diff --git a/src/thread.zig b/src/thread.zig
index 319a7d5..424b01a 100644
--- a/src/thread.zig
+++ b/src/thread.zig
@@ -327,7 +327,7 @@ pub fn test_create_user_from_code(a: *arena.Arena, code: []const u8) Thread.Erro
 
     log.info("Enter with sp = 0x{x}", .{sp});
 
-    const thread = Thread.create_user(a, address_space, 0x200000, sp, 0);
+    const thread = Thread.create_user(a, address_space, 0x200000, sp, 1234);
 
     return thread;
 }
diff --git a/user/arch.zig b/user/arch.zig
new file mode 100644
index 0000000..0b726e9
--- /dev/null
+++ b/user/arch.zig
@@ -0,0 +1,9 @@
+const builtin = @import("builtin");
+
+const impl = switch (builtin.cpu.arch) {
+    .riscv64 => @import("arch/riscv64.zig"),
+    .aarch64 => @import("arch/aarch64.zig"),
+    else => @compileError("Unsupported architecture"),
+};
+
+pub const syscall = impl.syscall;
diff --git a/user/arch/aarch64.zig b/user/arch/aarch64.zig
new file mode 100644
index 0000000..69b0c24
--- /dev/null
+++ b/user/arch/aarch64.zig
@@ -0,0 +1,20 @@
+pub const syscall = struct {
+    pub fn syscall1(func: usize, arg0: usize) usize {
+        return asm volatile ("svc #0"
+            : [result] "={x0}" (-> usize),
+            : [arg0] "{x0}" (arg0),
+              [func] "{x8}" (func),
+            : "memory"
+        );
+    }
+
+    pub fn syscall2(func: usize, arg0: usize, arg1: usize) usize {
+        return asm volatile ("svc #0"
+            : [result] "={x0}" (-> usize),
+            : [arg0] "{x0}" (arg0),
+              [arg1] "{x1}" (arg1),
+              [func] "{x8}" (func),
+            : "memory"
+        );
+    }
+};
diff --git a/user/arch/riscv64.zig b/user/arch/riscv64.zig
new file mode 100644
index 0000000..dcfc0a2
--- /dev/null
+++ b/user/arch/riscv64.zig
@@ -0,0 +1,20 @@
+pub const syscall = struct {
+    pub fn syscall1(func: usize, arg0: usize) usize {
+        return asm volatile ("ecall"
+            : [result] "={a0}" (-> usize),
+            : [func] "{a0}" (func),
+              [arg0] "{a1}" (arg0),
+            : "memory"
+        );
+    }
+
+    pub fn syscall2(func: usize, arg0: usize, arg1: usize) usize {
+        return asm volatile ("ecall"
+            : [result] "={a0}" (-> usize),
+            : [func] "{a0}" (func),
+              [arg0] "{a1}" (arg0),
+              [arg1] "{a2}" (arg1),
+            : "memory"
+        );
+    }
+};
diff --git a/user/log.zig b/user/log.zig
new file mode 100644
index 0000000..69209fe
--- /dev/null
+++ b/user/log.zig
@@ -0,0 +1,28 @@
+const std = @import("std");
+const syscall = @import("syscall.zig");
+
+pub const BUFFER_SIZE: usize = 512;
+
+pub const Writer = struct {
+    buffer: [BUFFER_SIZE]u8 = undefined,
+    position: usize = 0,
+
+    fn write(self: *Writer, bytes: []const u8) error{}!usize {
+        const amount = @min(BUFFER_SIZE - self.position, bytes.len);
+        if (amount != 0) {
+            @memcpy(self.buffer[self.position .. self.position + amount], bytes[0..amount]);
+            self.position += amount;
+        }
+        return amount;
+    }
+};
+
+pub fn println(comptime format: []const u8, args: anytype) void {
+    const W = std.io.GenericWriter(*Writer, error{}, Writer.write);
+    var context = Writer{};
+    var w = W{ .context = &context };
+    w.print(format, args) catch return;
+    if (context.position != 0) {
+        syscall.debug_print(context.buffer[0..context.position]);
+    }
+}
diff --git a/user/main.zig b/user/main.zig
index 9fd9b64..4d05b1a 100644
--- a/user/main.zig
+++ b/user/main.zig
@@ -1,59 +1,8 @@
-const builtin = @import("builtin");
+pub const arch = @import("arch.zig");
+pub const syscall = @import("syscall.zig");
+pub const log = @import("log.zig");
 
-const syscall = switch (builtin.cpu.arch) {
-    .aarch64 => struct {
-        pub fn syscall1(func: usize, arg0: usize) usize {
-            return asm volatile ("svc #0"
-                : [result] "={x0}" (-> usize),
-                : [arg0] "{x0}" (arg0),
-                  [func] "{x8}" (func),
-                : "memory"
-            );
-        }
-
-        pub fn syscall2(func: usize, arg0: usize, arg1: usize) usize {
-            return asm volatile ("svc #0"
-                : [result] "={x0}" (-> usize),
-                : [arg0] "{x0}" (arg0),
-                  [arg1] "{x1}" (arg1),
-                  [func] "{x8}" (func),
-                : "memory"
-            );
-        }
-    },
-    .riscv64 => struct {
-        pub fn syscall1(func: usize, arg0: usize) usize {
-            return asm volatile ("ecall"
-                : [result] "={a0}" (-> usize),
-                : [func] "{a0}" (func),
-                  [arg0] "{a1}" (arg0),
-                : "memory"
-            );
-        }
-
-        pub fn syscall2(func: usize, arg0: usize, arg1: usize) usize {
-            return asm volatile ("ecall"
-                : [result] "={a0}" (-> usize),
-                : [func] "{a0}" (func),
-                  [arg0] "{a1}" (arg0),
-                  [arg1] "{a2}" (arg1),
-                : "memory"
-            );
-        }
-    },
-    else => @compileError("Unsupported architecture"),
-};
-
-fn exit(code: u32) noreturn {
-    _ = syscall.syscall1(2, code);
-    unreachable;
-}
-
-fn debug_print(text: []const u8) void {
-    _ = syscall.syscall2(1, @intFromPtr(text.ptr), text.len);
-}
-
-export fn _start() linksection(".text.entry") callconv(.C) noreturn {
-    debug_print("Hello!");
-    exit(0);
+export fn _start(arg: usize) linksection(".text.entry") callconv(.C) noreturn {
+    log.println("arg=0x{x} ({})", .{ arg, arg });
+    syscall.exit(0);
 }
diff --git a/user/syscall.zig b/user/syscall.zig
new file mode 100644
index 0000000..7ac13d3
--- /dev/null
+++ b/user/syscall.zig
@@ -0,0 +1,12 @@
+const arch = @import("arch.zig");
+
+const sc = arch.syscall;
+
+pub fn exit(code: u32) noreturn {
+    _ = sc.syscall1(2, code);
+    unreachable;
+}
+
+pub fn debug_print(text: []const u8) void {
+    _ = sc.syscall2(1, @intFromPtr(text.ptr), text.len);
+}