...

src/boot.asm

@@ -1,5 +1,4 @@
 global _start
-extern kernel_main
 
 %define FLAGS       0b10
 %define MAGIC       0x1BADB002
@@ -11,21 +10,20 @@ section .multiboot
     dd FLAGS
     dd CHECKSUM
 
-section .bss
-    align 4096
-    pml4_table:
-        resb 4096
-    pdpt_table:
-        resb 4096
-    pd_table:
-        resb 4096
-
 section .bss
     align 16
-    stack_bottom:
         resb 16384
     stack_top:
 
+section .bss
+    align 4096
+    pml4:
+        resb 4096
+    pdpt:
+        resb 4096
+    pd:
+        resb 4096
+
 section .data
     align 8
     gdt64:
@@ -48,7 +46,7 @@ section .text
         cmp eax, 0x2BADB002
         jne error
 
-        ; Check if cpuid is available by flipping the 22-nth youngest bit
+        ; Check if cpuid is available by flipping bit 21
         ; in the eflags register and checking if the cpu flipped it back
         pushfd
         pop eax
@@ -74,33 +72,51 @@ section .text
 
         ; Check if long mode is available by calling cpuid with 0x80000001
         ; this will place the extended features of the cpu in edx
-        ; The 30-nth youngest bit tells us if long mode is supported or not
+        ; Bit 29 tells us if long mode is supported or not
         mov eax, 0x80000001
         cpuid
         test edx, 1 << 29
         jz error
 
-        ; todo(nub31): setup paging
-        ; todo(nub31): enter long mode
+        ; Enable PAE by setting bit 5 in cr4 to 1
+        mov eax, cr4
+        or eax, 1 << 5
+        mov cr4, eax
+
+        ; todo(nub31): Set up page tables
+
+        ; Load cr3 with the address of pml4
+        mov eax, pml4
+        mov cr3, eax
 
-        ; Load global descriptor table which is set up for 64 bit
         lgdt [gdt64.descriptor]
 
-        call kernel_main
-        jmp error
+        ; Enable long mode by setting bit 8 to 1 in EFER (Extended Feature Enable Register)
+        mov ecx, 0xc0000080
+        rdmsr
+        or eax, 1 << 8
+        wrmsr
+
+        ; Enable paging bt setting bit 31 in cr0 to 1
+        mov eax, cr0
+        or eax, 1 << 31
+        mov cr0, eax
+
+        jmp 0x8:long_mode
 
     error:
         cli
-        mov byte [0xb8000], 'B'
-        mov byte [0xb8002], 'O'
-        mov byte [0xb8004], 'O'
-        mov byte [0xb8006], 'T'
-        mov byte [0xb8008], ' '
-        mov byte [0xb800a], 'E'
-        mov byte [0xb800c], 'R'
-        mov byte [0xb800e], 'R'
-        mov byte [0xb8010], 'O'
-        mov byte [0xb8012], 'R'
-    hang:
+        mov byte [0xb8000], 'E'
+        mov byte [0xb8002], 'R'
+        mov byte [0xb8004], 'R'
+    .hang:
         hlt
-        jmp hang
+        jmp .hang
+
+section .text
+    bits 64
+    long_mode:
+        cli
+    .hang:
+        hlt
+        jmp .hang