diff --git a/Cargo.lock b/Cargo.lock
index 986e98c..b86f719 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -252,6 +252,7 @@ version = "0.1.0"
 dependencies = [
  "num-derive",
  "num-traits",
+ "utils",
 ]
 
 [[package]]
diff --git a/uapi/Cargo.toml b/uapi/Cargo.toml
index 4e9f8fb..6c2bc7f 100644
--- a/uapi/Cargo.toml
+++ b/uapi/Cargo.toml
@@ -4,5 +4,6 @@ version = "0.1.0"
 edition = "2021"
 
 [dependencies]
+utils = { path = "../lib/utils" }
 num-traits = { version = "0.2", default-features = false }
 num-derive = "0.4"
diff --git a/uapi/src/lib.rs b/uapi/src/lib.rs
index b00bfa1..d9519a5 100644
--- a/uapi/src/lib.rs
+++ b/uapi/src/lib.rs
@@ -6,4 +6,5 @@ extern crate num_derive;
 
 pub mod cap;
 pub mod error;
+pub mod syscall;
 pub mod vspace;
diff --git a/uapi/src/syscall.rs b/uapi/src/syscall.rs
new file mode 100644
index 0000000..ff24318
--- /dev/null
+++ b/uapi/src/syscall.rs
@@ -0,0 +1,71 @@
+use core::fmt::Debug;
+
+use num_traits::{FromPrimitive, ToPrimitive};
+use utils::MASK;
+
+use crate::error::SysError;
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq, FromPrimitive, ToPrimitive)]
+pub enum Syscall {
+    Invalid = 0,
+}
+
+/* in vanilla seL4, MessageInfo layout:
+ * > | label   | caps   | extra_caps | length |
+ * > | [63:12] | [11:9] | [8:7]      | [6:0]  |
+ *
+ * in out implementation, MessageInfo layout:
+ * > label: Syscall
+ * > length: args.len()
+ *
+ * Syscall convention:
+ * > Call:
+ * > > [a0]:     MessageInfo
+ * > > [a1]:     Capability Pointer
+ * > > [a3..a7]: extra arguments
+ * > > length = (a0..a?).len()
+ * > Reply:
+ * > TODO: figure out how vanilla seL4 handle reply
+ */
+
+#[derive(Clone, Copy)]
+pub struct MessageInfo(usize);
+
+impl MessageInfo {
+    const LABEL_MASK: usize = MASK!(63 - 12 + 1);
+
+    pub fn new(label: Syscall, length: usize) -> Self {
+        Self((label.to_usize().unwrap_or(0)) << 12 | (length & 0x7f))
+    }
+
+    pub fn label(&self) -> Syscall {
+        Syscall::from_usize((self.0 >> 12) & Self::LABEL_MASK).unwrap_or(Syscall::Invalid)
+    }
+
+    pub fn length(&self) -> usize {
+        self.0 & 0x7F
+    }
+
+    pub fn as_usize(&self) -> usize {
+        self.0
+    }
+}
+
+impl Debug for MessageInfo {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.debug_struct("MessageInfo")
+            .field("label", &self.label())
+            .field("length", &self.length())
+            .finish()
+    }
+}
+
+impl TryFrom<usize> for MessageInfo {
+    type Error = SysError;
+
+    fn try_from(value: usize) -> Result<Self, Self::Error> {
+        let syscall = Syscall::from_usize((value >> 12) & MessageInfo::LABEL_MASK).ok_or(SysError::InvalidArgument)?;
+        let length = value & 0x7F;
+        Ok(Self::new(syscall, length))
+    }
+}