Cleanup ACPI code, find lower tables

arch/x86_64/src/acpi/sdt.rs

@@ -1,6 +1,8 @@
+use core::mem;
+
 #[derive(Copy, Clone, Debug)]
 #[repr(packed)]
-pub struct SDTHeader {
+pub struct SDT {
   pub signature: [u8; 4],
   pub length: u32,
   pub revision: u8,
@@ -9,5 +11,23 @@ pub struct SDTHeader {
   pub oem_table_id: [u8; 8],
   pub oem_revision: u32,
   pub creator_id: u32,
-  pub creator_revision: u32,
+  pub creator_revision: u32
+}
+
+impl SDT {
+    /// Get the address of this tables data
+    pub fn data_address(&'static self) -> usize {
+        self as *const _ as usize + mem::size_of::<SDT>()
+    }
+
+    /// Get the length of this tables data
+    pub fn data_len(&'static self) -> usize {
+        let total_size = self.length as usize;
+        let header_size = mem::size_of::<SDT>();
+        if total_size >= header_size {
+            total_size - header_size
+        } else {
+            0
+        }
+    }
 }