diff --git a/Sming/System/include/m_printf.h b/Sming/System/include/m_printf.h
index ed36499842..46e74ea98a 100644
--- a/Sming/System/include/m_printf.h
+++ b/Sming/System/include/m_printf.h
@@ -37,10 +37,10 @@ nputs_callback_t m_setPuts(nputs_callback_t callback);
 extern "C" {
 #endif
 
-int m_vsnprintf(char* buf, size_t maxLen, const char* fmt, va_list args);
-int m_snprintf(char* buf, int length, const char* fmt, ...);
-int m_printf(char const*, ...);
-int m_vprintf(const char* format, va_list arg);
+int m_vsnprintf(char* buf, size_t maxLen, const char* fmt, va_list args) __attribute__((format(printf, 3, 0)));
+int m_snprintf(char* buf, int length, const char* fmt, ...) __attribute__((format(printf, 3, 4)));
+int m_printf(char const*, ...) __attribute__((format(printf, 1, 2)));
+int m_vprintf(const char* format, va_list arg) __attribute__((format(printf, 1, 0)));
 
 /** @brief output a single character
  *  @param c
diff --git a/Sming/System/m_printf.cpp b/Sming/System/m_printf.cpp
index 46b29172ac..de12db413a 100644
--- a/Sming/System/m_printf.cpp
+++ b/Sming/System/m_printf.cpp
@@ -28,6 +28,18 @@ static nputs_callback_t _puts_callback;
 #define is_digit(c) ((c) >= '0' && (c) <= '9')
 #define is_print(c) ((c) >= ' ' && (c) <= '~')
 
+static char to_upper(char c)
+{
+    return (c >= 'a' && c <= 'z') ? (c + 'A' - 'a') : c;
+}
+
+static void str_upper(char* s)
+{
+    for(; *s != '\0'; ++s) {
+        *s = to_upper(*s);
+    }
+}
+
 static int skip_atoi(const char **s)
 {
 	int i = 0;
@@ -125,6 +137,8 @@ int m_vsnprintf(char *buf, size_t maxLen, const char *fmt, va_list args)
         int8_t  precision   = -1;
         int8_t  width       = 0;
         char    pad         = ' ';
+        uint8_t length      = 0;
+        bool    upcase      = false;
 
         while (char f = *fmt) {
             if (f == '-')           minus = 1;
@@ -152,8 +166,19 @@ int m_vsnprintf(char *buf, size_t maxLen, const char *fmt, va_list args)
             if ( is_digit(*fmt) ) precision = skip_atoi(&fmt);
         }
 
-        //  ignore length
-        while (*fmt == 'l' || *fmt == 'h' || *fmt == 'L') fmt++;
+        // while (*fmt == 'l' || *fmt == 'h' || *fmt == 'L') fmt++;
+        
+        // process length
+        do {
+            if ( *fmt == 'l' ) {
+                ++length;
+                ++fmt;
+            } else if ( *fmt == 'h' || *fmt == 'L' ) {
+                ++fmt; // ignore
+            } else {
+                break;
+            }
+        } while(true);
 
         //  process type
         switch (char f = *fmt++) {
@@ -202,8 +227,12 @@ int m_vsnprintf(char *buf, size_t maxLen, const char *fmt, va_list args)
                 break;
 
             case 'x':
+                ubase = 16;
+                break;
+
             case 'X':
                 ubase = 16;
+                upcase = true;
                 break;
 
             case 'u':
@@ -217,7 +246,16 @@ int m_vsnprintf(char *buf, size_t maxLen, const char *fmt, va_list args)
         }
 
         //  format unsigned numbers
-        if (ubase) s = ultoa_wp(va_arg(args, unsigned int), tempNum, ubase, width, pad);
+        if (ubase != 0) {
+            if(length >= 2) {
+                s = ulltoa_wp(va_arg(args, uint64_t), tempNum, ubase, width, pad);
+            } else {
+                s = ultoa_wp(va_arg(args, uint32_t), tempNum, ubase, width, pad);
+            }
+            if (upcase) {
+                str_upper(tempNum);
+            }
+        }
 
         //  copy string to target
         while (*s) add(*s++);
diff --git a/tests/HostTests/modules/Libc.cpp b/tests/HostTests/modules/Libc.cpp
index 2ae0596fa1..73f0183068 100644
--- a/tests/HostTests/modules/Libc.cpp
+++ b/tests/HostTests/modules/Libc.cpp
@@ -66,6 +66,21 @@ class LibcTest : public TestGroup
 			REQUIRE(isRomPtr(strstr));
 		}
 #endif
+
+		TEST_CASE("64-bit mprintf")
+		{
+			char buffer[256];
+			m_snprintf(buffer, sizeof(buffer), "%x", 0x12345678);
+			REQUIRE_EQ(String(buffer), "12345678");
+			m_snprintf(buffer, sizeof(buffer), "%u", 12345678);
+			REQUIRE_EQ(String(buffer), "12345678");
+			m_snprintf(buffer, sizeof(buffer), "%llx", 0x123456789ABCDEFULL);
+			REQUIRE_EQ(String(buffer), "123456789abcdef");
+			m_snprintf(buffer, sizeof(buffer), "0x%016llX", 0x123456789ABCDEFULL);
+			REQUIRE_EQ(String(buffer), "0x0123456789ABCDEF");
+			m_snprintf(buffer, sizeof(buffer), "%llu", 123456789123456789ULL);
+			REQUIRE_EQ(String(buffer), "123456789123456789");
+		}
 	}
 };
 
diff --git a/tests/HostTests/modules/Stream.cpp b/tests/HostTests/modules/Stream.cpp
index 0f52776c96..d360be6d1d 100644
--- a/tests/HostTests/modules/Stream.cpp
+++ b/tests/HostTests/modules/Stream.cpp
@@ -95,7 +95,7 @@ class StreamTest : public TestGroup
 		TEST_CASE("ChunkedStream / StreamTransformer")
 		{
 			DEFINE_FSTR_LOCAL(FS_INPUT, "Some test data");
-			DEFINE_FSTR_LOCAL(FS_OUTPUT, "e\r\nSome test data\r\n0\r\n\r\n");
+			DEFINE_FSTR_LOCAL(FS_OUTPUT, "E\r\nSome test data\r\n0\r\n\r\n");
 			ChunkedStream chunked(new FlashMemoryStream(FS_INPUT));
 			MemoryDataStream output;
 			output.copyFrom(&chunked);