From b199b02578512e6ea43261a386b1029343f478cb Mon Sep 17 00:00:00 2001
From: Philipp Oppermann <dev@phil-opp.com>
Date: Tue, 11 Apr 2017 14:54:35 +0200
Subject: [PATCH] Set up page tables for identity mapping

---
 src/arch/x86_64/boot.asm | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/src/arch/x86_64/boot.asm b/src/arch/x86_64/boot.asm
index 3fb4c06..71703ee 100644
--- a/src/arch/x86_64/boot.asm
+++ b/src/arch/x86_64/boot.asm
@@ -9,6 +9,8 @@ start:
     call check_cpuid
     call check_long_mode
 
+    call set_up_page_tables
+
     ; print `OK` to screen
     mov dword [0xb8000], 0x2f4b2f4f
     hlt
@@ -74,6 +76,33 @@ check_long_mode:
     mov al, "2"
     jmp error
 
+set_up_page_tables:
+    ; map first P4 entry to P3 table
+    mov eax, p3_table
+    or eax, 0b11 ; present + writable
+    mov [p4_table], eax
+
+    ; map first P3 entry to P2 table
+    mov eax, p2_table
+    or eax, 0b11 ; present + writable
+    mov [p3_table], eax
+
+    ; map each P2 entry to a huge 2MiB page
+    mov ecx, 0         ; counter variable
+
+.map_p2_table:
+    ; map ecx-th P2 entry to a huge page that starts at address 2MiB*ecx
+    mov eax, 0x200000  ; 2MiB
+    mul ecx            ; start address of ecx-th page
+    or eax, 0b10000011 ; present + writable + huge
+    mov [p2_table + ecx * 8], eax ; map ecx-th entry
+
+    inc ecx            ; increase counter
+    cmp ecx, 512       ; if counter == 512, the whole P2 table is mapped
+    jne .map_p2_table  ; else map the next entry
+
+    ret
+
 ; Prints `ERR: ` and the given error code to screen and hangs.
 ; parameter: error code (in ascii) in al
 error:
@@ -84,6 +113,13 @@ error:
     hlt
 
 section .bss
+align 4096
+p4_table:
+    resb 4096
+p3_table:
+    resb 4096
+p2_table:
+    resb 4096
 stack_bottom:
     resb 64
 stack_top: