fix neg's sign and overflow flags

runtime/src/ops/math.rs

@@ -66,9 +66,13 @@ pub fn or<I: Int>(x: I, y: I, flags: &mut Flags) -> I {
 
 /// neg: Two's Complement Negation
 pub fn neg<I: Int>(x: I, flags: &mut Flags) -> I {
-    let (result, of) = I::zero().overflowing_sub(&x);
+    let (result, _) = I::zero().overflowing_sub(&x);
     flags.set(Flags::ZF, result.is_zero());
+    flags.set(Flags::SF, result.high_bit().is_one());
     flags.set(Flags::CF, !result.is_zero());
+    // Only the most negative value overflows: it is the one input whose
+    // negation is still negative.
+    let of = x.high_bit().is_one() && result.high_bit().is_one();
     flags.set(Flags::OF, of);
     flags.set(Flags::PF, result.low_byte().count_ones() % 2 == 0);
     result
@@ -180,6 +184,31 @@ mod tests {
         (result, flags)
     }
 
+    fn neg8(x: u8) -> (u8, Flags) {
+        let mut flags = Flags::default();
+        let result = neg(x, &mut flags);
+        (result, flags)
+    }
+
+    #[test]
+    fn neg_flags() {
+        let (result, flags) = neg8(0x00);
+        assert_eq!(result, 0x00);
+        assert_eq!(flags.to_string(), "PF ZF");
+
+        let (result, flags) = neg8(0x01);
+        assert_eq!(result, 0xff);
+        assert_eq!(flags.to_string(), "CF PF SF");
+
+        let (result, flags) = neg8(0xff);
+        assert_eq!(result, 0x01);
+        assert_eq!(flags.to_string(), "CF");
+
+        let (result, flags) = neg8(0x80);
+        assert_eq!(result, 0x80);
+        assert_eq!(flags.to_string(), "CF SF OF");
+    }
+
     #[test]
     fn sbb_edge_cases() {
         let (result, flags) = sbb8(0x00, 0x7f, true);