lesson 23, initial commit

23-fixes/boot/switch_pm.asm

@@ -0,0 +1,22 @@
+[bits 16]
+switch_to_pm:
+    cli ; 1. disable interrupts
+    lgdt [gdt_descriptor] ; 2. load the GDT descriptor
+    mov eax, cr0
+    or eax, 0x1 ; 3. set 32-bit mode bit in cr0
+    mov cr0, eax
+    jmp CODE_SEG:init_pm ; 4. far jump by using a different segment
+
+[bits 32]
+init_pm: ; we are now using 32-bit instructions
+    mov ax, DATA_SEG ; 5. update the segment registers
+    mov ds, ax
+    mov ss, ax
+    mov es, ax
+    mov fs, ax
+    mov gs, ax
+
+    mov ebp, 0x90000 ; 6. update the stack right at the top of the free space
+    mov esp, ebp
+
+    call BEGIN_PM ; 7. Call a well-known label with useful code