=== gdb/doc/gdb.texinfo
==================================================================
--- gdb/doc/gdb.texinfo	(/mirrors/gdb_mainline)	(revision 2094)
+++ gdb/doc/gdb.texinfo	(/patches/gdb/memcheck/gdb_mainline)	(revision 2094)
@@ -6894,6 +6894,26 @@
 Disable @value{GDBN} from caching target memory.  This is the default.
 @end table
 
+@subsection Memory Access Checking
+@value{GDBN} can be instructed to refuse accesses to memory that is
+not explicitly described.  This can be useful if accessing such
+regions has undesired effects for a specific target, or to provide
+better error checking.  The following commands control this behaviour.
+
+@table @code
+@kindex set mem inaccessible-by-default
+@item set mem inaccessible-by-default [on|off]
+If @code{on} is specified, make  @value{GDBN} treat memory not
+explicitly described by the memory ranges as non-existent and refuse accesses
+to such memory.  The checks are only performed if there's at least one
+memory range defined.  If @code{off} is specified, make @value{GDBN}
+treat the memory not explicitly described by the memory ranges as RAM.
+@kindex show mem inaccessible-by-default
+@item show mem inaccessible-by-default
+Show the current handling of accesses to unknown memory.
+@end table
+
+
 @c @subsubsection Memory Write Verification
 @c The memory write verification attributes set whether @value{GDBN}
 @c will re-reads data after each write to verify the write was successful.
=== gdb/target.c
==================================================================
--- gdb/target.c	(/mirrors/gdb_mainline)	(revision 2094)
+++ gdb/target.c	(/patches/gdb/memcheck/gdb_mainline)	(revision 2094)
@@ -1015,7 +1015,8 @@
 
   /* Try GDB's internal data cache.  */
   region = lookup_mem_region (memaddr);
-  if (memaddr + len < region->hi)
+  /* region->hi means there's no upper bound. */
+  if (memaddr + len < region->hi || region->hi == 0)
     reg_len = len;
   else
     reg_len = region->hi - memaddr;
@@ -1037,6 +1038,9 @@
       if (writebuf != NULL)
 	error (_("Writing to flash memory forbidden in this context"));
       break;
+
+    case MEM_NONE:
+      return -1;
     }
 
   if (region->attrib.cache)
@@ -1072,7 +1076,7 @@
   do
     {
       res = ops->to_xfer_partial (ops, TARGET_OBJECT_MEMORY, NULL,
-				  readbuf, writebuf, memaddr, len);
+				  readbuf, writebuf, memaddr, reg_len);
       if (res > 0)
 	return res;
 
=== gdb/memattr.c
==================================================================
--- gdb/memattr.c	(/mirrors/gdb_mainline)	(revision 2094)
+++ gdb/memattr.c	(/patches/gdb/memcheck/gdb_mainline)	(revision 2094)
@@ -29,6 +29,7 @@
 #include "language.h"
 #include "vec.h"
 #include "gdb_string.h"
+#include "gdb_locale.h"
 
 const struct mem_attrib default_mem_attrib =
 {
@@ -40,6 +41,17 @@
   -1 /* Flash blocksize not specified.  */
 };
 
+const struct mem_attrib unknown_mem_attrib =
+{
+  MEM_NONE,			/* mode */
+  MEM_WIDTH_UNSPECIFIED,
+  0,				/* hwbreak */
+  0,				/* cache */
+  0,				/* verify */
+  -1 /* Flash blocksize not specified.  */
+};
+
+
 VEC(mem_region_s) *mem_region_list, *target_mem_region_list;
 static int mem_number = 0;
 
@@ -53,6 +65,25 @@
    empty, then the target can't supply memory regions.  */
 static int target_mem_regions_valid;
 
+/* If this flag is set, gdb will assume that memory ranges not
+   specified by the memory map have type MEM_NONE, and will
+   emit errors on all accesses to that memory.  */
+static int inaccessible_by_default = 0;
+
+static void
+show_inaccessible_by_default (struct ui_file *file, int from_tty,
+			      struct cmd_list_element *c,
+			      const char *value)
+{
+  if (inaccessible_by_default)
+    fprintf_filtered (file, _("\
+Unknown memory addresses will be treated as inaccessible.\n"));
+  else
+    fprintf_filtered (file, _("\
+Unknown memory addresses will be treated as RAM.\n"));          
+}
+
+
 /* Predicate function which returns true if LHS should sort before RHS
    in a list of memory regions, useful for VEC_lower_bound.  */
 
@@ -215,13 +246,17 @@
   lo = 0;
   hi = 0;
 
-  /* If we ever want to support a huge list of memory regions, this
+  /* Either find memory range containing ADDRESS, or set LO and HI
+     to the nearest boundaries of an existing memory range.
+     
+     If we ever want to support a huge list of memory regions, this
      check should be replaced with a binary search (probably using
      VEC_lower_bound).  */
   for (ix = 0; VEC_iterate (mem_region_s, mem_region_list, ix, m); ix++)
     {
       if (m->enabled_p == 1)
 	{
+	  /* If address is in memory return, return that memory range.  */
 	  if (addr >= m->lo && (addr < m->hi || m->hi == 0))
 	    return m;
 
@@ -243,7 +278,15 @@
      was learned above.  */
   region.lo = lo;
   region.hi = hi;
-  region.attrib = default_mem_attrib;
+
+  /* When no memory map is defined at all, we always set 'default_mem_attrib',
+     so that we do not make the all memory inaccessible for targets that don't
+     provide a memory map.  */
+  if (inaccessible_by_default && !VEC_empty (mem_region_s, mem_region_list))
+    region.attrib = unknown_mem_attrib;
+  else
+    region.attrib = default_mem_attrib;
+
   return &region;
 }
 
@@ -674,9 +717,17 @@
 
   dont_repeat ();
 }
+
+static void
+dummy_cmd (char *args, int from_tty)
+{
+}
 
 extern initialize_file_ftype _initialize_mem; /* -Wmissing-prototype */
 
+static struct cmd_list_element *mem_set_cmdlist;
+static struct cmd_list_element *mem_show_cmdlist;
+
 void
 _initialize_mem (void)
 {
@@ -709,4 +760,25 @@
 
   add_info ("mem", mem_info_command,
 	    _("Memory region attributes"));
+
+  add_prefix_cmd ("mem", class_vars, dummy_cmd, _("\
+Memory regions settings"),
+		  &mem_set_cmdlist, "set mem ",
+		  0/* allow-unknown */, &setlist);
+  add_prefix_cmd ("mem", class_vars, dummy_cmd, _("\
+Memory regions settings"),
+		  &mem_show_cmdlist, "show mem  ",
+		  0/* allow-unknown */, &showlist);
+
+  add_setshow_boolean_cmd ("inaccessible-by-default", no_class,
+				  &inaccessible_by_default, _("\
+Set handling of unknown memory regions."), _("\
+Show handling of unknown memory regions."), _("\
+If on, and some memory map is defined, debugger will emit errors on\n\
+accesses to memory not defined in the memory map. If off, accesses to all\n\
+memory addresses will be allowed."),
+				NULL,
+				show_inaccessible_by_default,
+				&mem_set_cmdlist,
+				&mem_show_cmdlist);
 }
=== gdb/memattr.h
==================================================================
--- gdb/memattr.h	(/mirrors/gdb_mainline)	(revision 2094)
+++ gdb/memattr.h	(/patches/gdb/memcheck/gdb_mainline)	(revision 2094)
@@ -26,6 +26,7 @@
 
 enum mem_access_mode
 {
+  MEM_NONE,                     /* Memory that is not physically present. */
   MEM_RW,			/* read/write */
   MEM_RO,			/* read only */
   MEM_WO,			/* write only */
@@ -76,7 +77,10 @@
 
 struct mem_region 
 {
+  /* Lowest address in the region.  */
   CORE_ADDR lo;
+  /* Address past the highest address of the region. 
+     If 0, upper bound is "infinity".  */
   CORE_ADDR hi;
 
   /* Item number of this memory region. */