diff --git a/winsup/cygwin/heap.cc b/winsup/cygwin/heap.cc
index e18cd5508..b839c8cd4 100644
--- a/winsup/cygwin/heap.cc
+++ b/winsup/cygwin/heap.cc
@@ -34,9 +34,9 @@ eval_start_address ()
      executable starts at 0x1:00400000L, the Cygwin DLL starts at
      0x1:80040000L, other rebased DLLs are located in the region from
      0x2:00000000L up to 0x4:00000000L, -auto-image-based DLLs are located
-     in the region from 0x4:00000000L up to 0x6:00000000L.
-     So we let the heap start at 0x6:00000000L. */
-  uintptr_t start_address = 0x600000000L;
+     in the region from 0x4:00000000L up to 0x6:00000000L.  Leave another
+     8 Gigs slack space, so lets start the heap at 0x8:00000000L. */
+  uintptr_t start_address = 0x800000000L;
 #else
   /* Windows performs heap ASLR.  This spoils the entire region below
      0x20000000 for us, because that region is used by Windows to randomize
diff --git a/winsup/cygwin/mmap.cc b/winsup/cygwin/mmap.cc
index 9eb1643a0..8b1aedc48 100644
--- a/winsup/cygwin/mmap.cc
+++ b/winsup/cygwin/mmap.cc
@@ -801,8 +801,10 @@ mmap_worker (mmap_list *map_list, fhandler_base *fh, caddr_t base, size_t len,
 #ifdef __x86_64__
 
 /* The memory region used for memory maps */
-#define MMAP_STORAGE_LOW	0x00800000000L	/* Leave 8 Gigs for heap. */
-#define MMAP_STORAGE_HIGH	0x70000000000L	/* Leave enough room for OS. */
+#define MMAP_STORAGE_LOW	0x001000000000L	/* Leave 32 Gigs for heap. */
+/* Up to Win 8 only supporting 44 bit address space, starting with Win 8.1
+   48 bit address space. */
+#define MMAP_STORAGE_HIGH	wincap.mmap_storage_high ()
 
 /* FIXME?  Unfortunately the OS doesn't support a top down allocation with
 	   a ceiling value.  The ZeroBits mechanism only works for
diff --git a/winsup/cygwin/wincap.cc b/winsup/cygwin/wincap.cc
index 17e0cf1be..5c6e6428e 100644
--- a/winsup/cygwin/wincap.cc
+++ b/winsup/cygwin/wincap.cc
@@ -20,6 +20,7 @@ details. */
 
 wincaps wincap_vista __attribute__((section (".cygwin_dll_common"), shared)) = {
   def_guard_pages:1,
+  mmap_storage_high:0x070000000000LL,
   {
     is_server:false,
     needs_count_in_si_lpres2:true,
@@ -46,6 +47,7 @@ wincaps wincap_vista __attribute__((section (".cygwin_dll_common"), shared)) = {
 
 wincaps wincap_7 __attribute__((section (".cygwin_dll_common"), shared)) = {
   def_guard_pages:1,
+  mmap_storage_high:0x070000000000LL,
   {
     is_server:false,
     needs_count_in_si_lpres2:false,
@@ -72,6 +74,7 @@ wincaps wincap_7 __attribute__((section (".cygwin_dll_common"), shared)) = {
 
 wincaps wincap_8 __attribute__((section (".cygwin_dll_common"), shared)) = {
   def_guard_pages:2,
+  mmap_storage_high:0x070000000000LL,
   {
     is_server:false,
     needs_count_in_si_lpres2:false,
@@ -98,6 +101,7 @@ wincaps wincap_8 __attribute__((section (".cygwin_dll_common"), shared)) = {
 
 wincaps wincap_8_1 __attribute__((section (".cygwin_dll_common"), shared)) = {
   def_guard_pages:2,
+  mmap_storage_high:0x700000000000LL,
   {
     is_server:false,
     needs_count_in_si_lpres2:false,
@@ -124,6 +128,7 @@ wincaps wincap_8_1 __attribute__((section (".cygwin_dll_common"), shared)) = {
 
 wincaps  wincap_10_1507 __attribute__((section (".cygwin_dll_common"), shared)) = {
   def_guard_pages:2,
+  mmap_storage_high:0x700000000000LL,
   {
     is_server:false,
     needs_count_in_si_lpres2:false,
@@ -150,6 +155,7 @@ wincaps  wincap_10_1507 __attribute__((section (".cygwin_dll_common"), shared))
 
 wincaps wincap_10_1703 __attribute__((section (".cygwin_dll_common"), shared)) = {
   def_guard_pages:2,
+  mmap_storage_high:0x700000000000LL,
   {
     is_server:false,
     needs_count_in_si_lpres2:false,
@@ -176,6 +182,7 @@ wincaps wincap_10_1703 __attribute__((section (".cygwin_dll_common"), shared)) =
 
 wincaps wincap_10_1709 __attribute__((section (".cygwin_dll_common"), shared)) = {
   def_guard_pages:2,
+  mmap_storage_high:0x700000000000LL,
   {
     is_server:false,
     needs_count_in_si_lpres2:false,
@@ -202,6 +209,7 @@ wincaps wincap_10_1709 __attribute__((section (".cygwin_dll_common"), shared)) =
 
 wincaps wincap_10_1803 __attribute__((section (".cygwin_dll_common"), shared)) = {
   def_guard_pages:2,
+  mmap_storage_high:0x700000000000LL,
   {
     is_server:false,
     needs_count_in_si_lpres2:false,
@@ -228,6 +236,7 @@ wincaps wincap_10_1803 __attribute__((section (".cygwin_dll_common"), shared)) =
 
 wincaps wincap_10_1809 __attribute__((section (".cygwin_dll_common"), shared)) = {
   def_guard_pages:2,
+  mmap_storage_high:0x700000000000LL,
   {
     is_server:false,
     needs_count_in_si_lpres2:false,
diff --git a/winsup/cygwin/wincap.h b/winsup/cygwin/wincap.h
index 73b6f5ffc..ba01a1565 100644
--- a/winsup/cygwin/wincap.h
+++ b/winsup/cygwin/wincap.h
@@ -12,6 +12,7 @@ details. */
 struct wincaps
 {
   DWORD def_guard_pages;
+  int64_t mmap_storage_high;
   /* The bitfields must be 8 byte aligned on x86_64, otherwise the bitfield
      ops generated by gcc are off by 4 bytes. */
   struct  __attribute__ ((aligned (8))) {
@@ -71,6 +72,9 @@ public:
   {
     return ((wincaps *) this->caps)->def_guard_pages * page_size ();
   }
+#ifdef __x86_64__
+  intptr_t IMPLEMENT (mmap_storage_high)
+#endif
   bool  IMPLEMENT (is_server)
   bool	IMPLEMENT (needs_count_in_si_lpres2)
   bool	IMPLEMENT (needs_query_information)