diff --git a/rtl/inc/system.inc b/rtl/inc/system.inc
index acfebd611f..101fd788dc 100644
--- a/rtl/inc/system.inc
+++ b/rtl/inc/system.inc
@@ -44,7 +44,11 @@ const
 var
   { widechar, because also used by widestring -> pwidechar conversions }
   emptychar : widechar;public name 'FPC_EMPTYCHAR';
+{$ifndef FPC_NO_GENERIC_STACK_CHECK}
+  { if the OS does the stack checking, we don't need any stklen from the
+    main program }
   initialstklen : SizeUint;external name '__stklen';
+{$endif FPC_NO_GENERIC_STACK_CHECK}
 
 { checks whether the given suggested size for the stack of the current
  thread is acceptable. If this is the case, returns it unaltered.
diff --git a/rtl/win32/system.pp b/rtl/win32/system.pp
index 264d262876..22da670824 100644
--- a/rtl/win32/system.pp
+++ b/rtl/win32/system.pp
@@ -1083,7 +1083,7 @@ begin
    end;
 end;
 
-(* ProcessID cached to avoid repeated calls to GetCurrentProcess. *)
+{ ProcessID cached to avoid repeated calls to GetCurrentProcess. }
 
 var
   ProcessID: SizeUInt;
@@ -1093,9 +1093,10 @@ begin
  GetProcessID := ProcessID;
 end;
 
-function CheckInitialStkLen(stklen : SizeUInt) : SizeUInt;
-begin
-  result := stklen;
+function CheckInitialStkLen(stklen : SizeUInt) : SizeUInt;assembler;
+asm
+  movl  %fs:(4),%eax
+  subl  %fs:(8),%eax
 end;
 
 {
@@ -1105,7 +1106,8 @@ const
 }
 
 begin
-  StackLength := CheckInitialStkLen(InitialStkLen);
+  { pass dummy value }
+  StackLength := CheckInitialStkLen($1000000);
   StackBottom := StackTop - StackLength;
   { get some helpful informations }
   GetStartupInfo(@startupinfo);
diff --git a/rtl/win64/system.pp b/rtl/win64/system.pp
index 1bb190ce6e..a40cfb9e52 100644
--- a/rtl/win64/system.pp
+++ b/rtl/win64/system.pp
@@ -1136,17 +1136,19 @@ begin
 end;
 
 
-function CheckInitialStkLen(stklen : SizeUInt) : SizeUInt;
-begin
-  result := stklen;
+function CheckInitialStkLen(stklen : SizeUInt) : SizeUInt;assembler;
+asm
+  movq  %gs:(8),%rax
+  subq  %gs:(16),%rax
 end;
 
 
 begin
-  SysResetFPU;    
+  SysResetFPU;
   if not(IsLibrary) then
     SysInitFPU;
-  StackLength := CheckInitialStkLen(InitialStkLen);
+  { pass dummy value }
+  StackLength := CheckInitialStkLen($1000000);
   StackBottom := StackTop - StackLength;
   { get some helpful informations }
   GetStartupInfo(@startupinfo);