diff --git a/src/cpu/x86_ops_inc_dec.h b/src/cpu/x86_ops_inc_dec.h
index b33d02f45..8b9711aaa 100644
--- a/src/cpu/x86_ops_inc_dec.h
+++ b/src/cpu/x86_ops_inc_dec.h
@@ -56,16 +56,23 @@ opINCDEC_b_a16(uint32_t fetchdat)
     if (cpu_state.abrt)
         return 1;
 
-    if (rmdat & 0x38) {
-        seteab(temp - 1);
-        if (cpu_state.abrt)
-            return 1;
-        setsub8nc(temp, 1);
-    } else {
-        seteab(temp + 1);
-        if (cpu_state.abrt)
-            return 1;
-        setadd8nc(temp, 1);
+    switch (rmdat & 0x38) {
+        case 0: /* INC r/m8 */
+            seteab(temp + 1);
+            if (cpu_state.abrt)
+                return 1;
+            setadd8nc(temp, 1);
+            break;
+        case 0x8: /* DEC r/m8 */
+            seteab(temp - 1);
+            if (cpu_state.abrt)
+                return 1;
+            setsub8nc(temp, 1);
+            break;
+        default:
+            cpu_state.pc = cpu_state.oldpc;
+            x86illegal();
+            break;
     }
     CLOCK_CYCLES((cpu_mod == 3) ? timing_rr : timing_mm);
     PREFETCH_RUN((cpu_mod == 3) ? timing_rr : timing_mm, 2, rmdat, (cpu_mod == 3) ? 0 : 1, 0, (cpu_mod == 3) ? 0 : 1, 0, 0);
@@ -83,16 +90,23 @@ opINCDEC_b_a32(uint32_t fetchdat)
     if (cpu_state.abrt)
         return 1;
 
-    if (rmdat & 0x38) {
-        seteab(temp - 1);
-        if (cpu_state.abrt)
-            return 1;
-        setsub8nc(temp, 1);
-    } else {
-        seteab(temp + 1);
-        if (cpu_state.abrt)
-            return 1;
-        setadd8nc(temp, 1);
+    switch (rmdat & 0x38) {
+        case 0: /* INC r/m8 */
+            seteab(temp + 1);
+            if (cpu_state.abrt)
+                return 1;
+            setadd8nc(temp, 1);
+            break;
+        case 0x8: /* DEC r/m8 */
+            seteab(temp - 1);
+            if (cpu_state.abrt)
+                return 1;
+            setsub8nc(temp, 1);
+            break;
+        default:
+            cpu_state.pc = cpu_state.oldpc;
+            x86illegal();
+            break;
     }
     CLOCK_CYCLES((cpu_mod == 3) ? timing_rr : timing_mm);
     PREFETCH_RUN((cpu_mod == 3) ? timing_rr : timing_mm, 2, rmdat, (cpu_mod == 3) ? 0 : 1, 0, (cpu_mod == 3) ? 0 : 1, 0, 1);