diff --git a/zutil.h b/zutil.h
index 3b761d3..9441a1e 100644
--- a/zutil.h
+++ b/zutil.h
@@ -98,28 +98,38 @@ extern z_const char * const z_errmsg[10]; /* indexed by 2-zlib_error */
 #endif
 
 #ifdef AMIGA
-#  define OS_CODE  0x01
+#  define OS_CODE  1
 #endif
 
 #if defined(VAXC) || defined(VMS)
-#  define OS_CODE  0x02
+#  define OS_CODE  2
 #  define F_OPEN(name, mode) \
      fopen((name), (mode), "mbc=60", "ctx=stm", "rfm=fix", "mrs=512")
 #endif
 
+#ifdef __370__
+#  if __TARGET_LIB__ < 0x20000000
+#    define OS_CODE 4
+#  elif __TARGET_LIB__ < 0x40000000
+#    define OS_CODE 11
+#  else
+#    define OS_CODE 8
+#  endif
+#endif
+
 #if defined(ATARI) || defined(atarist)
-#  define OS_CODE  0x05
+#  define OS_CODE  5
 #endif
 
 #ifdef OS2
-#  define OS_CODE  0x06
+#  define OS_CODE  6
 #  if defined(M_I86) && !defined(Z_SOLO)
 #    include <malloc.h>
 #  endif
 #endif
 
 #if defined(MACOS) || defined(TARGET_OS_MAC)
-#  define OS_CODE  0x07
+#  define OS_CODE  7
 #  ifndef Z_SOLO
 #    if defined(__MWERKS__) && __dest_os != __be_os && __dest_os != __win32_os
 #      include <unix.h> /* for fdopen */
@@ -131,18 +141,24 @@ extern z_const char * const z_errmsg[10]; /* indexed by 2-zlib_error */
 #  endif
 #endif
 
-#ifdef TOPS20
-#  define OS_CODE  0x0a
+#ifdef __acorn
+#  define OS_CODE 13
 #endif
 
-#ifdef WIN32
-#  ifndef __CYGWIN__  /* Cygwin is Unix, not Win32 */
-#    define OS_CODE  0x0b
-#  endif
+#if defined(WIN32) && !defined(__CYGWIN__)
+#  define OS_CODE  10
 #endif
 
-#ifdef __50SERIES /* Prime/PRIMOS */
-#  define OS_CODE  0x0f
+#ifdef _BEOS_
+#  define OS_CODE  16
+#endif
+
+#ifdef __TOS_OS400__
+#  define OS_CODE 18
+#endif
+
+#ifdef __APPLE__
+#  define OS_CODE 19
 #endif
 
 #if defined(_BEOS_) || defined(RISCOS)
@@ -177,7 +193,7 @@ extern z_const char * const z_errmsg[10]; /* indexed by 2-zlib_error */
         /* common defaults */
 
 #ifndef OS_CODE
-#  define OS_CODE  0x03  /* assume Unix */
+#  define OS_CODE  3     /* assume Unix */
 #endif
 
 #ifndef F_OPEN