diff --git a/src/system/kernel/arch/x86/arch_cpu.cpp b/src/system/kernel/arch/x86/arch_cpu.cpp
index 4fc23e4487..bcc9050cba 100644
--- a/src/system/kernel/arch/x86/arch_cpu.cpp
+++ b/src/system/kernel/arch/x86/arch_cpu.cpp
@@ -113,15 +113,12 @@ extern int memcpy_generic_end;
 extern "C" void memset_generic(void* dest, int value, size_t count);
 extern int memset_generic_end;
 
-// TODO x86_64
-#ifndef __x86_64__
 x86_optimized_functions gOptimizedFunctions = {
 	memcpy_generic,
 	&memcpy_generic_end,
 	memset_generic,
 	&memset_generic_end
 };
-#endif
 
 
 static status_t
diff --git a/src/system/kernel/lib/arch/x86_64/Jamfile b/src/system/kernel/lib/arch/x86_64/Jamfile
index 2e43cfd19a..66f5fb864f 100644
--- a/src/system/kernel/lib/arch/x86_64/Jamfile
+++ b/src/system/kernel/lib/arch/x86_64/Jamfile
@@ -1,5 +1,9 @@
 SubDir HAIKU_TOP src system kernel lib arch x86_64 ;
 
+# find the generated asm_offsets.h
+SubDirHdrs [ FDirName $(TARGET_COMMON_DEBUG_OBJECT_DIR) system kernel arch
+	$(TARGET_KERNEL_ARCH) ] ;
+
 SEARCH_SOURCE += [ FDirName $(SUBDIR) $(DOTDOT) generic ] ;
 
 local librootSources = [ FDirName $(HAIKU_TOP) src system libroot ] ;
@@ -24,7 +28,12 @@ KernelMergeObject kernel_lib_posix_arch_$(TARGET_ARCH).o :
 	kernel_longjmp_return.c
 	kernel_setjmp_save_sigs.c
 
-	arch_string.cpp
+	arch_string.S
 
 	: $(TARGET_KERNEL_PIC_CCFLAGS)
 ;
+
+# Explicitly tell the build system that arch_string.S includes the generated
+# asm_offsets.h.
+Includes [ FGristFiles arch_string.S ]
+	: <src!system!kernel!arch!x86>asm_offsets.h ;
diff --git a/src/system/kernel/lib/arch/x86_64/arch_string.S b/src/system/kernel/lib/arch/x86_64/arch_string.S
new file mode 100644
index 0000000000..a24bbc8ff6
--- /dev/null
+++ b/src/system/kernel/lib/arch/x86_64/arch_string.S
@@ -0,0 +1,96 @@
+/*
+ * Copyright 2012, Alex Smith, alex@alex-smith.me.uk.
+ * Distributed under the terms of the MIT License.
+ */
+
+
+#include <asm_defs.h>
+
+#include "asm_offsets.h"
+
+
+.align 8
+FUNCTION(memcpy_generic):
+	push	%rbp
+	movq	%rsp, %rbp
+
+	// Preserve original destination address for return value.
+	movq	%rdi, %rax
+
+	// size -> %rcx
+	movq	%rdx, %rcx
+
+	// For small copies, always do it bytewise, the additional overhead is
+	// not worth it.
+	cmp		$24, %rcx
+	jl		.Lmemcpy_generic_byte_copy
+
+	// Do both source and dest have the same alignment?
+	movq	%rsi, %r8
+	xorq	%rdi, %r8
+	test	$7, %r8
+	jnz		.Lmemcpy_generic_byte_copy
+
+	// Align up to an 8-byte boundary.
+	movq	%rdi, %r8
+	andq	$7, %r8
+	jz		.Lmemcpy_generic_qword_copy
+	movq	$8, %rcx
+	subq	%r8, %rcx
+	subq	%rcx, %rdx			// Subtract from the overall count.
+	rep
+	movsb
+
+	// Get back the original count value.
+	movq	%rdx, %rcx
+.Lmemcpy_generic_qword_copy:
+	// Move by quadwords.
+	shrq	$3, %rcx
+	rep
+	movsq
+
+	// Get the remaining count.
+	movq	%rdx, %rcx
+	andq	$7, %rcx
+.Lmemcpy_generic_byte_copy:
+	// Move any remaining data by bytes.
+	rep
+	movsb
+
+	pop		%rbp
+	ret
+FUNCTION_END(memcpy_generic)
+SYMBOL(memcpy_generic_end):
+
+
+.align 8
+FUNCTION(memset_generic):
+	push	%rbp
+	movq	%rsp, %rbp
+
+	// Preserve original destination address for return value.
+	movq	%rdi, %r8
+
+	// size -> %rcx, value -> %al
+	movq	%rdx, %rcx
+	movl	%esi, %eax
+
+	// Move by bytes.
+	rep
+	stosb
+
+	movq	%r8, %rax
+	pop		%rbp
+	ret
+FUNCTION_END(memset_generic)
+SYMBOL(memset_generic_end):
+
+
+FUNCTION(memcpy):
+	jmp		*(gOptimizedFunctions + X86_OPTIMIZED_FUNCTIONS_memcpy)
+FUNCTION_END(memcpy)
+
+FUNCTION(memset):
+	jmp		*(gOptimizedFunctions + X86_OPTIMIZED_FUNCTIONS_memset)
+FUNCTION_END(memset)
+
diff --git a/src/system/kernel/lib/arch/x86_64/arch_string.cpp b/src/system/kernel/lib/arch/x86_64/arch_string.cpp
deleted file mode 100644
index a3d6c8b8a4..0000000000
--- a/src/system/kernel/lib/arch/x86_64/arch_string.cpp
+++ /dev/null
@@ -1,36 +0,0 @@
-/*
- * Copyright 2012, Alex Smith, alex@alex-smith.me.uk.
- * Distributed under the terms of the MIT License.
-*/
-
-// TODO: Replace these with optimized implementations.
-
-
-#include <string.h>
-
-
-void *
-memcpy(void *dest, const void *src, size_t count)
-{
-	const unsigned char *s = reinterpret_cast<const unsigned char *>(src);
-	unsigned char *d = reinterpret_cast<unsigned char *>(dest);
-
-	for (; count != 0; count--) {
-		*d++ = *s++;
-	}
-
-	return dest;
-}
-
-
-void *
-memset(void *dest, int val, size_t count)
-{
-	unsigned char *d = reinterpret_cast<unsigned char *>(dest);
-
-	for (; count != 0; count--) {
-		*d++ = static_cast<unsigned char>(val);
-	}
-
-	return dest;
-}