diff --git a/dlls/ntdll/signal_i386.c b/dlls/ntdll/signal_i386.c
index 635b8f4de81..59d9e1d0df9 100644
--- a/dlls/ntdll/signal_i386.c
+++ b/dlls/ntdll/signal_i386.c
@@ -504,13 +504,12 @@ USHORT WINAPI RtlCaptureStackBackTrace( ULONG skip, ULONG count, PVOID *buffer,
  */
 __ASM_GLOBAL_FUNC( signal_start_thread,
                    "movl 4(%esp),%esi\n\t"   /* context */
-                   "leal -12(%esi),%ecx\n\t"
+                   "leal -12(%esi),%edi\n\t"
                    /* clear the thread stack */
-                   "andl $~0xfff,%ecx\n\t"   /* round down to page size */
-                   "movl %fs:8,%edi\n\t"     /* NtCurrentTeb()->Tib.StackLimit */
-                   "addl $0x1000,%edi\n\t"
+                   "andl $~0xfff,%edi\n\t"   /* round down to page size */
+                   "movl $0xf0000,%ecx\n\t"
+                   "subl %ecx,%edi\n\t"
                    "movl %edi,%esp\n\t"
-                   "subl %edi,%ecx\n\t"
                    "xorl %eax,%eax\n\t"
                    "shrl $2,%ecx\n\t"
                    "rep; stosl\n\t"
diff --git a/dlls/ntdll/signal_x86_64.c b/dlls/ntdll/signal_x86_64.c
index d88f7ae553a..c5a6666f4d2 100644
--- a/dlls/ntdll/signal_x86_64.c
+++ b/dlls/ntdll/signal_x86_64.c
@@ -1544,9 +1544,7 @@ __ASM_GLOBAL_FUNC( signal_start_thread,
                    "movq %rcx,%rbx\n\t"        /* context */
                    /* clear the thread stack */
                    "andq $~0xfff,%rcx\n\t"     /* round down to page size */
-                   "movq %gs:0x30,%rax\n\t"
-                   "movq 0x10(%rax),%rdi\n\t"  /* NtCurrentTeb()->Tib.StackLimit */
-                   "addq $0x2000,%rdi\n\t"
+                   "leaq -0xf0000(%rcx),%rdi\n\t"
                    "movq %rdi,%rsp\n\t"
                    "subq %rdi,%rcx\n\t"
                    "xorl %eax,%eax\n\t"