diff --git a/src/arch/i386/firmware/pcbios/hidemem.c b/src/arch/i386/firmware/pcbios/hidemem.c
index 3a57fcba..3396e1e6 100644
--- a/src/arch/i386/firmware/pcbios/hidemem.c
+++ b/src/arch/i386/firmware/pcbios/hidemem.c
@@ -16,12 +16,41 @@
  */
 
 #include <realmode.h>
+#include <bios.h>
 #include <biosint.h>
 #include <gpxe/hidemem.h>
 
-/* Linker-defined symbols */
-extern char _text[];
-extern char _end[];
+/** Alignment for hidden memory regions */
+#define ALIGN_HIDDEN 4096   /* 4kB page alignment should be enough */
+
+/**
+ * A hidden region of Etherboot
+ *
+ * This represents a region that will be edited out of the system's
+ * memory map.
+ *
+ * This structure is accessed by assembly code, so must not be
+ * changed.
+ */
+struct hidden_region {
+	/* Physical start address */
+	physaddr_t start;
+	/* Physical end address */
+	physaddr_t end;
+};
+
+/**
+ * List of hidden regions
+ *
+ * Must be terminated by a zero entry.
+ */
+struct hidden_region __data16_array ( hidden_regions, [] ) = {
+	[TEXT] = { 0, 0 },
+	[BASEMEM] = { ( 640 * 1024 ), ( 640 * 1024 ) },
+	[EXTMEM] = { 0, 0 },
+	{ 0, 0, } /* Terminator */
+};
+#define hidden_regions __use_data16 ( hidden_regions )
 
 /** Assembly routine in e820mangler.S */
 extern void int15();
@@ -31,16 +60,50 @@ extern struct segoff __text16 ( int15_vector );
 #define int15_vector __use_text16 ( int15_vector )
 
 /**
- * List of hidden regions
+ * Hide region of memory from system memory map
  *
- * Must be terminated by a zero entry.
+ * @v start		Start of region
+ * @v end		End of region
  */
-struct hidden_region __data16_array ( hidden_regions, [] ) = {
-	[TEXT] = { 0, 0 },
-	[BASEMEM] = { 0, ( 640 * 1024 ) },
-	[EXTMEM] = { 0, 0 },
-	{ 0, 0, } /* Terminator */
-};
+void hide_region ( unsigned int region_id, physaddr_t start, physaddr_t end ) {
+	struct hidden_region *region = &hidden_regions[region_id];
+
+	/* Some operating systems get a nasty shock if a region of the
+	 * E820 map seems to start on a non-page boundary.  Make life
+	 * safer by rounding out our edited region.
+	 */	
+	region->start = ( start & ~( ALIGN_HIDDEN - 1 ) );
+	region->end = ( ( end + ALIGN_HIDDEN - 1 ) & ~( ALIGN_HIDDEN - 1 ) );
+	DBG ( "Hiding [%lx,%lx)\n", region->start, region->end );
+}
+
+/**
+ * Hide Etherboot text
+ *
+ */
+static void hide_text ( void ) {
+
+	/* The linker defines these symbols for us */
+	extern char _text[];
+	extern char _end[];
+
+	hide_region ( TEXT, virt_to_phys ( _text ), virt_to_phys ( _end ) );
+}
+
+/**
+ * Hide used base memory
+ *
+ */
+static void hide_basemem ( void ) {
+	uint16_t fbms;
+
+	/* Hide from the top of free base memory to 640kB.  Don't use
+	 * hide_region(), because we don't want this rounded to the
+	 * nearest page boundary.
+	 */
+	get_real ( fbms, BDA_SEG, BDA_FBMS );
+	hidden_regions[BASEMEM].start = ( fbms * 1024 );
+}
 
 /**
  * Hide Etherboot
@@ -49,16 +112,12 @@ struct hidden_region __data16_array ( hidden_regions, [] ) = {
  * returned by the BIOS.
  */
 void hide_etherboot ( void ) {
-	hidden_regions[TEXT].start = virt_to_phys ( _text );
-	hidden_regions[TEXT].end = virt_to_phys ( _end );
-	hidden_regions[BASEMEM].start = ( rm_cs << 4 );
 
-	DBG ( "Hiding [%lx,%lx) and [%lx,%lx)\n",
-	      ( unsigned long ) hidden_regions[TEXT].start,
-	      ( unsigned long ) hidden_regions[TEXT].end,
-	      ( unsigned long ) hidden_regions[BASEMEM].start,
-	      ( unsigned long ) hidden_regions[BASEMEM].end );
+	/* Initialise the hidden regions */
+	hide_text();
+	hide_basemem();
 
+	/* Hook INT 15 */
 	hook_bios_interrupt ( 0x15, ( unsigned int ) int15,
 			      &int15_vector );
 }
diff --git a/src/include/gpxe/hidemem.h b/src/include/gpxe/hidemem.h
index 89109004..547f8881 100644
--- a/src/include/gpxe/hidemem.h
+++ b/src/include/gpxe/hidemem.h
@@ -8,38 +8,16 @@
  *
  */
 
-/**
- * A hidden region of Etherboot
- *
- * This represents a region that will be edited out of the system's
- * memory map.
- *
- * This structure is accessed by assembly code, so must not be
- * changed.
- */
-struct hidden_region {
-	/* Physical start address */
-	physaddr_t start;
-	/* Physical end address */
-	physaddr_t end;
-};
-
 /**
  * Unique IDs for hidden regions
  */
-enum {
+enum hidemem_region_id {
 	TEXT = 0,
 	BASEMEM,
 	EXTMEM,
 };
 
-extern struct hidden_region __data16_array ( hidden_regions, [] );
-#define hidden_regions __use_data16 ( hidden_regions )
-
-static inline void hide_region ( unsigned int region,
-				 physaddr_t start, physaddr_t end ) {
-	hidden_regions[region].start = start;
-	hidden_regions[region].end = end;
-}
+extern void hide_region ( unsigned int region_id, physaddr_t start,
+			  physaddr_t end );
 
 #endif /* _GPXE_HIDEMEM_H */