bios_disk_load works

protected_mode.S

@@ -0,0 +1,64 @@
+/*
+# Protected mode
+
+TODO get working.
+
+- http://wiki.osdev.org/Journey_To_The_Protected_Land
+- http://wiki.osdev.org/Protected_Mode
+- http://stackoverflow.com/questions/28645439/how-do-i-enter-32-bit-protected-mode-in-nasm-assembly
+- https://github.com/chrisdew/xv6/blob/master/bootasm.S
+- https://thiscouldbebetter.wordpress.com/2011/03/17/entering-protected-mode-from-assembly/ FASM based. Did not word on first try, but looks real clean.
+
+## GDT
+
+Table in memory that gives properties of segment registers.
+
+Segment registers in protected mode point to entries of that table.
+
+The GDT modifies every memory access of a given segment by adding an offset to it.
+
+GDT is used as soon as we enter protected mode, so that's why we have to deal with it, but the preferred way of managing program memory spaces is paging.
+
+## GDTR
+
+## GDT register
+
+In 32-bit, a 6 byte register that holds:
+
+- 2 byte length of the GDT (TODO in bytes or number of entries?)
+- 4 byte address of the GDT in memory
+
+In 64 bit, makes 10 bytes, with the address having 8 bytes
+
+GRUB seems to setup one for you: http://www.jamesmolloy.co.uk/tutorial_html/4.-The%20GDT%20and%20IDT.html
+*/
+
+#include "common.h"
+BEGIN
+cli
+
+    /* Set the GDT register with start address of Global Descriptor Table */
+    lgdt gdt
+    mov %cr0, %eax
+    /* Set PE (Protection Enable) bit in CR0 (Control Register 0) */
+    or $1, %al
+    mov %eax, %cr0
+
+    /*
+    TODO why 8?
+
+    Perform far jump to selector 08h (offset into GDT,
+    pointing at a 32bit PM code segment descriptor)
+    to load CS with proper PM32 descriptor).
+    */
+    ljmp $0x08, $PModeMain
+
+    PModeMain:
+    /* TODO load DS, ES, FS, GS, SS, ESP. */
+
+    hlt
+
+    gdt:
+    .word 0x1234
+    .long 0x12345678
+END