diff --git a/src/arch/x86/assembly_entry.S b/src/arch/x86/assembly_entry.S
index 0d8307bb1f..31670c29b6 100644
--- a/src/arch/x86/assembly_entry.S
+++ b/src/arch/x86/assembly_entry.S
@@ -15,12 +15,22 @@
 	#define _STACK_TOP _ecar_stack
 #endif
 
+#ifdef __x86_64__
+.code64
+#else
+.code32
+#endif
+
 .section ".text._start", "ax", @progbits
 .global _start
 _start:
 
 	/* Migrate GDT to this text segment */
+#ifdef __x86_64__
+	call	gdt_init64
+#else
 	call	gdt_init
+#endif
 
 	/* reset stack pointer to CAR/EARLYRAM stack */
 	mov	$_STACK_TOP, %esp
diff --git a/src/arch/x86/gdt_init.S b/src/arch/x86/gdt_init.S
index 7dd4b94933..1558ac62c8 100644
--- a/src/arch/x86/gdt_init.S
+++ b/src/arch/x86/gdt_init.S
@@ -20,7 +20,20 @@ gdtptr:
 .section ".text._gdt64_", "ax", @progbits
 	.globl gdt_init64
 gdt_init64:
-	lgdt	gdtptr64
+	/* Workaround a bug in the assembler.
+	 * The following code doesn't work:
+	 * 	lgdt gdtptr64
+	 *
+	 * The assembler tries to save memory by using 32bit displacement addressing mode.
+	 * Displacements are using signed integers.
+	 * This is fine in protected mode, as the negative address points to the correct
+	 * address > 2GiB, but in long mode this doesn't work at all.
+	 * Tests showed that QEMU can gracefully handle it, but real CPUs can't.
+	 *
+	 * Use the movabs pseudo instruction to force using a 64bit absolute address.
+	 */
+	movabs	$gdtptr64, %rax
+	lgdt	(%rax)
 	ret
 
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