emu: RegHalf dispatcher to re-merge @r8 op directive

Keeps the macro recursion from branching. In effect this just moves a
conditional from @r8's output to the RegHalf wrapper, so this is
usually a wash except with less code generated.

src/emu/i8088.rs

@@ -3,7 +3,7 @@ use std::fmt::Debug;
 
 use super::byteorder::{ByteOrder, LittleEndian};
 
-use emu::operands::{FarPtr, Reg, RegHi, RegLo};
+use emu::operands::{FarPtr, Reg, RegHalf, RegHi, RegLo};
 use emu::operations as ops;
 use emu::flags::Flags;
 use emu::pc::Bus;
@@ -270,27 +270,19 @@ macro_rules! step {
   } };
 
   (@r8 $cookie:tt, $cpu:expr, $bus:expr, $prefix:tt, ($modrm_val:ident, $($modrm_rest:tt),*)) => { {
-    let modrm_shift = $modrm_val >> 3;
     // TODO: Should these also be passed into the macro like the modrm specs?
-    if modrm_shift & 0x4 == 0 {
-      let reg = match modrm_shift & 0x3 {
-        0 => RegLo { reg: &$cpu.a },
-        1 => RegLo { reg: &$cpu.c },
-        2 => RegLo { reg: &$cpu.d },
-        3 => RegLo { reg: &$cpu.b },
-        _ => unreachable!()
-      };
-      step!(@arg $cookie, reg)
-    } else {
-      let reg = match modrm_shift & 0x3 {
-        0 => RegHi { reg: &$cpu.a },
-        1 => RegHi { reg: &$cpu.c },
-        2 => RegHi { reg: &$cpu.d },
-        3 => RegHi { reg: &$cpu.b },
-        _ => unreachable!()
-      };
-      step!(@arg $cookie, reg)
-    }
+    let reg = match $modrm_val >> 3 & 0x7 {
+      0 => RegHalf::Lo(RegLo { reg: &$cpu.a }),
+      1 => RegHalf::Lo(RegLo { reg: &$cpu.c }),
+      2 => RegHalf::Lo(RegLo { reg: &$cpu.d }),
+      3 => RegHalf::Lo(RegLo { reg: &$cpu.b }),
+      4 => RegHalf::Hi(RegHi { reg: &$cpu.a }),
+      5 => RegHalf::Hi(RegHi { reg: &$cpu.c }),
+      6 => RegHalf::Hi(RegHi { reg: &$cpu.d }),
+      7 => RegHalf::Hi(RegHi { reg: &$cpu.b }),
+      _ => unreachable!()
+    };
+    step!(@arg $cookie, reg)
   } };
 
   (@reg=$reg:ident $cookie:tt, $cpu:expr, $bus:expr, $prefix:tt, $modrm:tt) => {

src/emu/operands.rs

@@ -177,6 +177,29 @@ impl RValue<u8> for RegLo<'_> {
   }
 }
 
+pub enum RegHalf<'a> {
+  Hi(RegHi<'a>),
+  Lo(RegLo<'a>)
+}
+
+impl LValue<u8> for RegHalf<'_> {
+  fn write(&mut self, val: u8) {
+    match self {
+      RegHalf::Hi(hi) => hi.write(val),
+      RegHalf::Lo(lo) => lo.write(val)
+    }
+  }
+}
+
+impl RValue<u8> for RegHalf<'_> {
+  fn read(&self) -> u8 {
+    match self {
+      RegHalf::Hi(hi) => hi.read(),
+      RegHalf::Lo(lo) => lo.read()
+    }
+  }
+}
+
 pub enum DynLValue<'a> {
   Reg(Reg<'a>),
   FarPtr { segment: u16, offset: u16 },