emu: dos exit service.

Just calls process::exit() for now

src/emu/dos.rs

@@ -1,9 +1,23 @@
 use emu::pc::Bus;
-use emu::i8088::{i8088, read_hi};
+use emu::i8088::{i8088, read_hi, read_lo, segoff_to_addr};
 
-pub fn interrupt(cpu: &mut i8088, _bus: &mut Bus) {
+pub fn interrupt(cpu: &mut i8088, bus: &mut Bus) {
   let svc = read_hi(cpu.a);
   match svc {
+    0x09 => print_string(cpu, bus),
+    0x4C => exit(read_lo(cpu.a)),
     _ => unimplemented!("dos service: AH={:02X}h\ncpu: {:#X?}", svc, cpu)
   }
 }
+
+fn print_string(cpu: &i8088, bus: &Bus) {
+  let addr = segoff_to_addr(cpu.ds, cpu.d);
+  for byte in bus.ram[addr..].iter().take_while(|byte| **byte as char != '$') {
+    print!("{}", *byte as char);
+  }
+}
+
+fn exit(code: u8) {
+  println!("");
+  std::process::exit(code as i32);
+}