diff --git a/.gitignore b/.gitignore
index fe8b1fd..975d0c4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,11 +1,10 @@
 build
 target
 /kernel/src/arch/x86_64/interrupt/vector.asm
-/crate/bit-allocator/Cargo.lock
-/crate/memory/Cargo.lock
-/crate/bbl/Cargo.lock
-/crate/sync/Cargo.lock
-/crate/process/Cargo.lock
+
+Cargo.lock
+!kernel/Cargo.lock
+!user/Cargo.lock
 
 .DS_Store
 
diff --git a/crate/atags/Cargo.lock b/crate/atags/Cargo.lock
deleted file mode 100644
index 42d11eb..0000000
--- a/crate/atags/Cargo.lock
+++ /dev/null
@@ -1,14 +0,0 @@
-[[package]]
-name = "atags"
-version = "0.1.0"
-dependencies = [
- "volatile 0.2.5 (registry+https://github.com/rust-lang/crates.io-index)",
-]
-
-[[package]]
-name = "volatile"
-version = "0.2.5"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-
-[metadata]
-"checksum volatile 0.2.5 (registry+https://github.com/rust-lang/crates.io-index)" = "d9ca391c55768e479d5c2f8beb40c136df09257292a809ea514e82cfdfc15d00"
diff --git a/kernel/Makefile b/kernel/Makefile
index ef57cea..3f4f76b 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -12,6 +12,7 @@
 #   d    = int | in_asm | ...	QEMU debug info
 #   mode = debug | release
 #   LOG  = off | error | warn | info | debug | trace
+#   SFSIMG = SFS image path of user programs
 #   smp                         SMP core number
 #   board = fpga				Only available on riscv32, build without bbl, run on board
 #         | raspi3				Only available on aarch64, run on Raspberry Pi 3 Model B/B+
@@ -31,6 +32,7 @@ user_dir := ../user
 user_bin_path := $(user_dir)/target/$(arch)-ucore/debug
 user_bins := $(patsubst $(user_bin_path)/%.d, $(user_bin_path)/%, $(wildcard $(user_bin_path)/*.d))
 user_obj := build/$(arch)/user.o
+sfsroot := $(user_dir)/build/sfsroot-$(arch)
 sfsimg := $(user_dir)/build/user-$(arch).img
 
 export ARCH = $(arch)
@@ -40,7 +42,7 @@ export SFSIMG = $(sfsimg)
 ifeq ($(arch), x86_64)
 qemu_opts := \
 	-drive format=raw,file=$(bootimage) \
-	-drive format=raw,file=$(sfsimg),media=disk,cache=writeback \
+	-drive format=raw,file=$(SFSIMG),media=disk,cache=writeback \
 	-smp cores=$(smp) \
 	-serial mon:stdio \
 	-device isa-debug-exit \
@@ -176,7 +178,7 @@ endif
 
 
 ### user programs ###
-$(sfsimg):
+$(sfsimg): $(sfsroot)
 	@cd $(user_dir) && make sfsimg
 
 # make user.o from binary files
diff --git a/kernel/build.rs b/kernel/build.rs
index ec94d4e..e85f6f3 100644
--- a/kernel/build.rs
+++ b/kernel/build.rs
@@ -1,23 +1,42 @@
 extern crate cc;
 
 use std::fs::File;
-use std::io::{Write, Result};
+use std::path::Path;
+use std::io::{Result, Write};
 
 fn main() {
-	if std::env::var("TARGET").unwrap().find("x86_64").is_some() {
-//		cc::Build::new()
-//			.file("src/arch/x86_64/driver/apic/lapic.c")
-//			.file("src/arch/x86_64/driver/keyboard/keyboard.c")
-//			.flag("-mcmodel=large")
-//			.compile("cobj");
-		gen_vector_asm().unwrap();
-	}
-	if std::env::var("TARGET").unwrap().find("riscv32").is_some() {
-		cc::Build::new()
-			.file("src/arch/riscv32/compiler_rt.c")
-			.flag("-march=rv32ia")
-			.flag("-mabi=ilp32")
-			.compile("atomic_rt");
+	println!("cargo:rerun-if-env-changed=LOG");
+
+	let arch: String = std::env::var("ARCH").unwrap();
+	match arch.as_str() {
+		"x86_64" => {
+			// cc::Build::new()
+			// 	.file("src/arch/x86_64/driver/apic/lapic.c")
+			// 	.file("src/arch/x86_64/driver/keyboard/keyboard.c")
+			// 	.flag("-mcmodel=large")
+			// 	.compile("cobj");
+			gen_vector_asm().unwrap();
+		}
+		"riscv32" => {
+			cc::Build::new()
+				.file("src/arch/riscv32/compiler_rt.c")
+				.flag("-march=rv32ia")
+				.flag("-mabi=ilp32")
+				.compile("atomic_rt");
+			if let Ok(file_path) = gen_sfsimg_asm() {
+				cc::Build::new()
+					.file(&file_path)
+					.flag("-march=rv32ia")
+					.flag("-mabi=ilp32")
+					.compile("sfsimg");
+			}
+		}
+		"aarch64" => {
+			if let Ok(file_path) = gen_sfsimg_asm() {
+				cc::Build::new().file(&file_path).compile("cobj");
+			}
+		}
+		_ => panic!("Unknown arch {}", arch),
 	}
 }
 
@@ -43,4 +62,28 @@ fn gen_vector_asm() -> Result<()> {
 		writeln!(f, "\t.quad vector{}", i)?;
 	}
 	Ok(())
-}
\ No newline at end of file
+}
+
+fn gen_sfsimg_asm() -> Result<std::path::PathBuf> {
+	let out_dir = std::env::var("OUT_DIR").unwrap();
+	let sfsimg = std::env::var("SFSIMG").unwrap();
+
+	let file_path = Path::new(&out_dir).join("sfsimg.S");
+	let mut f = File::create(&file_path).unwrap();
+
+	write!(f, "# generated by build.rs - do not edit")?;
+	write!(f, r#"
+	.section .rodata
+	.align 12
+	.global _user_img_start
+	.global _user_img_end
+_user_img_start:
+    .incbin "{}"
+_user_img_end:
+"#, sfsimg)?;
+
+	println!("cargo:rerun-if-changed={}", sfsimg);
+	println!("cargo:rerun-if-env-changed=SFSIMG");
+
+	Ok(file_path)
+}
diff --git a/kernel/src/fs.rs b/kernel/src/fs.rs
index 9fea7d7..a1a52f2 100644
--- a/kernel/src/fs.rs
+++ b/kernel/src/fs.rs
@@ -10,32 +10,19 @@ use crate::arch::driver::ide;
 use crate::sync::Condvar;
 use crate::sync::SpinNoIrqLock as Mutex;
 
-// Hard link user program
-#[cfg(target_arch = "riscv32")]
-global_asm!(r#"
-    .section .rodata
-    .align 12
-    .global _user_img_start
-    .global _user_img_end
-_user_img_start:
-    .incbin "../user/user-riscv.img"
-_user_img_end:
-"#);
-
 lazy_static! {
     pub static ref ROOT_INODE: Arc<INode> = {
-        #[cfg(target_arch = "riscv32")]
+        #[cfg(any(target_arch = "riscv32", target_arch = "aarch64"))]
         let device = {
             extern {
                 fn _user_img_start();
                 fn _user_img_end();
             }
+            // Hard link user program
             Box::new(unsafe { MemBuf::new(_user_img_start, _user_img_end) })
         };
         #[cfg(target_arch = "x86_64")]
         let device = Box::new(ide::IDE::new(1));
-        #[cfg(target_arch = "aarch64")]
-        let device = unimplemented!();
 
         let sfs = SimpleFileSystem::open(device).expect("failed to open SFS");
         sfs.root_inode()
diff --git a/kernel/src/shell.rs b/kernel/src/shell.rs
index 1dcd521..b8443f3 100644
--- a/kernel/src/shell.rs
+++ b/kernel/src/shell.rs
@@ -6,12 +6,15 @@ use crate::fs::{ROOT_INODE, INodeExt};
 use crate::process::*;
 
 pub fn run_user_shell() {
-    let inode = ROOT_INODE.lookup("sh").unwrap();
-    let data = inode.read_as_vec().unwrap();
-    processor().manager().add(ContextImpl::new_user(data.as_slice(), "sh".split(' ')), 0);
+    if let Ok(inode) = ROOT_INODE.lookup("sh") {
+        let data = inode.read_as_vec().unwrap();
+        processor().manager().add(ContextImpl::new_user(data.as_slice(), "sh".split(' ')), 0);
+    } else {
+        processor().manager().add(ContextImpl::new_kernel(shell, 0), 0);
+    }
 }
 
-pub fn shell() {
+pub extern fn shell(_arg: usize) -> ! {
     let files = ROOT_INODE.list().unwrap();
     println!("Available programs: {:?}", files);