Add basic types and a `load` function

src/lib.rs

@@ -1,3 +1,34 @@
-#[test]
-fn it_works() {
+#![feature(no_std)]
+#![no_std]
+
+pub unsafe fn load(address: usize) -> &'static Multiboot {
+    let multiboot = &*(address as *const Multiboot);
+    assert!(multiboot.has_valid_end_tag());
+    multiboot
+}
+
+#[repr(C)]
+pub struct Multiboot {
+    pub total_size: u32,
+    _reserved: u32,
+    // tags
+}
+
+impl Multiboot {
+    fn has_valid_end_tag(&self) -> bool {
+        const END_TAG: Tag = Tag{typ:0, size:8};
+
+        let self_ptr = self as *const _;
+        let end_tag_addr = self_ptr as usize + (self.total_size - END_TAG.size) as usize;
+        let end_tag = unsafe{&*(end_tag_addr as *const Tag)};
+
+        end_tag.typ == END_TAG.typ && end_tag.size == END_TAG.size
+    }
+}
+
+#[repr(C)]
+struct Tag {
+    typ: u32,
+    size: u32,
+    // tag specific fields
 }