emu: Factor out hi/lo register access

src/emu/dos.rs

@@ -1,9 +1,9 @@
 use emu::pc::Bus;
-use emu::i8088::i8088;
+use emu::i8088::{i8088, read_hi};
 
 pub fn interrupt(cpu: &mut i8088, _bus: &mut Bus) {
-  let svc = cpu.a;
+  let svc = read_hi(cpu.a);
   match svc {
-    _ => unimplemented!("dos service: {:#02X}\ncpu: {:#X?}", svc, cpu)
+    _ => unimplemented!("dos service: AH={:02X}h\ncpu: {:#X?}", svc, cpu)
   }
 }

src/emu/i8088.rs

@@ -120,18 +120,13 @@ struct RegHi<'a> {
 
 impl<'a> LValue<u8> for RegHi<'a> {
   fn write(&mut self, val: u8) {
-    let mut buf = [0; 2];
-    LittleEndian::write_u16(&mut buf, *self.reg);
-    buf[1] = val;
-    *self.reg = LittleEndian::read_u16(&buf);
+    write_hi(&mut self.reg, val);
   }
 }
 
 impl<'a> RValue<u8> for RegHi<'a> {
   fn read(&self) -> u8 {
-    let mut buf = [0; 2];
-    LittleEndian::write_u16(&mut buf, *self.reg);
-    buf[1] as u8
+    read_hi(*self.reg)
   }
 }
 
@@ -220,6 +215,32 @@ pub const fn segoff_to_addr(segment: u16, offset: u16) -> usize {
   segaddr + offset as usize
 }
 
+pub fn read_hi(val: u16) -> u8 {
+  let mut buf = [0; 2];
+  LittleEndian::write_u16(&mut buf, val);
+  buf[1] as u8
+}
+
+pub fn read_lo(val: u16) -> u8 {
+  let mut buf = [0; 2];
+  LittleEndian::write_u16(&mut buf, val);
+  buf[0] as u8
+}
+
+pub fn write_hi(reg: &mut u16, val: u8) {
+  let mut buf = [0; 2];
+  LittleEndian::write_u16(&mut buf, *reg);
+  buf[1] = val;
+  *reg = LittleEndian::read_u16(&buf);
+}
+
+pub fn write_lo(reg: &mut u16, val: u8) {
+  let mut buf = [0; 2];
+  LittleEndian::write_u16(&mut buf, *reg);
+  buf[1] = val;
+  *reg = LittleEndian::read_u16(&buf);
+}
+
 impl i8088 {
   pub fn run(&mut self, bus: &mut Bus) {
     loop {