From a3e0852ec782fe10e59ded42fe11a472a77458d4 Mon Sep 17 00:00:00 2001
From: Jenny Curle <thecurle12@gmail.com>
Date: Sat, 6 Apr 2019 20:00:11 +0100
Subject: [PATCH] Begin support for Global Descriptor Tables

---
 arch/i386/boot.s     | 18 ++++++++++++++++
 include/kernel/gdt.h |  4 ++++
 kernel/gdt.c         | 50 ++++++++++++++++++++++++++++++++++++++++++++
 kernel/kernel.c      |  5 ++++-
 makefile             |  3 ++-
 5 files changed, 78 insertions(+), 2 deletions(-)
 create mode 100755 include/kernel/gdt.h
 create mode 100755 kernel/gdt.c

diff --git a/arch/i386/boot.s b/arch/i386/boot.s
index 5504693..b1be172 100755
--- a/arch/i386/boot.s
+++ b/arch/i386/boot.s
@@ -10,12 +10,30 @@
 .long FLAGS
 .long CHECKSUM
 
+# C stack startup
 .section .bss
 .align 16
 stack_bottom:
 	.skip 16384
 stack_top:
 
+# GDT segment register setup
+.global _gdt_flush
+.extern _gp
+_gdt_flush:
+	lgdt (_gp)    #load the GDT with the special pointer
+	mov %ax, 0x10  #offset from GDT to data segment
+	mov %ds, ax
+	mov %es, ax
+	mov %fs, ax
+	mov %gs, ax
+	mov %ss, ax
+	jmp 0x08:flush2 #long jump
+flush2:
+	ret           #return to c code
+	
+
+# start function - calls kernel_main
 .section .text
 .global _start
 .type _start, @function
diff --git a/include/kernel/gdt.h b/include/kernel/gdt.h
new file mode 100755
index 0000000..780ba8f
--- /dev/null
+++ b/include/kernel/gdt.h
@@ -0,0 +1,4 @@
+#pragma once
+
+void gdt_set_gate(int, unsigned long, unsigned long, unsigned char, unsigned char);
+void gdt_install();
\ No newline at end of file
diff --git a/kernel/gdt.c b/kernel/gdt.c
new file mode 100755
index 0000000..f2bbd93
--- /dev/null
+++ b/kernel/gdt.c
@@ -0,0 +1,50 @@
+#include <kernel/utils.h>
+
+struct gdt_item {
+	unsigned short low_limit;
+	unsigned short low_base;
+	unsigned char middle_base;
+	unsigned char access;
+	unsigned char granular;
+	unsigned char high_base;
+} __attribute__((packed)); //Prevent compiler optimisation by packing
+
+struct gdt_ptr {
+	unsigned short limit;
+	unsigned int base;
+} __attribute__((packed));
+
+struct gdt_item gdt[3]; //3-entry gdt
+struct gdt_ptr gp;
+
+extern void gdt_flush();
+
+void gdt_set_gate(int num, unsigned long base,
+				unsigned long limit, unsigned char access,
+				unsigned char gran) {
+		//Implementation taken from osdever.net
+		gdt[num].low_base = (base & 0xFFFF);
+		gdt[num].middle_base = (base >> 16) & 0xFF;
+		gdt[num].high_base = (base >> 24) & 0xFF;
+		
+		gdt[num].low_limit = (limit & 0xFFFF);
+		gdt[num].granular = ((limit >> 16) & 0x0F);
+		
+		gdt[num].granular != (gran & 0xF0);
+		gdt[num].access = access;
+}
+
+void gdt_install() {
+	gp.limit = (sizeof(struct gdt_item) * 3) - 1;
+	gp.base = &gdt;
+	
+	gdt_set_gate(0, 0, 0, 0, 0); //NULL item
+	
+	//Code Segment - base 0, 4KiB granularity, 4GiB limit
+	gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);
+	
+	//Data Segment - ditto
+	gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
+	
+	//Apply changes
+	gdt_flush();
\ No newline at end of file
diff --git a/kernel/kernel.c b/kernel/kernel.c
index 893c8b3..77a7063 100755
--- a/kernel/kernel.c
+++ b/kernel/kernel.c
@@ -1,8 +1,11 @@
 //#include <stdio.h>
 
-#include "kernel/tty.h"
+#include <kernel/tty.h>
+#include <kernel/gdt.h>
 
 int kernel_main(void) {
+	//Prepare the GDT
+	gdt_install()
 	//Prepare the screen, and blank it out.
 	screen_initialize();
 	
diff --git a/makefile b/makefile
index cd00da8..060d5ca 100755
--- a/makefile
+++ b/makefile
@@ -30,6 +30,7 @@ LIBS:=$(LIBS) $(KERNEL_ARCH_LIBS)
 KERNEL_OBJS=           \
 $(KERNEL_ARCH_OBJS)    \
 kernel/utils.o         \
+kernel/gdt.o           \
 kernel/kernel.o        
 
 OBJS=\
@@ -93,4 +94,4 @@ gen-iso:
 		iso
 	
 -include $(OBJS:.o=.d)
-
+