@@ -27,7 +27,7 @@ typedef struct {
     PyObject * schedule_hook;                   /* the schedule callback function */
     slp_schedule_hook_func * schedule_fasthook; /* the fast C-only schedule_hook */
     PyThreadState * initial_tstate;             /* recording the main thread state */
-    int enable_softswitch;                      /* the flag which decides whether we try to use soft switching */
+    uint8_t enable_softswitch;                  /* the flag which decides whether we try to use soft switching */
 } PyStacklessInterpreterState;
 
 #define SPL_INTERPRETERSTATE_NEW(interp)       \
@@ -94,12 +94,10 @@ typedef struct _sts {
     PyObject *unwinding_retval;                 /* The return value during stack unwinding */
     Py_ssize_t frame_refcnt;                    /* The number of owned references to frames */
     int runcount;
-    /* trap recursive scheduling via callbacks */
-    int schedlock;
-    int runflags;                               /* flags for stackless.run() behaviour */
-    /* number of nested interpreters (1.0/2.0 merge) */
-    int nesting_level;
+    int nesting_level;                          /* number of nested interpreters */
     int switch_trap;                            /* if non-zero, switching is forbidden */
+    uint8_t schedlock;                          /* trap recursive scheduling via callbacks */
+    uint8_t runflags;                           /* flags for stackless.run() behaviour */
 #ifdef SLP_WITH_FRAME_REF_DEBUG
     struct _frame *next_frame;                  /* a ref counted copy of PyThreadState.frame */
 #endif
@@ -108,7 +106,7 @@ typedef struct _sts {
 #ifdef Py_BUILD_CORE
 
 /* internal macro to temporarily disable soft interrupts */
-#define PY_WATCHDOG_NO_SOFT_IRQ (1<<31)
+#define PY_WATCHDOG_NO_SOFT_IRQ (1U<<7)
 
 /* these macros go into pystate.c */
 #ifdef SLP_WITH_FRAME_REF_DEBUG
@@ -137,10 +135,10 @@ typedef struct _sts {
     tstate->st.unwinding_retval = NULL; \
     tstate->st.frame_refcnt = 0; \
     tstate->st.runcount = 0; \
-    tstate->st.schedlock = 0; \
     tstate->st.nesting_level = 0; \
-    tstate->st.runflags = 0; \
     tstate->st.switch_trap = 0; \
+    tstate->st.schedlock = 0; \
+    tstate->st.runflags = 0; \
     __STACKLESS_PYSTATE_NEW_NEXT_FRAME
 
 
 
@@ -500,7 +500,7 @@ generic_channel_cando(PyThreadState *ts, PyObject **result, PyChannelObject *sel
     PyTaskletObject *source = ts->st.current;
     PyTaskletObject *switchto, *target, *next;
     int interthread;
-    int oldflags, runflags = 0;
+    uint8_t oldflags, runflags = 0;
     int switched, fail;
 
     /* swap data and perform necessary scheduling */
 
@@ -1032,7 +1032,7 @@ slp_schedule_task_prepared(PyThreadState *ts, PyObject **result, PyTaskletObject
     int transfer_result;
 
     /* remove the no-soft-irq flag from the runflags */
-    int no_soft_irq = ts->st.runflags & PY_WATCHDOG_NO_SOFT_IRQ;
+    uint8_t no_soft_irq = ts->st.runflags & PY_WATCHDOG_NO_SOFT_IRQ;
     ts->st.runflags &= ~PY_WATCHDOG_NO_SOFT_IRQ;
 
     slp_schedule_soft_irq(ts, prev, &next, no_soft_irq);
 
@@ -372,7 +372,7 @@ enable_softswitch(PyObject *self, PyObject *flag)
     if (newflag == -1 && PyErr_Occurred())
         return NULL;
     ret = PyBool_FromLong(ts->interp->st.enable_softswitch);
-    ts->interp->st.enable_softswitch = newflag;
+    ts->interp->st.enable_softswitch = !!newflag;
     return ret;
 }
 
@@ -484,6 +484,11 @@ PyStackless_RunWatchdogEx(long timeout, int flags)
     long old_watermark = 0, old_interval = 0;
     int interrupt;
 
+    if (flags < 0 || flags >= (1U << (sizeof(ts->st.runflags) * CHAR_BIT))) {
+        PyErr_SetString(PyExc_ValueError, "flags must be in [0..255]");
+        return NULL;
+    }
+
     if (ts->st.main == NULL)
         return PyStackless_RunWatchdog_M(timeout, flags);