diff --git a/dlls/msvcrt/main.c b/dlls/msvcrt/main.c
index 272ba71f4c9..4ae52c70a57 100644
--- a/dlls/msvcrt/main.c
+++ b/dlls/msvcrt/main.c
@@ -123,15 +123,3 @@ BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved)
   }
   return TRUE;
 }
-
-/*********************************************************************
- *		$I10_OUTPUT (MSVCRT.@)
- * Function not really understood but needed to make the DLL work
- */
-void CDECL MSVCRT_I10_OUTPUT(void)
-{
-  /* FIXME: This is probably data, not a function */
-  /* no it is a function. I10 is an Int of 10 bytes */
-  /* also known as 80 bit floating point (long double */
-  /* for some compilers, not MSVC) */
-}
diff --git a/dlls/msvcrt/msvcrt.spec b/dlls/msvcrt/msvcrt.spec
index c7d66545b8c..1bf3f6db263 100644
--- a/dlls/msvcrt/msvcrt.spec
+++ b/dlls/msvcrt/msvcrt.spec
@@ -1,6 +1,6 @@
 # msvcrt.dll - MS VC++ Run Time Library
 
-@ cdecl $I10_OUTPUT() MSVCRT_I10_OUTPUT
+@ cdecl -norelay $I10_OUTPUT(double long long long ptr) MSVCRT_I10_OUTPUT
 @ cdecl -i386 -norelay ??0__non_rtti_object@@QAE@ABV0@@Z(ptr) __thiscall_MSVCRT___non_rtti_object_copy_ctor
 @ cdecl -i386 -norelay ??0__non_rtti_object@@QAE@PBD@Z(ptr) __thiscall_MSVCRT___non_rtti_object_ctor
 @ cdecl -i386 -norelay ??0bad_cast@@AAE@PBQBD@Z(ptr) __thiscall_MSVCRT_bad_cast_ctor
diff --git a/dlls/msvcrt/string.c b/dlls/msvcrt/string.c
index 21dc29f166b..8c993992223 100644
--- a/dlls/msvcrt/string.c
+++ b/dlls/msvcrt/string.c
@@ -26,6 +26,7 @@
 #include "wine/port.h"
 
 #include <stdlib.h>
+#include <stdio.h>
 #include <math.h>
 #include <limits.h>
 #include <errno.h>
@@ -669,3 +670,98 @@ int CDECL MSVCRT__ui64toa_s(unsigned __int64 value, char *str,
     memcpy(str, pos, buffer-pos+65);
     return 0;
 }
+
+#define I10_OUTPUT_MAX_PREC 21
+/* Internal structure used by $I10_OUTPUT */
+struct _I10_OUTPUT_DATA {
+    short pos;
+    char sign;
+    BYTE len;
+    char str[I10_OUTPUT_MAX_PREC+1]; /* add space for '\0' */
+};
+
+/*********************************************************************
+ *              $I10_OUTPUT (MSVCRT.@)
+ * ld - long double to be printed to data
+ * prec - precision of part, we're interested in
+ * flag - 0 for first prec digits, 1 for fractional part
+ * data - data to be populated
+ *
+ * return value
+ *      0 if given double is NaN or INF
+ *      1 otherwise
+ *
+ * FIXME
+ *      Native sets last byte of data->str to '0' or '9', I don't know what
+ *      it means. Current implementation sets it always to '0'.
+ */
+int CDECL MSVCRT_I10_OUTPUT(MSVCRT__LDOUBLE ld, int prec, int flag, struct _I10_OUTPUT_DATA *data)
+{
+    static const char inf_str[] = "1#INF";
+    static const char nan_str[] = "1#QNAN";
+
+    double d = ld.x;
+    char format[8];
+    char buf[I10_OUTPUT_MAX_PREC+9]; /* 9 = strlen("0.e+0000") + '\0' */
+    char *p;
+
+    TRACE("(%lf %d %x %p)\n", d, prec, flag, data);
+
+    if(d<0) {
+        data->sign = '-';
+        d = -d;
+    } else
+        data->sign = ' ';
+
+    if(isinf(d)) {
+        data->pos = 1;
+        data->len = 5;
+        memcpy(data->str, inf_str, sizeof(inf_str));
+
+        return 0;
+    }
+
+    if(isnan(d)) {
+        data->pos = 1;
+        data->len = 6;
+        memcpy(data->str, nan_str, sizeof(nan_str));
+
+        return 0;
+    }
+
+    if(flag&1) {
+        int exp = 1+floor(log10(d));
+
+        prec += exp;
+        if(exp < 0)
+            prec--;
+    }
+    prec--;
+
+    if(prec+1 > I10_OUTPUT_MAX_PREC)
+        prec = I10_OUTPUT_MAX_PREC-1;
+    else if(prec < 0) {
+        d = 0.0;
+        prec = 0;
+    }
+
+    sprintf(format, "%%.%dle", prec);
+    sprintf(buf, format, d);
+
+    buf[1] = buf[0];
+    data->pos = atoi(buf+prec+3);
+    if(buf[1] != '0')
+        data->pos++;
+
+    for(p = buf+prec+1; p>buf+1 && *p=='0'; p--);
+    data->len = p-buf;
+
+    memcpy(data->str, buf+1, data->len);
+    data->str[data->len] = '\0';
+
+    if(buf[1]!='0' && prec-data->len+1>0)
+        memcpy(data->str+data->len+1, buf+data->len+1, prec-data->len+1);
+
+    return 1;
+}
+#undef I10_OUTPUT_MAX_PREC
diff --git a/dlls/msvcrt/tests/misc.c b/dlls/msvcrt/tests/misc.c
index d8b1b88b91a..68f47f8b309 100644
--- a/dlls/msvcrt/tests/misc.c
+++ b/dlls/msvcrt/tests/misc.c
@@ -24,6 +24,7 @@
 
 static int (__cdecl *prand_s)(unsigned int *);
 static int (__cdecl *memcpy_s)(void *, MSVCRT_size_t, void*, MSVCRT_size_t);
+static int (__cdecl *pI10_OUTPUT)(long double, int, int, void*);
 
 static void init(void)
 {
@@ -31,6 +32,7 @@ static void init(void)
 
     prand_s = (void *)GetProcAddress(hmod, "rand_s");
     memcpy_s = (void*)GetProcAddress(hmod, "memcpy_s");
+    pI10_OUTPUT = (void*)GetProcAddress(hmod, "$I10_OUTPUT");
 }
 
 static void test_rand_s(void)
@@ -104,10 +106,97 @@ static void test_memcpy_s(void)
     ok(dest[0] == '\0', "dest[0] != \'\\0\'\n");
 }
 
+typedef struct _I10_OUTPUT_data {
+    short pos;
+    char sign;
+    BYTE len;
+    char str[100];
+} I10_OUTPUT_data;
+
+typedef struct _I10_OUTPUT_test {
+    long double d;
+    int size;
+    int flags;
+
+    I10_OUTPUT_data out;
+    int ret;
+    const char *remain;
+} I10_OUTPUT_test;
+
+static const I10_OUTPUT_test I10_OUTPUT_tests[] = {
+    /* arg3 = 0 */
+    { 0.0, 10, 0, {0, ' ', 1, "0"}, 1, "" },
+    { 1.0, 10, 0, {1, ' ', 1, "1"}, 1, "000000009" },
+    { -1.0, 10, 0, {1, '-', 1, "1"}, 1, "000000009" },
+    { 1.23, 10, 0, {1, ' ', 3, "123"}, 1, "0000009" },
+    { 1e13, 10, 0, {14, ' ', 1, "1"}, 1, "000000009" },
+    { 1e30, 30, 0, {31, ' ', 21, "100000000000000001988"}, 1, "" },
+    { 1e-13, 10, 0, {-12, ' ', 1, "1"}, 1, "000000000" },
+    { 0.25, 10, 0, {0, ' ', 2, "25"}, 1, "00000000" },
+    { 1.0000001, 10, 0, {1, ' ', 8, "10000001"}, 1, "00" },
+    /* arg3 = 1 */
+    { 0.0, 10, 1, {0, ' ', 1, "0"}, 1, "" },
+    { 1.0, 10, 1, {1, ' ', 1, "1"}, 1, "0000000009" },
+    { -1.0, 10, 1, {1, '-', 1, "1"}, 1, "0000000009" },
+    { 1.23, 10, 1, {1, ' ', 3, "123"}, 1, "00000009" },
+    { 1e13, 10, 1, {14, ' ', 1, "1"}, 1, "00000000000000000009" },
+    { 1e30, 30, 1, {31, ' ', 21, "100000000000000001988"}, 1, "" },
+    { 1e-13, 10, 1, {0, ' ', 1, "0"}, 1, "" },
+    { 1e-7, 10, 1, {-6, ' ', 1, "1"}, 1, "09" },
+    { 0.25, 10, 1, {0, ' ', 2, "25"}, 1, "00000000" },
+    { 1.0000001, 10, 1, {1, ' ', 8, "10000001"}, 1, "000" },
+    /* too small buffer */
+    { 0.0, 0, 0, {0, ' ', 1, "0"}, 1, "" },
+    { 0.0, 0, 1, {0, ' ', 1, "0"}, 1, "" },
+    { 123.0, 2, 0, {3, ' ', 2, "12"}, 1, "" },
+    { 123.0, 0, 0, {0, ' ', 1, "0"}, 1, "" },
+    { 123.0, 2, 1, {3, ' ', 3, "123"}, 1, "09" },
+    { 0.99, 1, 0, {1, ' ', 1, "1"}, 1, "" },
+    { 1264567.0, 2, 0, {7, ' ', 2, "13"}, 1, "" },
+    { 1264567.0, 2, 1, {7, ' ', 7, "1264567"}, 1, "00" },
+    { 1234567891.0, 2, 1, {10, ' ', 10, "1234567891"}, 1, "09" }
+};
+
+static void test_I10_OUTPUT(void)
+{
+    I10_OUTPUT_data out;
+    int i, j, ret;
+
+    if(!pI10_OUTPUT) {
+        win_skip("I10_OUTPUT not available\n");
+        return;
+    }
+
+    for(i=0; i<sizeof(I10_OUTPUT_tests)/sizeof(I10_OUTPUT_test); i++) {
+        memset(out.str, '#', sizeof(out.str));
+
+        ret = pI10_OUTPUT(I10_OUTPUT_tests[i].d, I10_OUTPUT_tests[i].size, I10_OUTPUT_tests[i].flags, &out);
+        ok(ret == I10_OUTPUT_tests[i].ret, "%d: ret = %d\n", i, ret);
+        ok(out.pos == I10_OUTPUT_tests[i].out.pos, "%d: out.pos = %hd\n", i, out.pos);
+        ok(out.sign == I10_OUTPUT_tests[i].out.sign, "%d: out.size = %c\n", i, out.sign);
+        ok(out.len == I10_OUTPUT_tests[i].out.len, "%d: out.len = %d\n", i, (int)out.len);
+        ok(!strcmp(out.str, I10_OUTPUT_tests[i].out.str), "%d: out.str = %s\n", i, out.str);
+
+        j = strlen(I10_OUTPUT_tests[i].remain);
+        if(j && I10_OUTPUT_tests[i].remain[j-1]=='9')
+            todo_wine ok(!strncmp(out.str+out.len+1, I10_OUTPUT_tests[i].remain, j),
+                    "%d: &out.str[%d] = %.25s...\n", i, out.len+1, out.str+out.len+1);
+        else
+            ok(!strncmp(out.str+out.len+1, I10_OUTPUT_tests[i].remain, j),
+                    "%d: &out.str[%d] = %.25s...\n", i, out.len+1, out.str+out.len+1);
+
+
+        for(j=out.len+strlen(I10_OUTPUT_tests[i].remain)+1; j<sizeof(out.str); j++)
+            if(out.str[j] != '#')
+                ok(0, "%d: out.str[%d] = %c (expected \'#\')\n", i, j, out.str[j]);
+    }
+}
+
 START_TEST(misc)
 {
     init();
 
     test_rand_s();
     test_memcpy_s();
+    test_I10_OUTPUT();
 }