diff --git a/dmd/mars.c b/dmd/mars.c
index dc49a9dd..491211ba 100644
--- a/dmd/mars.c
+++ b/dmd/mars.c
@@ -231,9 +231,6 @@ int main(int argc, char *argv[])
     Module *m;
     int status = EXIT_SUCCESS;
     int argcstart = argc;
-    char* tt_arch = 0;
-    char* tt_os = 0;
-    char* data_layout = 0;
     bool very_verbose = false;
 
     // Check for malformed input
@@ -312,24 +309,15 @@ int main(int argc, char *argv[])
     VersionCondition::addPredefinedGlobalIdent("LLVMDC");
 #endif
 
+    // setup default target os to be build os
 #if _WIN32
-    VersionCondition::addPredefinedGlobalIdent("Windows");
-    VersionCondition::addPredefinedGlobalIdent("Win32");
-    VersionCondition::addPredefinedGlobalIdent("mingw32");
-    global.params.isWindows = 1;
-    tt_os = "-pc-mingw32";
+    global.params.os = OSWindows;
 #elif linux
-    VersionCondition::addPredefinedGlobalIdent("linux");
-    global.params.isLinux = 1;
-    tt_os = "-pc-linux-gnu";
+    global.params.os = OSLinux;
 #else
 #error
 #endif /* linux */
 
-    // !win32 == posix for now
-    if (!global.params.isWindows)
-        VersionCondition::addPredefinedGlobalIdent("Posix");
-
     //VersionCondition::addPredefinedGlobalIdent("D_Bits");
     VersionCondition::addPredefinedGlobalIdent("all");
 
@@ -700,8 +688,8 @@ int main(int argc, char *argv[])
         global.params.isLE = true;
         global.params.is64bit = false;
         global.params.cpu = ARCHx86;
-        tt_arch = "i686";
-        data_layout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-f80:32:32-v64:64:64-v128:128:128-a0:0:64";
+        global.params.tt_arch = "i686";
+        global.params.data_layout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-f80:32:32-v64:64:64-v128:128:128-a0:0:64";
         if (global.params.useInlineAsm) {
             VersionCondition::addPredefinedGlobalIdent("D_InlineAsm");
             VersionCondition::addPredefinedGlobalIdent("D_InlineAsm_X86");
@@ -712,24 +700,24 @@ int main(int argc, char *argv[])
         global.params.isLE = true;
         global.params.is64bit = true;
         global.params.cpu = ARCHx86_64;
-        tt_arch = "x86_64";
-        data_layout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64";
+        global.params.tt_arch = "x86_64";
+        global.params.data_layout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64";
     }
     else if (strcmp(global.params.llvmArch,"ppc32")==0) {
         VersionCondition::addPredefinedGlobalIdent("PPC");
         global.params.isLE = false;
         global.params.is64bit = false;
         global.params.cpu = ARCHppc;
-        tt_arch = "powerpc";
-        data_layout = "E-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64";
+        global.params.tt_arch = "powerpc";
+        global.params.data_layout = "E-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64";
     }
     else if (strcmp(global.params.llvmArch,"ppc64")==0) {
         VersionCondition::addPredefinedGlobalIdent("PPC64");
         global.params.isLE = false;
         global.params.is64bit = true;
         global.params.cpu = ARCHppc_64;
-        tt_arch = "powerpc64";
-        data_layout = "E-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64";
+        global.params.tt_arch = "powerpc64";
+        global.params.data_layout = "E-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64";
     }
     else {
         assert(0 && "Invalid arch");
@@ -750,12 +738,27 @@ int main(int argc, char *argv[])
         VersionCondition::addPredefinedGlobalIdent("LLVM64");
     }
 
-    assert(tt_arch != 0);
-    assert(tt_os != 0);
-    assert(data_layout != 0);
-    global.params.tt_arch = tt_arch;
-    global.params.tt_os = tt_os;
-    global.params.data_layout = data_layout;
+
+    // setup version idents and tt_os for chosen target os
+    switch(global.params.os)
+    {
+    case OSWindows:
+	VersionCondition::addPredefinedGlobalIdent("Windows");
+	VersionCondition::addPredefinedGlobalIdent("Win32");
+	VersionCondition::addPredefinedGlobalIdent("mingw32");
+	global.params.tt_os = "-pc-mingw32";
+	break;
+
+    case OSLinux: 
+	VersionCondition::addPredefinedGlobalIdent("linux");
+	VersionCondition::addPredefinedGlobalIdent("Posix");
+	global.params.tt_os = "-pc-linux-gnu";
+	break;
+
+    default:
+	assert(false && "Target OS not supported");
+    }
+
 
     // Initialization
     Type::init();
diff --git a/dmd/mars.h b/dmd/mars.h
index 1c22a997..bd8ff6ec 100644
--- a/dmd/mars.h
+++ b/dmd/mars.h
@@ -37,6 +37,22 @@
 
 struct Array;
 
+// LLVMDC
+enum ARCH
+{
+    ARCHx86,
+    ARCHx86_64,
+    ARCHppc,
+    ARCHppc_64
+};
+
+enum OS
+{
+    OSLinux,
+    OSWindows,
+    OSMacOSX
+};
+
 // Put command line switches in here
 struct Param
 {
@@ -48,11 +64,10 @@ struct Param
     char symdebug;	// insert debug symbolic information
     char optimize;	// run optimizer
     char optimizeLevel; // optimization level
-    char cpu;		// target CPU
+    ARCH cpu;		// target CPU
+    OS   os;		// target OS
     char is64bit;	// generate 64 bit code
     char isLE;      // generate little endian code
-    char isLinux;	// generate code for linux
-    char isWindows;	// generate code for Windows
     char scheduler;	// which scheduler to use
     char useDeprecated;	// allow use of deprecated features
     char useAssert;	// generate runtime code for assert()'s
@@ -305,15 +320,6 @@ enum MATCH
     MATCHexact		// exact match
 };
 
-// LLVMDC
-enum ARCH
-{
-    ARCHx86,
-    ARCHx86_64,
-    ARCHppc,
-    ARCHppc_64
-};
-
 void error(Loc loc, const char *format, ...);
 void verror(Loc loc, const char *format, va_list);
 void fatal();
diff --git a/gen/linker.cpp b/gen/linker.cpp
index 2654b586..399c0a39 100644
--- a/gen/linker.cpp
+++ b/gen/linker.cpp
@@ -72,7 +72,7 @@ int linkExecutable(const char* argv0)
         else
             exestr = "a.out";
     }
-    if (global.params.isWindows)
+    if (global.params.os == OSWindows)
         exestr.append(".exe");
 
     std::string outopt = "-o=" + exestr;
@@ -151,13 +151,13 @@ int linkExecutable(const char* argv0)
     }
 
     // default libs
-    if(global.params.isLinux)
+    if(global.params.os == OSLinux)
     {
         args.push_back("-lpthread");
         args.push_back("-ldl");
         args.push_back("-lm");
     }
-    else if (global.params.isWindows)
+    else if (global.params.os == OSWindows)
     {
         // FIXME: I'd assume kernel32 etc
     }
@@ -174,12 +174,11 @@ int linkExecutable(const char* argv0)
     std::string runtime_path(global.params.runtimePath);
 
     // path seperator can be \ on windows, but we check for /
-    if (global.params.isWindows)
-    {
-        int i=0;
-        while ((i = runtime_path.find("\\", i)) > 0)
-            runtime_path.replace(i, 1, "/");
-    }
+#if _WIN32
+    int i=0;
+    while ((i = runtime_path.find("\\", i)) > 0)
+        runtime_path.replace(i, 1, "/");
+#endif
 
     if (*runtime_path.rbegin() != '/')
         runtime_path.append("/");
diff --git a/gen/tollvm.cpp b/gen/tollvm.cpp
index c4fded43..de4f0a7c 100644
--- a/gen/tollvm.cpp
+++ b/gen/tollvm.cpp
@@ -705,7 +705,7 @@ const LLStructType* DtoMutexType()
         return gIR->mutexType;
 
     // win32
-    if (global.params.isWindows)
+    if (global.params.os == OSWindows)
     {
         // CRITICAL_SECTION.sizeof == 68
         std::vector<const LLType*> types(17, LLType::Int32Ty);