From 9703f2484c975900a2b4f20e5a9dc00ca2ac9f3f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nicol=C3=A1s=20Ortega=20Froysa?=
 <nortega@themusicinnoise.net>
Date: Wed, 14 Mar 2018 17:10:51 +0100
Subject: [PATCH] Added GDT.

Unfortunately I can't do it in C, but whatever.
---
 Makefile                   |  2 +-
 src/kernel/arch/x86/boot.s | 15 +++++++++
 src/kernel/arch/x86/gdt.s  | 68 ++++++++++++++++++++++++++++++++++++++
 src/kernel/arch/x86/tss.h  |  2 +-
 4 files changed, 85 insertions(+), 2 deletions(-)
 create mode 100644 src/kernel/arch/x86/gdt.s

diff --git a/Makefile b/Makefile
index de11eb2..445fb2d 100644
--- a/Makefile
+++ b/Makefile
@@ -20,7 +20,7 @@ ARCH=$(shell echo $(TARGET) | sed s/i.86/x86/)
 CROSSFLAGS=--target=$(TARGET)-pc-none-elf -march=$(TARGET)
 # Assembly options
 AS=nasm
-AFLAGS=-felf32
+AFLAGS=-felf32 -Isrc/kernel/arch/$(ARCH)/ -Isrc/
 # C options
 CC=clang
 CFLAGS?=-O0 -g
diff --git a/src/kernel/arch/x86/boot.s b/src/kernel/arch/x86/boot.s
index 4e32d72..46705ac 100644
--- a/src/kernel/arch/x86/boot.s
+++ b/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
diff --git a/src/kernel/arch/x86/gdt.s b/src/kernel/arch/x86/gdt.s
new file mode 100644
index 0000000..c420b90
--- /dev/null
+++ b/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
diff --git a/src/kernel/arch/x86/tss.h b/src/kernel/arch/x86/tss.h
index c3e8d45..ab65b58 100644
--- a/src/kernel/arch/x86/tss.h
+++ b/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