[boot] implement page-table mapping in pure Rust

moves code from 05d24c8 into this branch

boot/src/lib.rs

@@ -2,7 +2,7 @@
 //  SOS: the Stupid Operating System
 //  by Eliza Weisman (hi@hawkweisman.me)
 //
-//  Copyright (c) 2016 Eliza Weisman
+//  Copyright (c) 2016-2017 Eliza Weisman
 //  Released under the terms of the MIT license. See `LICENSE` in the root
 //  directory of this repository for more information.
 //
@@ -11,6 +11,7 @@
 #![crate_type = "staticlib"]
 #![feature(asm)]
 #![feature(lang_items)]
+#![feature(naked_functions)]
 #![no_std]
 
 const TABLE_LENGTH: usize = 512;
@@ -25,37 +26,37 @@ extern fn panic_fmt(fmt: Arguments, file: &'static str, line: u32) -> ! {
 }
 
 extern {
-	static mut pml4_table: Table;
-	static mut pdp_table: Table;
-	static mut pd_table: Table;
-	static mut page_table: Table;
+    static mut pml4_table: Table;
+    static mut pdp_table: Table;
+    static mut pd_table: Table;
+    static mut page_table: Table;
     //
-	// static low_end: u8;
-	// static kernel_size: u8; // &kernel_size == kernel size in pages
+    // static low_end: u8;
+    // static kernel_size: u8; // &kernel_size == kernel size in pages
 }
 
 #[no_mangle]
+#[naked]
 pub unsafe extern fn create_page_tables() {
+    const HUGE_PAGE_SIZE: u64 = 2 * 1024 * 1024; // 2 MiB
+
+    //-- map the PML4 and PDP tables -----------------------------------------
     // recursive map last PML4 entry
     pml4_table[511] = (&pml4_table as *const Table as u64) | 3;
     // map first PML4 entry to PDP table
     pml4_table[0] = (&pdp_table as *const Table as u64) | 3;
     // map first PDPT entry to PD table
     pdp_table[0] = (&pd_table as *const Table as u64) | 3;
 
-    // map the PD table
-    // TODO: implement this in Rust
-    asm!("
-        // map each PD table entry to its own 2mB page
-        mov         ecx, 0
-    .pd_table_map: // maps the PD table ----------------------------------------
-        mov     eax, 0x200000   // 2 mB
-        mul     ecx             // times the start address of the page
-        or      eax, 0b10000011 // check if present + writable + huge
-        mov     [pd_table + ecx * 8], eax // map nth entry from pd -> own page
-        // increment counter and check if done
-        inc     ecx
-        cmp     ecx, 512
-        jne     .pd_table_map
-        " :::: "volatile", "intel");
+    //-- map the PD table ----------------------------------------------------
+    for (number, entry) in pd_table.iter_mut().enumerate() {
+        // set each PD entry equal to the start address of the page (the page
+        // number times the page's size)
+        let addr = number as u64 * HUGE_PAGE_SIZE;
+        // with the appropriate flags (present + writable + huge)
+        // TODO: do we want to do this using bitflags, or is that too
+        //       heavyweight for the boot module?
+        //          - eliza, 1/23/2017
+        *entry = addr | 0b10000011;
+    }
 }