diff --git a/src/arch/i386/prefix/libprefix.S b/src/arch/i386/prefix/libprefix.S
index df71a9d7..6a23a96c 100644
--- a/src/arch/i386/prefix/libprefix.S
+++ b/src/arch/i386/prefix/libprefix.S
@@ -512,13 +512,24 @@ install_prealloc:
 
 	/* Open up access to payload */
 #ifndef KEEP_IT_REAL
-	/* Flatten real mode */
+	/* Access high memory */
 	pushw	%cs
 	pushw	$1f
 	pushw	%ax
-	pushw	$flatten_real_mode
+	pushw	$access_highmem
 	lret
-1:
+1:	/* Die if we could not access high memory */
+	jnc	3f
+	movw	$a20_death_message, %si
+	xorw	%di, %di
+	call	print_message
+2:	jmp	2b
+	.section ".prefix.data", "aw", @progbits
+a20_death_message:
+	.asciz	"Gate A20 stuck - cannot continue\n"
+	.size	a20_death_message, . - a20_death_message
+	.previous
+3:
 #endif
 
 	/* Calculate physical address of payload (i.e. first source) */
@@ -570,13 +581,13 @@ install_prealloc:
 	popl	%edx /* discard */
 
 	/* Copy code to new location */
+	pushl	%edi
 	xorw	%ax, %ax
 	movw	%ax, %es
-	movl	%ebp, %edi
 	es rep addr32 movsb
+	popl	%edi
 
 	/* Initialise librm at new location */
-	movl	%ebp, %edi
 	lcall	*init_librm_vector
 #endif
 
diff --git a/src/arch/i386/transitions/libflat.S b/src/arch/i386/transitions/libflat.S
index 9cb4c8af..9062b74f 100644
--- a/src/arch/i386/transitions/libflat.S
+++ b/src/arch/i386/transitions/libflat.S
@@ -24,7 +24,7 @@ FILE_LICENCE ( GPL2_OR_LATER )
 #define CR0_PE 1
 
 /****************************************************************************
- * flatten_real_mode (real-mode far call)
+ * flatten_real_mode
  *
  * Set up 4GB segment limits
  *
@@ -63,7 +63,6 @@ flatten_saved_gdt:
 
 	.section ".text16.early", "awx", @progbits
 	.code16
-	.globl	flatten_real_mode
 flatten_real_mode:
 	/* Preserve registers and flags */
 	pushfl
@@ -126,7 +125,7 @@ flatten_real_mode:
 	popw	%si
 	popl	%eax
 	popfl
-	lret
+	ret
 	.size flatten_real_mode, . - flatten_real_mode
 
 	.section ".text16.early", "awx", @progbits
@@ -139,3 +138,281 @@ set_seg_base:
 	andb	$0x0f, 4(%si)
 	ret
 	.size set_seg_base, . - set_seg_base
+
+/****************************************************************************
+ * test_a20_short, test_a20_long
+ *
+ * Check to see if A20 line is enabled
+ *
+ * Parameters:
+ *   none
+ * Returns:
+ *   CF set if A20 line is not enabled
+ * Corrupts:
+ *   none
+ ****************************************************************************
+ */
+#define TEST_A20_SHORT_MAX_RETRIES 0x20
+#define TEST_A20_LONG_MAX_RETRIES 0x200000
+	.section ".text16.early", "awx", @progbits
+	.code16
+test_a20_short:
+	pushl	%ecx
+	movl	$TEST_A20_SHORT_MAX_RETRIES, %ecx
+	jmp	1f
+	.size	test_a20_short, . - test_a20_short
+test_a20_long:
+	pushl	%ecx
+	movl	$TEST_A20_LONG_MAX_RETRIES, %ecx
+1:	pushw	%ax
+
+	/* Flatten real mode so we can access the test pattern's 1MB offset */
+	call	flatten_real_mode
+
+2:	/* Modify and check test pattern; succeed if we see a difference */
+	incw	%cs:test_a20_data
+	addr32 movw %cs:(test_a20_data + 0x100000 ), %ax
+	cmpw	%cs:test_a20_data, %ax
+	clc
+	jnz	99f
+
+	/* Delay and retry */
+	outb	%al, $0x80
+	addr32 loop 2b
+	stc
+
+99:	/* Restore registers and return */
+	popw	%ax
+	popl	%ecx
+	ret
+	.size	test_a20_long, . - test_a20_long
+
+	.section ".text16.early.data", "aw", @progbits
+	.align	2
+test_a20_data:
+	.word	0xdead
+	.size	test_a20_data, . - test_a20_data
+
+/****************************************************************************
+ * enable_a20_bios
+ *
+ * Try enabling A20 line via BIOS
+ *
+ * Parameters:
+ *   none
+ * Returns:
+ *   CF set if A20 line is not enabled
+ * Corrupts:
+ *   none
+ ****************************************************************************
+ */
+	.section ".text16.early", "awx", @progbits
+	.code16
+enable_a20_bios:
+	/* Preserve registers */
+	pushw	%ax
+
+	/* Attempt INT 15,2401 */
+	movw	$0x2401, %ax
+	int	$0x15
+	jc	99f
+
+	/* Check that success was really successful */
+	call	test_a20_short
+
+99:	/* Restore registers and return */
+	popw	%ax
+	ret
+	.size	enable_a20_bios, . - enable_a20_bios
+
+/****************************************************************************
+ * enable_a20_kbc
+ *
+ * Try enabling A20 line via keyboard controller
+ *
+ * Parameters:
+ *   none
+ * Returns:
+ *   CF set if A20 line is not enabled
+ * Corrupts:
+ *   none
+ ****************************************************************************
+ */
+#define KC_RDWR		0x60
+#define KC_RDWR_SET_A20		0xdf
+#define	KC_CMD		0x64
+#define KC_CMD_WOUT		0xd1
+#define KC_CMD_NULL		0xff
+#define KC_STATUS	0x64
+#define KC_STATUS_OBUF_FULL	0x01
+#define KC_STATUS_IBUF_FULL	0x02
+#define KC_MAX_RETRIES	100000
+	.section ".text16.early", "awx", @progbits
+	.code16
+enable_a20_kbc:
+	/* Preserve registers */
+	pushw	%ax
+
+	/* Try keyboard controller */
+	call	empty_kbc
+	movb	$KC_CMD_WOUT, %al
+	outb	%al, $KC_CMD
+	call	empty_kbc
+	movb	$KC_RDWR_SET_A20, %al
+	outb	%al, $KC_RDWR
+	call	empty_kbc
+	movb	$KC_CMD_NULL, %al
+	outb	%al, $KC_CMD
+	call	empty_kbc
+
+	/* Check to see if it worked */
+	call	test_a20_long
+
+	/* Restore registers and return */
+	popw	%ax
+	ret
+	.size	enable_a20_kbc, . - enable_a20_kbc
+
+	.section ".text16.early", "awx", @progbits
+	.code16
+empty_kbc:
+	/* Preserve registers */
+	pushl	%ecx
+	pushw	%ax
+
+	/* Wait for KBC to become empty */
+	movl	$KC_MAX_RETRIES, %ecx
+1:	outb	%al, $0x80
+	inb	$KC_STATUS, %al
+	testb	$( KC_STATUS_OBUF_FULL | KC_STATUS_IBUF_FULL ), %al
+	jz	99f
+	testb	$KC_STATUS_OBUF_FULL, %al
+	jz	2f
+	outb	%al, $0x80
+	inb	$KC_RDWR, %al
+2:	addr32 loop 1b
+
+99:	/* Restore registers and return */
+	popw	%ax
+	popl	%ecx
+	ret
+	.size	empty_kbc, . - empty_kbc
+
+/****************************************************************************
+ * enable_a20_fast
+ *
+ * Try enabling A20 line via "Fast Gate A20"
+ *
+ * Parameters:
+ *   none
+ * Returns:
+ *   CF set if A20 line is not enabled
+ * Corrupts:
+ *   none
+ ****************************************************************************
+ */
+#define SCP_A 0x92
+	.section ".text16.early", "awx", @progbits
+	.code16
+enable_a20_fast:
+	/* Preserve registers */
+	pushw	%ax
+
+	/* Try "Fast Gate A20" */
+	inb	$SCP_A, %al
+	orb	$0x02, %al
+	andb	$~0x01, %al
+	outb	%al, $SCP_A
+
+	/* Check to see if it worked */
+	call	test_a20_long
+
+	/* Restore registers and return */
+	popw	%ax
+	ret
+	.size	enable_a20_fast, . - enable_a20_fast
+
+/****************************************************************************
+ * enable_a20
+ *
+ * Try enabling A20 line via any available method
+ *
+ * Parameters:
+ *   none
+ * Returns:
+ *   CF set if A20 line is not enabled
+ * Corrupts:
+ *   none
+ ****************************************************************************
+ */
+#define ENABLE_A20_RETRIES 255
+	.section ".text16.early", "awx", @progbits
+	.code16
+enable_a20:
+	/* Preserve registers */
+	pushl	%ecx
+	pushw	%ax
+
+	/* Check to see if A20 is already enabled */
+	call	test_a20_short
+	jnc	99f
+
+	/* Use known working method, if we have one */
+	movw	%cs:enable_a20_method, %ax
+	testw	%ax, %ax
+	jz	1f
+	call	*%ax
+	jmp	99f
+1:
+	/* Try all methods in turn until one works */
+	movl	$ENABLE_A20_RETRIES, %ecx
+2:	movw	$enable_a20_bios, %ax
+	movw	%ax, %cs:enable_a20_method
+	call	*%ax
+	jnc	99f
+	movw	$enable_a20_kbc, %ax
+	movw	%ax, %cs:enable_a20_method
+	call	*%ax
+	jnc	99f
+	movw	$enable_a20_fast, %ax
+	movw	%ax, %cs:enable_a20_method
+	call	*%ax
+	jnc	99f
+	addr32 loop 2b
+	/* Failure; exit with carry set */
+	movw	$0, %cs:enable_a20_method
+	stc
+
+99:	/* Restore registers and return */
+	popw	%ax
+	popl	%ecx
+	ret
+
+	.section ".text16.early.data", "aw", @progbits
+	.align	2
+enable_a20_method:
+	.word	0
+	.size	enable_a20_method, . - enable_a20_method
+
+/****************************************************************************
+ * access_highmem (real mode far call)
+ *
+ * Open up access to high memory in flat real mode with A20 enabled
+ *
+ * Parameters:
+ *   none
+ * Returns:
+ *   CF set if high memory could not be accessed
+ * Corrupts:
+ *   none
+ ****************************************************************************
+ */
+	.section ".text16.early", "awx", @progbits
+	.code16
+	.globl	access_highmem
+access_highmem:
+	/* Enable A20 line */
+	call	enable_a20
+	/* CPU will be in flat real mode as a result of this call */
+	lret
+	.size	access_highmem, . - access_highmem