Overflow should simply be bit 6's value

src/mos6502.bits.c

@@ -51,7 +51,19 @@ DEFINE_INST(asl)
  */
 DEFINE_INST(bit)
 {
-    mos6502_modify_status(cpu, MOS_NVZ, oper, oper);
+    // We're just relying on the modify_status function to do negative
+    // and zero; we also need to do overflow, but overflow is checked in
+    // a slightly different way with BIT...
+    mos6502_modify_status(cpu, MOS_NZ, oper, oper);
+
+    // Normally, overflow is handled by checking if bit 7 flipped from 0
+    // to 1 or vice versa, and that's done by comparing the result to
+    // the operand. But in the case of BIT, all we want to know is if
+    // bit 6 is high.
+    cpu->P &= ~MOS_OVERFLOW;
+    if (oper & 0x40) {
+        cpu->P |= MOS_OVERFLOW;
+    }
 }
 
 /*