diff --git a/cores/esp8266/WString.cpp b/cores/esp8266/WString.cpp
index aee4b1c94..048704074 100644
--- a/cores/esp8266/WString.cpp
+++ b/cores/esp8266/WString.cpp
@@ -185,7 +185,7 @@ unsigned char String::changeBuffer(unsigned int maxStrLen) {
         size_t oldSize = capacity() + 1; // include NULL.
         if (isSSO()) {
             // Copy the SSO buffer into allocated space
-            memmove(newbuffer, sso.buff, sizeof(sso.buff));
+            memmove_P(newbuffer, sso.buff, sizeof(sso.buff));
         }
         if (newSize > oldSize)
         {
@@ -210,7 +210,7 @@ String & String::copy(const char *cstr, unsigned int length) {
         return *this;
     }
     setLen(length);
-    memmove(wbuffer(), cstr, length + 1);
+    memmove_P(wbuffer(), cstr, length + 1);
     return *this;
 }
 
@@ -228,7 +228,7 @@ String & String::copy(const __FlashStringHelper *pstr, unsigned int length) {
 void String::move(String &rhs) {
     if(buffer()) {
         if(capacity() >= rhs.len()) {
-            memmove(wbuffer(), rhs.buffer(), rhs.length() + 1);
+            memmove_P(wbuffer(), rhs.buffer(), rhs.length() + 1);
             setLen(rhs.len());
 	    rhs.invalidate();
             return;
@@ -241,7 +241,7 @@ void String::move(String &rhs) {
     }
     if (rhs.isSSO()) {
         setSSO(true);
-        memmove(sso.buff, rhs.sso.buff, sizeof(sso.buff));
+        memmove_P(sso.buff, rhs.sso.buff, sizeof(sso.buff));
     } else {
         setSSO(false);
         setBuffer(rhs.wbuffer());
@@ -313,7 +313,7 @@ unsigned char String::concat(const String &s) {
             return 1;
         if (!reserve(newlen))
             return 0;
-        memmove(wbuffer() + len(), buffer(), len());
+        memmove_P(wbuffer() + len(), buffer(), len());
         setLen(newlen);
         wbuffer()[len()] = 0;
         return 1;
@@ -330,12 +330,7 @@ unsigned char String::concat(const char *cstr, unsigned int length) {
         return 1;
     if(!reserve(newlen))
         return 0;
-    if (cstr >= wbuffer() && cstr < wbuffer() + len())
-        // compatible with SSO in ram #6155 (case "x += x.c_str()")
-        memmove(wbuffer() + len(), cstr, length + 1);
-    else
-        // compatible with source in flash #6367
-        memcpy_P(wbuffer() + len(), cstr, length + 1);
+    memmove_P(wbuffer() + len(), cstr, length + 1);
     setLen(newlen);
     return 1;
 }
@@ -739,21 +734,21 @@ void String::replace(const String& find, const String& replace) {
     char *foundAt;
     if(diff == 0) {
         while((foundAt = strstr(readFrom, find.buffer())) != NULL) {
-            memmove(foundAt, replace.buffer(), replace.len());
+            memmove_P(foundAt, replace.buffer(), replace.len());
             readFrom = foundAt + replace.len();
         }
     } else if(diff < 0) {
         char *writeTo = wbuffer();
         while((foundAt = strstr(readFrom, find.buffer())) != NULL) {
             unsigned int n = foundAt - readFrom;
-            memmove(writeTo, readFrom, n);
+            memmove_P(writeTo, readFrom, n);
             writeTo += n;
-            memmove(writeTo, replace.buffer(), replace.len());
+            memmove_P(writeTo, replace.buffer(), replace.len());
             writeTo += replace.len();
             readFrom = foundAt + find.len();
             setLen(len() + diff);
         }
-        memmove(writeTo, readFrom, strlen(readFrom)+1);
+        memmove_P(writeTo, readFrom, strlen(readFrom)+1);
     } else {
         unsigned int size = len(); // compute size needed for result
         while((foundAt = strstr(readFrom, find.buffer())) != NULL) {
@@ -767,9 +762,9 @@ void String::replace(const String& find, const String& replace) {
         int index = len() - 1;
         while(index >= 0 && (index = lastIndexOf(find, index)) >= 0) {
             readFrom = wbuffer() + index + find.len();
-            memmove(readFrom + diff, readFrom, len() - (readFrom - buffer()));
+            memmove_P(readFrom + diff, readFrom, len() - (readFrom - buffer()));
 	    int newLen = len() + diff;
-            memmove(wbuffer() + index, replace.buffer(), replace.len());
+            memmove_P(wbuffer() + index, replace.buffer(), replace.len());
             setLen(newLen);
             wbuffer()[newLen] = 0;
             index--;
@@ -797,7 +792,7 @@ void String::remove(unsigned int index, unsigned int count) {
     char *writeTo = wbuffer() + index;
     unsigned int newlen = len() - count;
     setLen(newlen);
-    memmove(writeTo, wbuffer() + index + count, newlen - index);
+    memmove_P(writeTo, wbuffer() + index + count, newlen - index);
     wbuffer()[newlen] = 0;
 }
 
@@ -829,7 +824,7 @@ void String::trim(void) {
     unsigned int newlen = end + 1 - begin;
     setLen(newlen);
     if(begin > buffer())
-        memmove(wbuffer(), begin, newlen);
+        memmove_P(wbuffer(), begin, newlen);
     wbuffer()[newlen] = 0;
 }
 
diff --git a/tests/host/sys/pgmspace.h b/tests/host/sys/pgmspace.h
index be5046bd9..1b357d2e3 100644
--- a/tests/host/sys/pgmspace.h
+++ b/tests/host/sys/pgmspace.h
@@ -60,6 +60,7 @@ inline size_t strlen_P(const char *s) { return strlen(s); }
 inline int vsnprintf_P(char *str, size_t size, const char *format, va_list ap) { return vsnprintf(str, size, format, ap); }
 
 #define memcpy_P memcpy
+#define memmove_P memmove
 #define strncpy_P strncpy
 #define strcmp_P strcmp
 #define memccpy_P memccpy
diff --git a/tools/sdk/libc/xtensa-lx106-elf/include/sys/string.h b/tools/sdk/libc/xtensa-lx106-elf/include/sys/string.h
index 159b2d802..e19c3d843 100644
--- a/tools/sdk/libc/xtensa-lx106-elf/include/sys/string.h
+++ b/tools/sdk/libc/xtensa-lx106-elf/include/sys/string.h
@@ -25,6 +25,7 @@ extern "C" {
 int 	 _EXFUN(memcmp_P,(const _PTR, const _PTR, size_t));
 _PTR	 _EXFUN(memmem_P, (const _PTR, size_t, const _PTR, size_t));
 _PTR 	 _EXFUN(memcpy_P,(_PTR __restrict, const _PTR __restrict, size_t));
+_PTR 	 _EXFUN(memmove_P,(_PTR __restrict, const _PTR __restrict, size_t));
 _PTR	 _EXFUN(memccpy_P,(_PTR __restrict, const _PTR __restrict, int, size_t));
 _PTR     _EXFUN(memchr_P,(const _PTR, int, size_t));
 
diff --git a/tools/sdk/libc/xtensa-lx106-elf/lib/libc.a b/tools/sdk/libc/xtensa-lx106-elf/lib/libc.a
index aa8a59bd9..9c879cdeb 100644
Binary files a/tools/sdk/libc/xtensa-lx106-elf/lib/libc.a and b/tools/sdk/libc/xtensa-lx106-elf/lib/libc.a differ