name($($args),*);
   };
 
   // Inductive case: decode next arg to be placed in list
   (@code ( $($args:tt)* ),
-         $cpu:ident, $bus:ident, $name:ident, $cycles:literal,
+         $cpu:expr, $bus:expr, $name:ident, $cycles:literal,
          ($next:ident $(= $nextrhs:ident)? $($rest:ident $(= $restrhs:ident)?)*)) => {
     step!(@$next$(= $nextrhs)?
           ( ($($args)*), $cpu, $bus, $name, $cycles, ($($rest $(= $restrhs)?)*) ),
@@ -145,41 +153,50 @@ macro_rules! step {
   };
 
   // accept an argument from a decoder and recur to look for next arg
-  (@arg ( ($($args:tt)*), $cpu:ident, $bus:ident, $name:ident, $cycles:literal, ( $($rest:tt)* ) ),
+  (@arg ( ($($args:tt)*), $cpu:expr, $bus:expr, $name:ident, $cycles:literal, ( $($rest:tt)* ) ),
         $arg:expr) => {
     step!(@code ($($args)* $arg), $cpu, $bus, $name, $cycles, ( $($rest)* ))
   };
 
   // Argument Decoders
 
-  (@d8 $cookie:tt, $cpu:ident, $bus:ident) => { {
-    step!(@arg $cookie, &i8088::next_ip($cpu.cs, &mut $cpu.ip, $bus))
+  (@bus $cookie:tt, $cpu:expr, $bus:expr) => { {
+    step!(@arg $cookie, $bus)
   } };
 
-  (@d16 $cookie:tt, $cpu:ident, $bus:ident) => { {
+  (@cpu $cookie:tt, $cpu:expr, $bus:expr) => { {
+    step!(@arg $cookie, $cpu)
+  } };
+
+  (@d8 $cookie:tt, $cpu:expr, $bus:expr) => { {
+    let d8 = i8088::next_ip($cpu.cs, &mut $cpu.ip, $bus);
+    step!(@arg $cookie, &d8)
+  } };
+
+  (@d16 $cookie:tt, $cpu:expr, $bus:expr) => { {
     let mut buf = [0; 2];
     buf[0] = i8088::next_ip($cpu.cs, &mut $cpu.ip, $bus);
     buf[1] = i8088::next_ip($cpu.cs, &mut $cpu.ip, $bus);
     step!(@arg $cookie, &LittleEndian::read_u16(&buf))
   } };
 
-  (@mem $cookie:tt, $cpu:ident, $bus:ident) => {
+  (@mem $cookie:tt, $cpu:expr, $bus:expr) => {
     step!(@arg $cookie, &mut $bus.ram)
   };
 
-  (@reg=$reg:ident $cookie:tt, $cpu:ident, $bus:ident) => {
+  (@reg=$reg:ident $cookie:tt, $cpu:expr, $bus:expr) => {
     step!(@arg $cookie, &mut $cpu.$reg);
   };
 
-  (@reghi=$reg:ident $cookie:tt, $cpu:ident, $bus:ident) => {
+  (@reghi=$reg:ident $cookie:tt, $cpu:expr, $bus:expr) => {
     step!(@arg $cookie, &mut RegHi { reg: &mut $cpu.$reg });
   };
 
-  (@regval=$reg:ident $cookie:tt, $cpu:ident, $bus:ident) => {
+  (@regval=$reg:ident $cookie:tt, $cpu:expr, $bus:expr) => {
     step!(@arg $cookie, $cpu.$reg);
   };
 
-  (@rel16 $cookie:tt, $cpu:ident, $bus:ident) => { {
+  (@rel16 $cookie:tt, $cpu:expr, $bus:expr) => { {
     let mut buf = [0; 2];
     buf[0] = i8088::next_ip($cpu.cs, &mut $cpu.ip, $bus);
     buf[1] = i8088::next_ip($cpu.cs, &mut $cpu.ip, $bus);
@@ -187,7 +204,7 @@ macro_rules! step {
   } };
 
   // Entry Point
-  (($cpu:ident, $bus:ident) => { $( $code:literal => $name:ident[$($args:ident $(= $argrhs:ident)?),*] / $cycles:literal ),*$(,)? } )  => {
+  (($cpu:expr, $bus:expr) => { $( $code:literal => $name:ident[$($args:ident $(= $argrhs:ident)?),*] / $cycles:literal ),*$(,)? } )  => {
     {
       let opcode = i8088::next_ip($cpu.cs, &mut $cpu.ip, $bus);
       match(opcode) {
@@ -210,6 +227,7 @@ impl i8088 {
         0xB4 => mov[reghi=a, d8] / 4,
         0xBA => mov[reg=d, d16] / 4,
         0xC3 => ret[reg=ip, regval=ss, reg=sp, mem] / 20,
+        0xCD => int[cpu, bus, d8] / 71,
         0xE8 => call[reg=ip, regval=ss, reg=sp, mem, rel16] / 23,
       })
     }
diff --git a/src/emu/mod.rs b/src/emu/mod.rs
index d9a9281..66dbcf2 100644
--- a/src/emu/mod.rs
+++ b/src/emu/mod.rs
@@ -1,4 +1,5 @@
 extern crate byteorder;
 
-pub mod pc;
+pub mod dos;
 pub mod i8088;
+pub mod pc;