From 92ad44ce3af9b000146d324e5c0f5dfce16927f9 Mon Sep 17 00:00:00 2001
From: Dennis Kobert <dennis@kobert.dev>
Date: Fri, 25 Oct 2019 03:59:21 +0200
Subject: Start 64bit mode

---
 kernel/src/boot.asm           | 41 +++++++++++++++++++++++++++++++++++++++--
 kernel/src/long_mode_init.asm | 17 +++++++++++++++++
 2 files changed, 56 insertions(+), 2 deletions(-)
 create mode 100644 kernel/src/long_mode_init.asm

diff --git a/kernel/src/boot.asm b/kernel/src/boot.asm
index 3d79096..e3484c1 100644
--- a/kernel/src/boot.asm
+++ b/kernel/src/boot.asm
@@ -1,4 +1,5 @@
 global start
+extern long_mode_start
 
 section .text
 bits 32
@@ -9,8 +10,12 @@ start:
 	call check_cpuid
 	call check_long_mode
 
-	; print 'OK'
-	mov dword [0xb8000], 0x2f4b2f4f
+	call setup_page_tables
+	call enable_paging
+	; load the 64-bit GDT
+    lgdt [gdt64.pointer]
+	jmp gdt64.code:long_mode_start
+
 	hlt
 
 check_multiboot:
@@ -105,6 +110,38 @@ setup_page_tables:
     jne .map_p2_table  ; else map the next entry
 
 	ret
+
+enable_paging:
+	mov eax, p4_table
+	mov cr3, eax
+
+    ; enable PAE-flag in cr4 (Physical Address Extension)
+    mov eax, cr4
+    or eax, 1 << 5
+    mov cr4, eax
+
+    ; set the long mode bit in the EFER MSR (model specific register)
+    mov ecx, 0xC0000080
+    rdmsr
+    or eax, 1 << 8
+    wrmsr
+
+    ; enable paging in the cr0 register
+    mov eax, cr0
+    or eax, 1 << 31
+    mov cr0, eax
+
+    ret
+
+section .rodata
+gdt64:
+    dq 0 ; zero entry
+.code: equ $ - gdt64
+    dq (1<<43) | (1<<44) | (1<<47) | (1<<53) ; code segment
+.pointer:
+    dw $ - gdt64 - 1
+    dq gdt64
+
 ; Prints 'ERR: ' and the given error code to the screen and halts
 ; parameter: error code letter (ascii) in al
 error:
diff --git a/kernel/src/long_mode_init.asm b/kernel/src/long_mode_init.asm
new file mode 100644
index 0000000..f105388
--- /dev/null
+++ b/kernel/src/long_mode_init.asm
@@ -0,0 +1,17 @@
+global long_mode_start
+
+section .text
+bits 64
+long_mode_start:
+	; load 0 into all data segment registers
+    mov ax, 0
+    mov ss, ax
+    mov ds, ax
+    mov es, ax
+    mov fs, ax
+    mov gs, ax
+
+    ; print `Uff!` to screen
+    mov rax, 0x8521846683668255
+    mov qword [0xb8000], rax
+    hlt
-- 
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