diff --git a/src/lib.rs b/src/lib.rs
index b4ca55ac9c..1186760eae 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,5 +1,21 @@
 #![no_std]
-#![feature(alloc_error_handler, core_intrinsics, lang_items, panic_info_message)]
+#![feature(
+    asm_const,
+    asm_sym,
+    alloc_error_handler,
+    core_intrinsics,
+    lang_items,
+    naked_functions,
+    panic_info_message
+)]
+
+#[cfg(target_arch = "x86")]
+#[path = "i686.rs"]
+pub mod arch;
+
+#[cfg(target_arch = "x86_64")]
+#[path = "x86_64.rs"]
+pub mod arch;
 
 pub mod exec;
 pub mod initfs;
diff --git a/src/x86_64.asm b/src/x86_64.asm
deleted file mode 100644
index e8e49136c8..0000000000
--- a/src/x86_64.asm
+++ /dev/null
@@ -1,42 +0,0 @@
-BITS 64
-
-section .text
-
-global ustart
-extern start
-
-ustart:
-  ; Setup a stack.
-  mov rax, 0x21000384 ; SYS_CLASS_FILE+SYS_ARG_SLICE+SYS_MMAP
-  mov rdi, 0xFFFFFFFFFFFFFFFF ; dummy `fd`, indicates anonymous map
-  mov rsi, map ; pointer to Map struct
-  mov rdx, map_size ; size of Map struct
-  syscall
-
-  ; Test for success (nonzero value).
-  cmp rax, 0
-  jg .continue
-  ; (failure)
-  ud2
-.continue:
-  ; Subtract 16 since all instructions seem to hate non-canonical RSP values :)
-  lea rsp, [rax+size-16]
-  mov rbp, rsp
-
-  ; Stack has the same alignment as `size`.
-  call start
-  ; `start` must never return.
-  ud2
-
-section .rodata
-map:
-  dq 0 ; offset (unused)
-  dq size
-  dq flags
-  dq address - size
-
-map_size equ $ - map
-
-size equ 65536 ; 64 KiB
-address equ 0x0000800000000000 ; highest possible (normal) user address, why not
-flags equ 0x0006000E ; PROT_READ+PROT_WRITE, MAP_PRIVATE+MAP_FIXED_NOREPLACE
diff --git a/src/x86_64.rs b/src/x86_64.rs
new file mode 100644
index 0000000000..f90f651ed8
--- /dev/null
+++ b/src/x86_64.rs
@@ -0,0 +1,53 @@
+use core::mem;
+use syscall::{
+    data::Map,
+    flag::MapFlags,
+    number::SYS_FMAP,
+};
+
+const STACK_SIZE: usize = 64 * 1024; // 64 KiB
+static MAP: Map = Map {
+    offset: 0,
+    size: STACK_SIZE,
+    flags: MapFlags::PROT_READ
+            .union(MapFlags::PROT_WRITE)
+            .union(MapFlags::MAP_PRIVATE)
+            .union(MapFlags::MAP_FIXED_NOREPLACE),
+    address: 0x0000_8000_0000_0000 - STACK_SIZE, // highest possible user address
+};
+
+#[naked]
+#[no_mangle]
+pub unsafe extern "C" fn ustart() {
+    core::arch::asm!(
+        "
+        # Setup a stack.
+        mov rax, {number}
+        mov rdi, {fd}
+        lea rsi, {map} # pointer to Map struct
+        mov rdx, {map_size} # size of Map struct
+        syscall
+
+        # Test for success (nonzero value).
+        cmp rax, 0
+        jg 1f
+        # (failure)
+        ud2
+        1:
+        # Subtract 16 since all instructions seem to hate non-canonical RSP values :)
+        lea rsp, [rax+{stack_size}-16]
+        mov rbp, rsp
+
+        # Stack has the same alignment as `size`.
+        call start
+        # `start` must never return.
+        ud2
+        ",
+        fd = const usize::MAX, // dummy fd indicates anonymous map
+        map = sym MAP,
+        map_size = const mem::size_of::<Map>(),
+        number = const SYS_FMAP,
+        stack_size = const STACK_SIZE,
+        options(noreturn),
+    );
+}
\ No newline at end of file