Added GDT.

Unfortunately I can't do it in C, but whatever.

src/kernel/arch/x86/boot.s

@@ -32,12 +32,27 @@ stack_bottom:
 	resb 16384  ; 16KiB
 stack_top:
 
+section .data
+%include "gdt.s"
+
 section .text
 global _start:function (_start.end - _start)
 _start:
 	; setup the stack
 	mov esp, stack_top
 
+	; TODO: setup GDT, IDT, paging, etc. here
+	cli
+	lgdt [gdtr]
+	jmp 0x08:.reload_segs
+.reload_segs:
+	mov ax, 0x10
+	mov ds, ax
+	mov es, ax
+	mov fs, ax
+	mov gs, ax
+	mov ss, ax
+
 	; call the kernel
 	extern kernel_main
 	call kernel_main

src/kernel/arch/x86/gdt.s

@@ -0,0 +1,68 @@
+; Copyright (C) 2018  Ortega Froysa, Nicolás <nortega@themusicinnoise.net>
+; Author: Ortega Froysa, Nicolás <nortega@themusicinnoise.net>
+;
+; This program is free software: you can redistribute it and/or modify
+; it under the terms of the GNU General Public License as published by
+; the Free Software Foundation, either version 3 of the License, or
+; (at your option) any later version.
+;
+; This program is distributed in the hope that it will be useful,
+; but WITHOUT ANY WARRANTY; without even the implied warranty of
+; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+; GNU General Public License for more details.
+;
+; You should have received a copy of the GNU General Public License
+; along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+gdt_start:
+
+gdt_null:
+	dd 0x0
+	dd 0x0
+
+gdt_kcode:
+	dw 0xFFFF     ; limit (bits 0-15)
+	dw 0x0        ; base (bits 0-15)
+
+	db 0x0        ; base (bits 16-23)
+	db 10011010b  ; 1st flags | type flags
+	db 11001111b  ; 2nd flags | limit (bits 16-19)
+	db 0x0        ; base (bits 24-31)
+
+gdt_kdata:
+	dw 0xFFFF
+	dw 0x0
+
+	db 0x0
+	db 10010010b
+	db 11001111b
+	db 0x0
+
+gdt_ucode:
+	dw 0xFFFF
+	dw 0x0
+
+	db 0x0
+	db 11111010b
+	db 11001111b
+	db 0x0
+
+gdt_udata:
+	dw 0xFFFF
+	dw 0x0
+
+	db 0x0
+	db 11110010b
+	db 11001111b
+	db 0x0
+
+; TODO: fill this in later
+gdt_tss:
+	dd 0x0
+	dd 0x0
+
+gdt_end:
+
+gdtr:
+	dw gdt_end  - gdt_start - 1
+	dd gdt_start

src/kernel/arch/x86/tss.h

@@ -18,7 +18,7 @@
 
 #pragma once
 
-volatile struct tss {
+struct tss {
 	uint16_t link;
 	uint16_t link_r;    // reserved