diff --git a/cores/esp8266/debug.cpp b/cores/esp8266/debug.cpp
index d8bf8bb11d..eff43dfdba 100644
--- a/cores/esp8266/debug.cpp
+++ b/cores/esp8266/debug.cpp
@@ -23,14 +23,23 @@
 
 void ICACHE_RAM_ATTR hexdump(const void *mem, uint32_t len, uint8_t cols) {
     const uint8_t* src = (const uint8_t*) mem;
-    os_printf("\n[HEXDUMP] Address: 0x%08X len: 0x%X (%d)", (ptrdiff_t)src, len, len);
+    ::printf((PGM_P)F("\n[HEXDUMP] Address: 0x%08X len: 0x%X (%d)"), (ptrdiff_t)src, len, len);
     for(uint32_t i = 0; i < len; i++) {
         if(i % cols == 0) {
-            os_printf("\n[0x%08X] 0x%08X: ", (ptrdiff_t)src, i);
+            ::printf((PGM_P)F("\n[0x%08X] 0x%08X: "), (ptrdiff_t)src, i);
         	yield();
         }
-        os_printf("%02X ", *src);
+        ::printf((PGM_P)F("%02X "), *src);
         src++;
     }
     os_printf("\n");
 }
+
+#ifndef NDEBUG
+void assert_iram (uintptr_t address) {
+    if (address < 0x40100000 || address >= 0x40110000) {
+        ::printf((PGM_P)F("all ISR must be in IRAM with ICACHE_RAM_ATTR\r\n"));
+        panic();
+    }
+}
+#endif
diff --git a/cores/esp8266/debug.h b/cores/esp8266/debug.h
index fab128253c..abac756832 100644
--- a/cores/esp8266/debug.h
+++ b/cores/esp8266/debug.h
@@ -29,5 +29,11 @@ void __panic_func(const char* file, int line, const char* func) __attribute__((n
 }
 #endif
 
+#ifdef NDEBUG
+void assert_iram (uintptr_t iram_function_addr) { }
+#else
+void assert_iram (uintptr_t iram_function_addr);
+#endif
+#define check_function_is_in_iram(f) do { assert_iram((intptr_t)f); } while (0)
 
 #endif//ARD_DEBUG_H
diff --git a/libraries/SPISlave/src/SPISlave.cpp b/libraries/SPISlave/src/SPISlave.cpp
index a88915b518..e621cba5d5 100644
--- a/libraries/SPISlave/src/SPISlave.cpp
+++ b/libraries/SPISlave/src/SPISlave.cpp
@@ -93,18 +93,22 @@ void SPISlaveClass::setStatus(uint32_t status)
 }
 void SPISlaveClass::onData(SpiSlaveDataHandler cb)
 {
+    check_function_is_in_iram(cb);
     _data_cb = cb;
 }
 void SPISlaveClass::onDataSent(SpiSlaveSentHandler cb)
 {
+    check_function_is_in_iram(cb);
     _data_sent_cb = cb;
 }
 void SPISlaveClass::onStatus(SpiSlaveStatusHandler cb)
 {
+    check_function_is_in_iram(cb);
     _status_cb = cb;
 }
 void SPISlaveClass::onStatusSent(SpiSlaveSentHandler cb)
 {
+    check_function_is_in_iram(cb);
     _status_sent_cb = cb;
 }